LOKO I‘A A Manual on Hawaiian Fishpond Restoration and Management (2007)

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In-pond transport

A barge provides a platform for group viewing, feeding,

carrying supplies and materials around the fishpond, and

transfering and harvesting fish. The barge in this photo

is constructed of six plastic floats, is 8 ft by 12 ft, and

can carry approximately 2000 pounds.

A small dinghy is a useful vehicle for transportation

to and from pen culture systems for feeding and water

quality testing. The boat also services the production

operations by transporting supplies and materials. The

boat in this photo is also equipped with a small, 12-volt

electric motor (not pictured).

Tide calendar

A tide calendar shows daily tide predictions for specific

areas. These calendars are essential for fishpond opera-

tions, because the monthly projections help in scheduling

activities. By understanding tidal heights and durations

at various stages, pond operators can determine the best

scenarios for harvesting, transporting fish, and other

tasks. Tide charts need to be adjusted for each locality,

as they differ slightly in timing from one site to another.

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Net-Pen Production

awai‘i’s land, ocean, and cultural environment

These systems are not without risk, however, and

has changed irrevocably since the times when fish-

if production is too intensive, negative impacts (such

pond aquaculture flourished, and the conditions under

as high stock densities and feeding rates and increased

which those marvelous innovations were traditionally

levels of animal waste pollution) can result. In ancient

managed are gone.

times, a fishpond’s natural productivity was carefully

Managers of the ancient fishponds could call upon

nurtured rather than overly exploited. Traditional fish-

the entire populace of an ahupua‘a for assistance with

pond production was “extensive,” meaning fish culture

construction, repair, and maintenance. Today, significant

was conducted with a minimum impact or expense to the

effort is required to rebuild fishpond walls and renew

system. This resulted in low yields (somewhere between

pond ecosystems, but the social structure that once pro-

400 and 600 pounds per acre per year). By comparison,

vided that labor is also generally gone.

in today’s “intensive” systems, the practice would aim

In ancient times the shoreline ecosystem that was

to produce over 2000 pounds per acre per year, with in-

modified by Hawaiians to create an environment to raise

creased labor, capital, and system impacts, both physical

fish and other seafood products was relatively pristine.

and biological.

Today, environmental factors such as invasive plant and

Traditional Hawaiian production utilized mullet

marine species, land development for agriculture and

and milkfish as the primary culture species. These two

urban uses, and redirection of freshwater streams have

herbivores are, at the same time, the most important

contributed to degradation of shoreline conditions to

biological maintenance tools of the fishpond. Feeding

various degrees.

directly into the primary productivity level of photo-

synthetic and benthic (ocean bottom) organisms, these

Modern net-pen technology

species maintain the fishpond’s environmental integrity.

The methods illustrated in this guidebook utilize modern

The mullet/milkfish production rates proposed in

net-pen materials in fishponds and combine traditional

this guidebook are conservative and would appear to

pond management methods with applicable contempo-

be a feasible starting point within a typical fishpond

rary technology.

system, ocean and weather conditions considered. More

Net-pens for aquaculture enclose areas within a

site-specific variations in chosen culture species, water

fishpond and confine fish to a particular location. Their

character, water quality, and management style should

use is about as old as aquaculture itself, starting with

be equally assessed throughout the production run and

carp growers in China 2000 years ago. Today, net-pen

adjusted where applicable.

production in the open ocean and bays makes a sub-

Net-pen culture within a protected area can func-

stantial contribution to the total amount of farm-raised

tion well with other aquaculture activities that may go

seafood, thus proving the success of the methodology.

on in adjacent locations within fishponds, such as the

The benefits of net-pens include the

cultivation of limu, coral, or even aquarium species.

• ability to culture species within ponds that can no

The materials to be used in construction of the net-pen

longer provide a secure, safe enclosure for the fish

are readily available, and the methods of construction

• ease of production management (e.g., stocking, sam-

require only hand tools and basic construction skills.

pling, harvesting)

• transportability of materials to almost any desirable

Net-pen design parameters

site for construction

The fundamental purpose of any net-pen is to provide

• manageability of invasive and predatory species.

a secure, safe enclosure for the species being cultured.

Developing a diagram on paper is recommended before

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doing any work in the water. The diagram should take

into consideration measurements and parameters such

Considerations before building a net-pen

as pen height, surface area, stocking density and water

When building a net-pen, the following practices

depth. Special consideration should be given to plan for

should be followed or taken into account:

the highest possible tide, because the top of the net-pen

• wear solid footwear to protect feet

• wear shirt, hat, gloves, eye protection, and long-

should never go below the water level, which would allow

lasting sunscreen to reduce sun exposure

fish to escape. The average water depth in a fishpond is

• wear a back brace for support when lifting

approximately 4 feet. Tide fluctuations can range another

• two or more people are needed, depending on

2¹⁄2

feet once or twice annually. The net-pen can be as

pen size

large as the total pond area will allow, but using a size

• size of final pen depends on projected production

that serves production “workability” is most practical.

Before building a net-pen, gather the following

Two types of net-pens have been used in production

equipment and materials:

trials on Moloka‘i. Rectangular net-pens were con-

• rolls of ¹⁄4-1-inch plastic mesh (11 guage)

structed for nursery culture, from the initial stocking to

• boat, raft

• post pounder

about ¹⁄4-¹⁄2 pound size. Then the fish were transferred to

• rope, ¹⁄4 inch or more thick and about 20 ft longer

large, round pens for growing out to a harvestable size

than the radius of the circle

of over 1 pound. Thus there were two different net-pen

• ultraviolet-resistant plastic cable ties, 11 inches

shapes and production management styles.

and 7-8 inches (for example, a 300-foot cir-

As a general rule, net-pen culture requires a total

cumference pond needs about 1000 ties, 300

11-inch ties and 700 7-9-inch ties)

pond area approximately 10 times the area in which the

• wire clippers

fish are confined. For example, a rectangular net-pen

• T-posts (“T-stands”), 8-ft or 10-ft; ⁵⁄8-inch rebar

50 by 100 feet has an area of 5000 square feet. A single

to extend posts if necessary

net-pen of 5000 square feet requires approximately one

acre (45,600 square feet) of water surface area. In other

words, in a pond one acre in size, one net-pen of 5000 sq

ft area could be constructed. In a pond 10 acres in size,

up to 10 net-pens of that size could be constructed.

Circle calculations

A circular net-pen with an 85-foot diameter has an

area of 7,880 square feet. In terms of the area confined

circumference = 2 π r = π d = 3.1416 d

within a given length of net, a circular net-pen is more

economical and efficient because it provides 36 percent

area = π r²

more pen area than a square one.

r = radius, d = diameter, π = 3.1416

circumference

Rectangle calculations

area = a x b

perimeter = 2 (a + b)

if square, a = b

diagonal =

a² + b²

area (shaded)

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Check the site for level floor (step 1).

Net-pen construction

Net-pen construction usually takes place in water that

can be as much as chest deep, and it is very difficult to

Drive in the center post (step 2).

illustrate the actual process as it takes place underwater.

Therefore, we constructed a demonstration net-pen on

land to better show the details of this process.

Step 2

Step 1

With the net-pen area chosen, drive a T-post in where

If possible, identify an area within the fishpond that is

you want the center of the pen to be.

1¹⁄2-2 times the size of the planned net-pen, because ad-

justments in the pen’s placement may need to be made.

Step 3a

After identifying a site that may meet the biological

Tie a loop in the end of the rope and put it over the post.

and physical needs for production, make a thorough

Measure the pen radius distance from the center post, and

inspection of the site using a mask and snorkel. Next,

tie another loop in the rope at the length of the radius.

slowly walk over the projected area; for a large area, it

Holding the rope near the center post and gradually let-

helps to have several people, 10-15 feet apart. Although

ting it pass through your hands, walk around the center

bare feet can allow a better feel for the bottom, injuries

post in expanding circles until you reach the circumfer-

are highly likely, so boots or shoes should be worn at

ence, at the end of the radius. This allows you to check

all times. Probe the bottom with a rod or stick to “feel”

the floor area within the circumference for any problems,

depths below any layer of sediment on the pond floor.

particularly for imperfections near the circumference that

The ideal site placement for a net-pen should have

may hinder the flat lay of the pen’s side.

the following characteristics:

• bottom contour: flat and level

• bottom type: coarse sand

• water quality: best possible or high in dissolved

oxygen

• water flow: light, consistent flow with tide

• wind currents: open to prevailing tradewind

• water depth: about 4-6 feet

• accessibility: close to the work and storage facility.

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Use the rope find the location of the second circumference

post (see diagram).

Step 4

Make a third loop in the measuring rope to define the

chosen distance between circumference posts (10 ft

or less). Using the first circumference post as a start-

ing point, put the “distance” loop over it and pull the

radius loop so that the two parts of the rope are tight,

Position the first circumference post at the limit of the

radius.

loop-to-loop. This will define the location of the next

post on the circumference. Continue clockwise around

Step 3b

the circle, moving the distance loop to each successive

Pull the rope out to the circumference and hold it tight

post pounded in, and using the radius loop to find the

and level. Drive the first circumference post straight

location for the next post.

down to a depth that has a solid foundation and still

provides enough post height to keep fish in at the highest

tide level. Circumference posts should be spaced about

2. Put the “distance” loop on the first circumference post

10 feet apart, at most. Because the ocean bottom and

and pull the radius loop tight to find the next circumfer-

ence post. Then, keep shifting the “distance” loop clock-

netting materials will differ with the circumstances, the

wise to complete the circle of circumference posts.

posts may need to be less than 10 feet apart to provide

the wire mesh with sufficient support. Determine the

number of circumference posts needed by dividing the

Circumference

desired circumference of the pen by 10 feet, or whatever

post #3

other distance is desired between posts.

1. Drive the first circumference post at the end of the

Circumference

radius.

Center

post #2

Center

post

“Distance”

post

Radius

loop

~10 ft

between

Center

posts

loop

Center

Circumference

loop

post #1

“Distance”

Circumference

Radius

Rope with

loop

post #1

loop

three loops

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Finish driving in the circumference posts (step 5).

Lay out the mesh near the circumference posts (step 6).

Step 5

Repeat Step 4 to find the place to drive each successive

circumference post. Continue until the circle of circum-

ference posts is complete. Don’t hesitate to adjust the

distance between circumfrence posts if you hit a problem

area and the post is hard to drive into the pond bottom.

In general, putting the posts closer together is preferred

to their being farther apart, because it increases the net’s

stability. However, when adjusting post-to-post distance,

it is best to keep the radius rope tight so the circumfer-

ence remains circular. Don’t worry about having the

distance between the last and the first posts be the same

as between the other circumference posts, because it

Start lifting the mesh up and wrapping it against the posts

should always be shorter.

(step 7).

Step 6

Collect the materials for attaching the plastic-coated

wire mesh to the posts. This outer screen, of about 11

gauge plastic with a 1 x 1 inch mesh and 4 feet wide, is

a heavy-duty material to keep out crab and eel predators

and to add integrity to the pen structure. It may be ad-

Step 7

vantageous to have a few people assisting in this activity.

Do not try to lift the wire mesh upright until its bottom

Also useful is a boat, raft, or other water vehicle to carry

edge is brought as close as possible to the circumference

materials and supplies to the net-pen site.

support posts. If a level site has been chosen, the top

Start to unroll the wire mesh next to the net-pen site.

edge of the wire mesh will be level. If an uneven site has

The wire mesh will partially sink when this is done in

been chosen, the wire mesh will either lean in or lean

the water. Pull the front edge of the wire mesh and lay

out. Level the site bottom manually, using a shovel; if

it along one side of the pen circumference. If not many

the bottom cannot be made level, it may be necessary to

people are helping, hold the mesh upright as it unrolls;

move the pen to a better location. Continue to adjust the

otherwise it will tend to lie flat on the pond floor.

wire mesh around the circumference posts.

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Bring mesh upright against the circumference posts

Tie mesh to the circumference posts (step 9).

(step 8).

Step 8

Starting on one side of the pen, stand the wire mesh up

against each circumference post and continue around

the pen until the mesh is upright and tight against all

the circumference posts. Recheck to make sure the wire

mesh is flush against the circumference post from top to

bottom. Use plastic cable ties to attach the wire mesh to

the post, placing one every 8-10 inches.

Step 9

Shown at right above is the appropriate method for at-

taching the mesh to the post with plastic cable ties. When

attaching the cable ties, one person is needed to stand

inside the net-pen to help secure the cable tie, which is

Overlap the mesh together to complete the pen (step 10).

inserted through the wire mesh and around the post by the

person standing on the outside. Start from the bottom and

secure ties about 8-10 inches apart, moving upward. If

another panel of wire mesh will be added to increase pen

height, leave the top of the lower panel loose (no cable

Step 10

ties) to allow for the upper panel to be tucked in between

Pull the wire mesh ends together and overlap them a

the T-post and the inside of the lower panel. Now, if it is

minimum of 1¹⁄2-2 feet on the last post. The overlap

necessary to increase the height, attach the plastic mesh,

should be level along the top and bottom, then secured

rebar, and additional upper panels to the T-posts using

tight to the circumference posts. It is important to have

11-inch ties. Continue this process until all posts but the

posts at the area of overlap for strength; if a post is not

last one has mesh on it.

there already, add one.

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Pound rebar into the ocean bottom to add height to the

Add mesh to increase pen wall height (step 12).

mesh wall (step 11).

Step 11

Step 12

Often an additional panel may be needed to add height to

Any mesh panel needed to extend the height of the pen

the top of the net-pen for periods when tides are at their

should be inserted between the rebar and T-post sup-

highest. To increase the net-pen height, the circumfer-

ports and the lower mesh panel. Cable ties should be

ence post must be tall enough so that the wire mesh can

connected only to the bottom half of the support post

be securely connected at the top. If the post is not tall

during the first wiring. The extension panel is slid down

enough, a piece of ⁵⁄8-inch rebar longer than the post can

between the T-post and the lower mesh panel until it is

be attached to raise the overall height of the post. To at-

at the desired height. The two panels need to be as tight

tach the rebar to the post, the post must first be embedded

as possible so as not to leave gaps at the overlap where

in the ocean bottom about 8-10 inches. With the post

animals can enter or exit the net-pen. Working on two

pounder over the rebar, carefully pull the post out of the

to three posts at a time, always be vigilant to maintain

way and pound down the rebar. Note: if it is known that

the top edge of the extension panel as level as possible

additional height is necessary, this step can be also done

all the way around the net-pen. The lower mesh panel

when the original T-post is driven into the ocean bottom

can usually be used as a guide when the pen is on a level

and before the first mesh panel is attached.

location, because the lower panel should already be level

and even with the ocean bottom. Continue securing the

top panel, as described, until the net-pen is completed.

Additional lengths of extension panel that need to be

added are “spliced” in at the side edges first, then secured

to the bottom panel.

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Add more ties to bind the layers of mesh.

Finishing up

Initially, cable ties are used to loosely secure the shape

of the net-pen and all its parts. Follow the initial tying

with additional cable ties, making sure there are no gaps.

When the water is clear, check the pen’s “attachment” to

the ocean floor by diving to see if there are any gaps be-

tween the mesh panel and the ocean bottom. These gaps

can be easily closed with small stones and sand. Perform

all checks well in advance of stocking the net-pen. It is

essential to re-check the net-pen for holes, broken ties,

etc., every few days, because fish can escape quickly

through holes in the net-pen.

Shown here are two mesh panels being secured

laterally. A two-person cable-tying team is needed

Here a cable tie is being pushed through for connection.

to poke the ties through the wire mesh to wrap them

At this depth underwater, connecting is done mostly by

around the T-posts. Use ties that are at least 7 inches

feel because, even with a mask, the water will be turbid

long. Each section, from one post to the next, will need

from the work activity. Bending the tie in half will keep

between 20 and 40 cable ties. This will depend on mesh

it in a “U” shape while poking it through. Attach the ties

size, pen height, a good top-to-bottom panel seal, and

as close to the post as possible from both sides.

desired strength. It is most economical to purchase ties

in packs of 250-500 per package. Heavy-duty 11-inch

ties in black (UV resistant) are preferred for post ty-

ing, because the ties connect both post and mesh at the

same time. 7-8-inch UV-resistant cable ties are used to

secure panels to one another where no post is available.

Expect to use three to four times as many small ties as

big ones. A 300-foot circumference pond needs about

1000 ties, 300 of the 11-inch ties and 700 of the shorter

ties. Always have plenty of extra ties in both sizes for

general maintenance and in case of emergencies.

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Gap to

Once the outer mesh is installed, an inner mesh needs

avoid

to be installed in the same manner using 7-inch cable

ties. Appropriate mesh sizes range from ¹⁄4 to 1 inch,

depending on the size of the animals to be contained.

Begin with a ¹⁄4-inch inner mesh material for fry (fish

¹⁄2-1 inch long) then change to a ¹⁄2-inch or ³⁄4-inch mesh

for fingerlings (fish 3-4 inches long). Final grow-out

The final net-pen placement should be carefully surveyed

should be conducted in net-pen material that is 1-inch

for flatness and any obstructions in the path of its edge.

mesh, assuming it is appropriate for the species. The

It is common to overlook dips or depressions along the

interior PVC netting will need to be changed periodi-

bottom edge of the mesh panel. Try to avoid this as much

cally. As the fish grow, they produce more organic matter,

as possible. If a gap is unavoidable, fill it with small rocks

which accumulates more rapidly on the tighter-weaved

and sand. The fill should cover the gap and more, along

netting material. Changing the mesh size allows more

its length and beyond its sides, and be mounded above

water filtration, decreases the potential for biofouling,

the depression itself. These spots need to be checked

and promotes better water conditions.

often to ensure the fill does not wash away.

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Net-pens at ‘Ualapu‘e: rectangular pens in the foreground; two circular pens in the background.

Seedstock acquisition and pond seeding

through May. Milkfish fry occur in late summer to fall.

Cultured seedstock may be available from aquaculture

Equipment for fry collecting includes ¹⁄8-inch seine nets,

facilities such as the Oceanic Institute at Makapu‘u

¹⁄8-inch mesh throw nets, scoop nets, a small battery-

on O‘ahu, the Natural Energy Laboratory in Kona, or

powered aerator, and 5-gallon buckets (see the chapter on

the state’s Anuenue Fisheries Research Center in Ho-

Equipment for Pond Operations). Experienced fishpond

nolulu. The fry stock should be about ¹⁄2-1 inch long

operators claim that it is easier to capture 100 1-inch fry

at a minimum. Researchers at Oceanic Institute have

than one 3-inch fingerling. The larger fish tend not to

always experienced better transfer and survival rates

school like the younger fry, making it more difficult and

with larger fish.

time consuming to capture enough to stock a net-pen.

Animals are boxed, transported in insulated contain-

Limu culture should be included in the production

ers, and transferred from their source to neighboring

strategy both for water quality benefits and economic

islands via air cargo. A small truck and trailer takes

return. Limu seedstock has been developed and can ob-

shipments to selected fishponds for acclimation and

tained from Ke Kua ‘Aina Hanauna Hou at Pūko‘o on

stocking. Initially, between 10,000 and 15,000 fry can

Moloka‘i. Limu “patches” of sporulated stones can be set

be stocked into a nursery pen system, then transferred

up inside and outside pen enclosures. Production areas

to larger-mesh pens as they grow.

can be visually monitored for growth (see the chapter

Natural recruitment, or collecting fry from the wild,

on Limu Production).

is economical but must be done with a collection per-

mit from the Hawaii Department of Land and Natural

Fish food

Resources, Division of Aquatic Resources, Aquaculture

High-protein fish foods manufactured by companies such

Development Program. Mullet fry can be found around

as Moore-Clark are excellent for fish, but these feeds are

brackish water riverbeds and estuaries from February

expensive and may be difficult to obtain. Rangen and

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Limu seedstock on mesh “pillow.”

Fish stock finglerings in bag.

Purina are proven brands of fish food that are generally

bottom substrate and the net-pen fenceline. Observations

available at reasonable prices. Fish food with high soy

of fish shape, size, and condition, and periodic weight

or grain meal content is not recommended. We have

sampling, allow the operator to adjust to lower feed

found that the grain feeds, although cheaper, are less

amounts through the grow-out phase. This saves money

nutritious. In some cases we also experienced the feed

and helps creat less organic waste.

going rancid quicker in Hawai‘i’s humid climate. It is

not recommended to ever give fish food that you think

Data recording

may be going rancid.

The net-pen’s production activities should be recorded

Purina Trout and Salmon Starter, #00 to #3 size pel-

in two separate log books. One log should be designated

let, can be fed to fry and fingerlings. Purina 350 floating

specifically for recording water quality, feeding, weather

feed can be used for feeding larger fish to market size.

conditions, and animal observations. A second log should

Newly stocked fry can be fed twice a day, with the daily

record the day’s work, such as maintenance and repair,

feed ration weighing five percent of their total body

sampling, predator control, fish transfer, limu harvest, etc.

weight, as determined at the previous monthly sampling.

Basic water quality parameters to monitor twice

Determine this by taking a few random samples of the

daily are dissolved oxygen, tide, temperature, and water

animals, weighing each sample, and dividing by the

salinity. Other water quality parameters such as ammo-

number of animals in the sample; take the average of

nia, nitrate, nitrite, and pH should be monitored weekly

the samples measured. Multiply the average weight per

using a test kit designed for salt-water aquaculture farm-

animal by the estimated total number of animals in the

ers. This type of test kit can be purchased for under $50

culture pen to obtain the “total standing biomass” (TSB).

and includes several months worth of testing materials.

The TSB is then multiplied by 0.05 to give a daily feed

Animals should be sampled monthly for growth and

amount which is divided by the number of feedings.

general health. Feed rates can be adjusted based on size,

Re-check this monthly during sampling activities. For

weight, and total standing biomass calculations.

growing out fish, feed once to twice a day at three percent

Data and recordkeeping become an important man-

of their total body weight.

agement tool for the pond operator and should indicate

It is important to note that these feeding rates are

seasonal, biological, or cyclical trends. These trends,

for a closed system, without the benefits of supple-

once determined, allow the operator to make production

mental foods. Fish cultured in net-pens often obtain a

decisions that may impact business profitability, fish

large amount of supplemental foods, such as micro- or

culture management, and operational methods.

macro-phytoplankton and zooplankton, from the pond

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Optimizing Pond Health

The biological environment

Food web and food pyramids

To better understand how to optimize pond health in ac-

Although a traditional fishpond may have had a diversity

tive fishponds, it is useful to study the aquaculture meth-

of species, the dominate species of fish were milkfish and

ods developed by the ancient Hawaiians. Their fishpond

mullet. These fish are herbivores, feeding on phytoplank-

site choice was often based on freshwater input, as the

ton. Other fish, such as barracuda, a predator, depend for

brackish water environment caused by mixing fresh and

survival on a more complex web of interactions among

ocean water created a natural fish nursery environment.

other species—barracuda eat lizardfish that eat shrimp that

The use of mullet and milkfish, species adapted to brakish

eat polychaete worms that eat phytoplankton. This type

conditions, as the main cultured species was an efficient

of web has much interdependency and less independence.

way to maximize productivity. The naturally occurring

Herbivores, on the other hand, have only one step from

currents and seasonal tides of the location defined times

the primary productivity of algae.

for stock recruitment or harvesting.

Using this subsystem, traditional Hawaiian fishponds

The complex aquatic food web begins with a pri-

made a great leap in production yield by eliminating

mary chain of productivity that transforms organic and

biomass inefficiency and excess links in the food web

inorganic detritus and other “nutrients” into some form

in order to seek their end products. In a food pyramid

of plankton (i.e., phytoplankton, unicellular algae, mi-

there is a normal energy loss of 90 percent for respira-

crobenthos, etc.). This primary productivity depends on

tion and body functions, which leaves only 10 percent

sunlight and the photosynthetic process for its growth.

for the next level. Therefore, a system using milkfish

Green and blue-green filamentous algae found in Hawai-

and mullet, which feed at the bottom of the food chain,

ian fishponds are also light-limited. Turbidity and light

enabled more productivity of the food pyramid (i.e.,

play an important part in this growth, which is the base

1000 pounds of algae and detritus makes 100 pounds of

for the food chain and also determines the productivity

herbivorous fish, which makes 10 pounds of humans).

of fish that are feeding on the algae.

Therefore, the greatest yield in pounds of fish is

Animal plankton (zooplankton) is also another key

harvested from the herbivorous links of the food chain.

factor adding to the pond biomass. Zooplankton feed on

Early Hawaiians exploited this and knew to increase

production of microbenthos and larger benthic algae by

bacteria or phytoplankton, and in turn are fed upon by

applying organic fertilizers (i.e., greenery or starch foods)

larger fish and crustaceans. This food chain goes through

and making shallow ponds for maximum light penetra-

many intermediate steps as it progresses until it reaches

tion. This system, applied through practical conservation

humans.

and management techniques, set Hawaiians and their

Because the species raised in ancient Hawaiian

fishponds apart as early pioneers of aquaculture.

fishponds were mostly autotrophic (depending on self-

nourishment) their productivity depended on what was

Traditional water quality environment

happening at the base of the food chain—plankton and

Water quality in traditional Hawaiian fishponds was

algae. And because fishponds are estuary-like, they

somewhat static, in that there was little fluctuation. In a

have high yields of primary productivity. The traditional

totally controlled monoculture system, most parameters

Hawaiian fishpond operators were able to efficiently

of importance, i.e., dissolved oxygen, water exchanges,

exploit this high productivity by “cultivating” milkfish

temperature, nitrogen, etc., are monitored and manipu-

and mullet, both herbivorous fishes. This was how the

lated to some degree to meet the desired conditions for

traditional fishpond operation allowed a consistent yield

the species cultured for harvest. By comparison, a

of fish with little or no need for fertilization or supple-

traditional Hawaiian fishpond had very little room for

mental feeds.

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water quality control and manipulation, which forced

clays and coarse suspensions of soil particles

traditional aquaculturists to better understand the “total

• suspended organic materials: living phytoplankton,

system.” This big-picture approach looked at

zooplankton, fungi, bacteria, and colloidal or sus-

• the interrelationship of species cultured in the pond

pended remains of organisms in stages of decay.

• the cultured species’ life cycle in relation to seasons,

moon phases, and other animals

Current water quality environment

• water flow and exchanges resulting from moon phases

There are dozens of biological and physical factors

and changes in tides

(“parameters”) that can be sampled and monitored in a

• seasonal recruitment of new stocking material and

fishpond. Of these, and in general, most fishpond moni-

harvests timed to moon phases

toring should look at the most “impactful” parameters.

• polyculture of complementary and beneficial species

It would be beneficial in the initial start-up year to regu-

for pond health and fish food production.

larly test as many parameters as possible and to develop

site-specific baseline data and records of the fishpond

Today, as modern pioneers of this traditional art form,

over a production season or year. If the fishpond is the

we have the ability to better understand and tradition-

receptacle of industrial or urban upland run-off, being

ally culture fish using modern technology and sciences.

able to test for fluorocarbons, pesticides, agricultural

run-off, etc., is important.

The basics

The most important parameters to monitor regularly,

The basic life needs of any cultured species, plant or ani-

as often as daily, are temperature, dissolved oxygen, sa-

mal, need to be available in the environment they live in.

linity, and pH; the ions ammonium (NH₄₊), nitrite (NO_2-),

Therefore, the quality of water and the understanding of

and nitrate (NO_3-) can be checked weekly. Changes in

various physical and biological properties or parameters

these parameters happen quickly and will impact the

is a primary concern. These properties must be tested and

system quickly due to the interrelationships among them.

measured regularly to assist the aquaculturist in main-

For instance, high oxygen and temperature will cause

taining a healthy growth environment for the cultured

more respiration by animals, which then cause chemical

species. In addition, any physical or biological changes

changes in the system that impact the fish. Less important

that happen in the culture will directly affect the cultured

water parameters include hardness, phosphates, carbon,

species and the system itself, as they are interconnected

suspended solids, and dissolved solids.

via the water interface.

For each parameter there is an optimum level for

the cultured species to thrive. Range of tolerance is the

Disclaimer: The aquaculture instruments identified

limit within which the cultured species can survive. Toxic

in the section Equipment for Pond Operations are the

levels are lethal. Note that not all species have the same

instruments used with our project, but they are not the

culture needs and ranges. If a polyculture situation is

only instruments available. Ours choices were based

desired, the culture parameters of the primary species

in part on economic considerations; some high-priced,

should be cross-checked and noted.

multi-parameter testing meters cost as much as a new car.

Temperature is one of the prime factors affecting

growth. Each species has a specific temperature range

Physical and biological components

it can tolerate and an optimum range for growth. For

• gases: oxygen (O₂), carbon dioxide (CO₂), nitrogen

efficient conversion of food to fish weight, water tem-

(N₂), hydrogen sulfide (H₂SO₄) and methane (CH₄)

perature must be kept as close to optimum as possible.

• minerals: calcium (Ca), manganese (Mn), sodium

Water temperature affects feeding, reproduction, im-

(Na), potassium (K), magnesium (Mg), aluminum

munity, and metabolic rates in aquatic animals.

(Al), zinc (Zn), copper (Cu), molybdenum (Mo),

Factors that affect water temperature include: am-

cobalt (Co), carbon (C), phosphorus (P), and others

bient air temperature, direct sunlight, depth of water,

• soluble organic compounds: sugars, fatty acids, hu-

mic acids, vitamins, amino acids, peptides, proteins,

urea, plant pigments, and others

Temperature Conversion

°F = (9/5 °C) + 32

°C = 5/9 (°F - 32)

• suspended inorganic materials: suspended colloidal

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circulation and currents. Abrupt changes in temperature

day sunlight causes photosynthesis, and at night O₂ is

can stress fish and even result in disease problems. It is

used for respiration. Signs of low DO levels affecting

important to monitor and record temperature on a regular

fish include

basis, usually first thing in the morning and again in the

• sluggish movement

afternoon. Temperature can be measured using a ther-

• not eating

mometer; either Celsius or Fahrenheit will do. To prevent

• gasping for breath

breakage, purchase a thermometer with an encased body.

• grouped near water inlet

Also, many dissolved oxygen instruments already have

• slow growth

a temperature function.

• disease and parasite problems.

Dissolved oxygen (DO) is the amount of gaseous

oxygen dissolved in water and is measured in milligrams

Salinity is the concentration of dissolved sodium

per liter (mg/l) or parts per million (ppm). All fish need

chloride (NaCl) measured as grams (g) of salt per kilo-

oxygen to survive, and monitoring this parameter is the

gram (1000 g) of water or parts per thousand (⁰/₀₀). Pure

most important daily activity a pond operator should

seawater in Hawaii varies from 32 to 34 ppt, although

perform. The DO meter is any pond operator’s best

some systems can become hyper saline at 40 ppt. A

friend. These instruments cost from $400 upward.

refractometer is the simplest and quickest method to

A DO concentration of 5 ppm is considered opti-

measured salinity.

mum. Fish begin to experience stress when DO levels

pH is a measure of the acidity or alkalinity of the

fall below 4 ppm, and they die after prolonged exposure

water, expressed on a scale of 1 to 14. It is based on the

to levels of 1 to 0.3 ppm. Under certain conditions water

number of active hydrogen ions, (H⁺) in solution. A pH

can become supersaturated with oxygen, above 15 ppm,

of 7.0 is neutral, values below 7 indicate acidity, and

and this also will cause stress in fish.

values above 7 indicate alkalinity. Pure water is neutral,

Temperature and salinity both affect the oxygen-

tap water is 7.7-8, clean rainwater is about 5.6, “acid

holding capacity in water. As temperature and salinity

rain” is 3.5-5.5. pH is an important parameter affecting

of the water increases, the amount of available D.O.

several key changes in chemical processes and param-

decreases. Other factors influencing DO levels include

eters, such as the nitrogen cycle. Most fish survive over

stocking density, weather and climate, fish activity (feed-

a pH range between 6 and 10, but each species has its

ing and respiration), photosynthesis by algae, run-off,

ideal pH level for optimum growth and good health.

decomposing organic material, etc.

pH can be measured quickly and precisely using a

Oxygen comes into the water when molecules of

handheld, battery-powered pH meter. Less expensive but

oxygen (O₂) gas from the atmosphere diffuse into the

less accurate measurements can be attained using litmus

water, and through photosynthesis. In outdoor culture

paper. Other methods include pH test kits (See section

systems DO can fluctuate greatly, because during the

on Equipment for Pond Operations).

Ammonia (NH₄) is a waste product of protein me-

tabolism or breakdown by aquatic animals, but it is also

naturally present in small amounts in water bodies. In

certain forms and concentrations it can be toxic and can

Dissolved Oxygen Chart (ppm)

increase in proportion to pH and/or temperature. It can

be measured with a basic saltwater chemical test kit.

Nitrite (NO₂) is the basis of organic matter de-

composition by nitrifying bacteria oxidizing ammonia.

Nitrite levels above 0.55 ppm can be toxic to “oxygen-

Legend

to-blood transfer” processes in fish. Nitrite levels may

1.0-0.3 ppm - fish die with prolonged exposure

also increase due to over-feeding, high fish density,

5 ppm - optimum

phytoplankton “crashes,” etc. Nitrite can be measured

< 4 ppm - fish stressed due to low DO

with a basic saltwater chemical test kit.

> 15 ppm - supersaturation stress

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Nitrate (NO₃) is the final product in the nitrification

allows several months’ worth of testing.

process and does not typically have any toxic effect on

Animals should be sampled monthly for growth and

culture animals, but it will influence plant growth by

general health. Feeding rates will be adjusted based on

acting as a fertilizer. It can be measured with a basic

size, weight and total standing biomass calculations,

saltwater chemical test kit.

which are reviewed by analyzing data. At the end of

Turbidity is a term used to describe suspended sol-

this section are four samples of logs and worksheets

ids, such as silt. These small particles prevent sunlight

that can be easily duplicated and used to keep records

from reaching phytoplankton that produces oxygen

for managing a fishpond.

and food. Wind, bottom-feeders, run-off, inorganic silt,

phytoplankton, erosion, rain, etc. can cause turbidity.

Summary

Turbidity can be measured with a turbidity meter, but

Over time and with diligent recordkeeping, a fishpond

the more common and cheaper method is using a sec-

operator will see that conditions in a fishpond are some-

chi disk. The depth of the water surface to the point in

what constant and predictable, much like the coastal

which the disk is not visible is measured in cm from the

conditions outside of it. Natural seasonal changes, as

surface. A measure of 80 cm is not very turbid whereas

well as tide and moon phases, provide cycles for pro-

a measure of 15 cm and less is turbid.

duction management. Unpredictable storm weather or

other climatic phenomena are less probable and are not

Important Things to Remember

a good gauge for anything other than crisis and catas-

Records and data keeping become an important man-

trophe planning.

agement tool for the pond operator and will indicate

It is important to monitor the water quality, as it af-

seasonal, biological or cyclical trends. These trends,

fects the fish. It will be valuable also to integrate other

once determined, allows the operator to forecast produc-

“natural mechanisms” that provide clues and insights

tion decisions that may impact business profitability,

about pond conditions. Ancient Hawaiians had no instru-

culture management and operational methods. Keep

mentation, yet they produced a sufficient food supply over

daily records of the day’s work, such as maintenance and

centuries using similar systems and culture methods.

repairs, sampling, predator control, fish transfer, limu

Step back and take a big-picture approach to connect

harvest, etc. A daily log identifies routine tasks, such as

relationships between instruments, production animals,

water quality, feeding, weather conditions, and animal

observed daily fishpond conditions, and your five senses.

observations. Basic water quality parameters to monitor

Over time, and with keen observation, these rhythms and

twice daily are DO, tide, temperature, and salinity.

patterns will become more obvious and expected. You

More specific water quality parameters such as

will find less need for the rigors of daily water quality

ammonia, nitrate, nitrite, and pH should be monitored

collecting and will “depend on your fish or the weather

weekly using an aquaculture farmer’s saltwater test kit.

to tell you how things are.”

This type of test kit can be purchased for under $50 and

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Troubleshooting Fish Culture Problems

The working fishpond

Changes to the fishpond’s environment and water

One of the biggest challenges for any fishpond opera-

quality, along with weather conditions, are usually con-

tion is to identify a problem and be able to remedy it.

sidered external impacts. The animals’ health is the next

Preferably, the fishpond operator will be proactive and

obvious condition to monitor. Lastly, there are a number

avoid as many problems as possible. Unlike land-based

of other ways to manage potential production losses,

agriculture, aquaculture occurs under water, making it

including checking for the following occurrences:

difficult to regularly “inspect” production, as would be

• feed that is rancid or moldy, not fresh

possible on land. For example, trade winds stir up par-

• predators in the pond, such as barracuda, eels, and

ticulate matter from the pond floor, and activities such

crabs

harvesting or cleaning also create turbidity that makes the

• fencelines with holes or split seams

pond water less clear. Add to this the typical shy nature of

• evidence of poaching, which can be detected by

fish, and observation for any purpose becomes difficult.

checking for bruises on the fish caused when escaping

Several natural opportunities aid observation of the

capture nets.

health of the operation. These opportunities occur

• under weather conditions with no wind

Behavioral observations

• in the early morning before the daily wind starts

Observable indicators of healthy feeding behavior in-

• during extremely low tides

clude fish that

• after a phytoplankton “crash” (die-off).

•

“swarm” close to the feed site

•

“hit the surface” as soon as feed is tossed

By far the best and most consistent times to observe

• actively feed together as a school (group)

fish are when they feed. They need to eat, are conditioned

• display a feeding pattern that is consistent over time.

to a feeding schedule, and should readily come to the

surface at feeding times.

Observed indicators of animals under stress or problems

If there is no supplemental feeding with a prepared

include fish that are

pelleted feed, then much of the feeding observed will

• not feeding at all

not take place. If you are not going to feed, a periodic

• eating less than the normal amount

fish sampling will show how well or fast the animals

• dead or separated from the larger group

are growing. Observation can be done during sampling,

•

“gulping air” at the surface

as well as when there are sunny, windless days. Also,

• showing discoloration or spots on their scales

polarized sunglasses can greatly enhance visibility of

• swimming erratically and quickly (predators inside pen)

fish from above the water surface.

• display other non-typical behaviors.

As an observation method, sampling fish has some

limitations. A sample group represents a small number

These are general indicators of something wrong

of the total, and it may not truly be representative of

within the system. In the beginning, problem-solving

the conditions of the population. The act of sampling

will be complex and will need to be addressed as soon

(i.e., handling) is very stressful to the fish and may lead

as possible. In general, allowing a problem to occur for

to diseases being contracted or spread. Therefore, it is

too long will result in catastrophe. In your log book,

best to learn how to make close observations and to

write down your analysis of the problem and list solu-

start recognizing behavior and changes at feeding times.

tions or reasons for solving your problem. Don’t hesitate

In general, these changes in behavior are indicators of

to consult someone knowledgeable—your stock may

something else happening to the animals.

depend on it.

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behaviors are usually signs of disease or parasites. A

few dead fish each day usually indicates some type of

slowly spreading disease or parasite problem. Progres-

sively more dead fish each day is a sign of a very seri-

ous disease problem. Dying fish or fish with suspected

diseases or parasites should be diagnosed, or sent for

diagnosis, immediately.

Hawai‘i is supportive of aquaculture and the state

has facilities to assist with diagnostics through its state-

run fisheries, disease diagnostics lab, and University of

Hawai‘i aquaculture extension programs. Contact your

nearest aquaculture extension office or state fisheries

specialist for information on the proper way to package

and ship fish samples. Take a live sample of fish and a

Active swarming indicates a healthy feeding population.

water sample to the nearest fish disease diagnostic lab

and have it tested. Besides diagnosing the problem, the

extension agents may be able to suggest treatments for

your problem. In some cases, as with internal diseases,

a medicated feed could be purchased and fed to the

Over time, and with good recordkeeping, problems

animals.

will be easier to identify. The following sections give

some common answers to typical situations. This is by

Water quality problems

no means a comprehensive discussion, as these issues

Excessive nutrients in the water, possibly caused by

are complex in nature and there are site-specific, animal-

too much feed, can create an unhealthy or toxic water

specific, and weather-specific conditions that must also

quality situation in your culture system. This is usually

be accounted for.

identified by visual changes in the water’s color. Rapid

color changes, surface scum, intense color, odors, and

Non-instrumental observations

extreme weed growth are associated with water quality

If there has been a period of heavily overcast and wind-

problems. These conditions are symptomatic of exces-

less days, phytoplankton production will be reduced,

sive nutrients from feed, sediment in runoff from the

creating a situation of low dissolved oxygen (DO) levels.

land, or overstocking. Monitor water quality and try

If there are cloudless days for some time without wind,

to keep readings of nitrogen (ammonia and nitrate) at

phytoplankton may “crash,” consuming oxygen in the

acceptable water quality parameters. If need be, reduce

process. This may be temporary but is stressful none-

feeding, transfer overstocked fish to another pen (if pos-

theless. If this occurs, stop feeding, wait for the tide to

sible), and continue to monitor water quality. Be careful

bring in new water, and avoid DO levels below 3 ppm.

not to transfer sick fish.

Fishpond operators will find that the animals them-

Strong odors are an indicator of decaying plant

selves provide the best information on how they feel

material, which consumes oxygen as part of the decom-

and what they want. This intimate connection between

position process. Other than reducing feeding, adding

operator and animal will be the best relationship to

oxygen to the system may be warranted. When aeration

develop. Again, for example, fish that do not eat when

is extremely necessary but not readily available, a small

they normally would suggests an oxygen problem. Fish

outboard motor can be used to create propeller aeration.

that are “gulping for air” early in the morning suggest

Another solution is introducing new water via tidal ex-

critically low DO levels. If this occurs, do not feed, check

changes, with the highest exchanges occurring with the

the tide charts for the next high tide for water exchange,

new and full moons.

and monitor the DO level.

Dissolved oxygen stress is the primary cause for fish

Diseases and parasites are other problem areas that

culture problems. Make it a habit to measure DO levels

may be detected during feeding. Skin discolorations,

twice a day. Testing first thing in the morning measures

spots, fin erosion, erratic swimming, or other strange

DO availability without photosynthesis. Testing again at

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Scrubbing netpens reduces biofouling.

Scum growth on the pond walls indicates water quality

problems.

the end of the day, before sunset, can provide an indica-

tion of the level of DO as a result of photosynthesis.

The critical concern for any fishpond operator will be

to recognize and prevent potential problems before they

arise. This will happen over time and with familiarity

with the site and culture systems. In general, it will take

a full year of observation, because every season has its

own characteristics. Learning this will assist in formulat-

ing culture schedules based on the animals’ spawning,

growing, and maturation rates.

Various water quality monitoring instruments and test kits.

LOKO I‘A A Manual on Hawaiian Fishpond Restoration and Management (2007) - page 2