Silver perch - breeding

Fertilisation and Plankton Management of Silver Perch Larval Rearing Ponds

Introduction

The culture of fish larvae requires specific management of ponds to enhance phytoplankton and hence zooplankton production. Fish larvae feed on zooplankton through to the transition to fingerlings. The dynamic characteristics of zooplankton populations have led researchers and commercial hatcheries to use a range of fertilisation regimes. The aims of the management techniques are to maintain high densities of desirable zooplankton species until the fish are harvested or weaned onto commercial feeds.

Types of Fertiliser

Fertilisers are classified as organic or inorganic (chemical). Inorganic fertilisers are man-made and are readily available eg. superphosphate, urea, potash etc. Organic fertilisers are manures, plant meals and other natural products eg. lucerne hay, grasses, poultry manure and cottonseed meal. The latter provide relatively lower levels of nutrients (Table 1) compared to inorganic fertilisers (Table 2) but they serve as a substrate for the growth of bacertia, protozoans and zooplankton. It is advisable to use both organic and inorganic fertilisers because the combination provides a broad base to stimulate the variety of zooplankton that will be consumed by fish larvae.

Nitrogen, phosphorous and potassium (K) are the primary nutrients in inorganic fertilisers. The grade of inorganic fertiliser refers to the percentages by weight of nitrogen (as N), phosphorous (as P₂O₅) and potassium (as K₂o also called potash). For example a 20-20-5 grade of fertiliser contains 20% N, 20% P and 5% K. Impurities and inert materials are also added to bulk out of the final weight.

Controlling Algae Productivity

Different algal (phytoplankton) species have widely varying abilities and demands for nutrient uptake and light utilisation. The purpose of fertilisation is to promote an algal "bloom" without necessarily trying to promote a particular alga species. Algae will continue to grow rapidly if the following requirements are met:

• Primary nutrients (an inorganic carbon source eg. carbon dioxide, bicarbonate; Nitrogen and Phosphorous), hydrogen and oxygen; K is not as important as P and N;
• Minor and micronutrients;
• Sufficient light energy; and
• Suitable water temperatures for growth.

From a pond management perspective, however, only N, P, K and carbon (C) are controllable. The supply of essential micronutrients has never been shown to limit algal growth in fertilised ponds and oxygen and hydrogen are supplied via the water.

Selecting a Fertilisation Regime

Most often a regime selected prior to filling the larval pond is based on previous fertilisation research and subsequent zooplankton monitoring and larval survival. However due to the many ecological differences between ponds (eg. climate, soil type, aspect) fixed-rate recipes are often not applicable from one region to another, or even between ponds on the same farm. A degree of trial and error is required when first applying a fertilisation recipe.

Algae require a supply of inorganic nutrients. Phosphorous is the single most essential element. It is relatively scarce in the aquatic environment and can be quickly absorbed into the pond bottom. Some nitrogen may be needed as well, especially in new ponds and in combination with organic fertilisers to hasten decomposition. Potassium is rarely required.

Alkalinity is another requirement to assist in promoting plankton growth. Alkalinity stabilises pH and facilitates the uptake of inorganic carbon by algae. Waters having less than 20 mg/l total alkalinity will need liming. Carbon can also be supplied to the algae when carbon dioxide is released following the decomposition of organic fertilisers. As a general guide inorganic fertiliser is usually applied to warm, freshwater ponds at the rate of:

• 113 kg/ha of an 8:8:2 fertiliser mixture or
• 45 kg/ha of a 20:20:5 mixture.

These fertilising rates will result in the application of approximately 9 kg/ha of N, 9 kg/ha of P and 2.2 kg/ha of potassium (K). The following examples of fertilisation regimes approximate these inorganic nutrient levels.

1. Pond size: 1.0 ha; applied weekly.
   15 kg Sulphate of Ammonia
   40 kg Diammonium phosphate (DAP)
   5 kg Potassium chloride (Muriate of potash)
   150 kg lucerne chaff
2. Pond size: 1.0 ha; applied weekly.
   20 kg Urea
   46 kg Superphosphate
   5 kg Potassium chloride
   250 kg Poultry manure initially then 80 kg weekly.
   
3. Pond size: 1.0 ha; applied weekly
   33 kg Sulphate of Ammonia
   18 kg Triple superphosphate
   5 kg Potassium chloride
   300 kg cottonseed meal.
4. Pond size: 1.0 ha; applied weekly
   40 kg Diammonium phosphate
   10 kg Potassium chloride
   5 bales lucerne hay initially and then every 14 days
   60-kg poultry manure

Applying Fertilisers

It is important the fertilisers do not come into contact with the pond bottom because the nutrients (especially phosphorous) can become bound up in the mud. Placing the fertiliser on a submerged platform allows the nutrients to go into solution in the top waters where photosynthetic activity is greatest. Suspending the fertilisers in porous bags is another option. Alternatively fertilisers can also be dissolved on the pond bank and the liquid broadcast around the pond.

Organic fertilisers should have small particle sizes (eg. lucerne chaff) to aid decomposition. They can be broadcast over the pond or placed in porous bags to soak prior to releasing into the water; this will help prevent all the fertiliser from floating to a side or corner of the pond.

When not to Fertilise

• If the total alkalinity is below 20 mg/l (phosphorous solubility will be reduced).
• If rooted aquatic vegetation or filamentous algae are present.
• If the soils are acidic or very fertile.
• If the pond is receiving a flow through of water.
• If the secchi disk reading is less than 30 cm.
• If early morning dissolved oxygen is below 3 mg/l.
• If the afternoon pH exceeds 9.5.
• If the water has clay turbidity.

Failure of Fertilisation Programs

• Inadequate quantity of fertiliser or specific nutrients.
• Acidic water or low alkalinity.
• Very hard water and pH above 8 which causes phosphates to precipitate out.
• Waters with very high clay turbidity.
• Excessive weed or filamentous algae growth.
• Low light intensity.
• Low water temperatures.

Management of Clay Turbid Ponds

Muddy fresh water can often be cleared, prior to fertilisation, with applications of gypsum at rates of 200-500 mg/l (Boyd 1990). Other treatments have included aluminium sulphate (alum) which must be mixed over the entire pond surface and mixed as rapidly and thoroughly with the water as possible. Alum rates of 15-25 mg/l have been used successfully (Boyd 1990). Alum will however deplete the pond alkalinity and lower the pH, markedly.

Timing of Stocking Larvae after Fertilisation

The extensive larval rearing of silver perch is very much a timing exercise. Mis-timing the stocking of larvae can result in poor survival and/or reduced growth. Ideally, the larvae should be stocked when temperatures are above 21ºC and the pond has an established bloom of algae and suitable sized zooplankton. Stocking larvae too early runs the risk of the pond having poorly established zooplankton populations. Conversely, stocking into ponds that have been filled for 3-4 weeks could cause high larval mortality due to the lack of small zooplankton and the presence of aquatic insects that prey on the larvae.

When ponds are filled and fertilised, the plant and animal populations go through predictable changes in both predominant types and sizes, a process called ecological succession. Understanding succession will greatly contribute toward success in larval culture. This can be achieved with regular plankton sampling using a plankton net and observing the plankton under a microscope.

Stocking silver perch larvae when rotifer populations begin to rapidly grow (7 to 10 days after filling) generally results in a successful rearing program. By the time the populations of rotifers decline the larvae have grown large enough to consume larger zooplankton. Table 3 outlines the major zooplankton groups and their approximate sizes and life spans.

Silver perch larvae are stocked into the rearing ponds on the first day that yolk-sac absorption is complete. This is usually 5 days following the commencement of egg hatching at 23-25ºC. The yolk will be absorbed slower in cooler water and faster in warmer water (4 days at 25ºC). Larvae are stocked at densities between 50 and 100 larvae/m2 of pond surface area.

Maintaining Zooplankton and Larval Weaning

The aims of pond management are to co-ordinate the release of first feeding larvae with high concentrations of food organisms, and then maintain high zooplankton concentrations for as long as possible. The continuous application of a suitable fertilisation regime is paramount to this goal. Major influences that affect the longevity of zooplankton blooms are:

• Die-off of the algae bloom (cold, overcast weather or poor fertilisation regime).
• Overgrazing of the algae by the zooplankton.
• Predation of the zooplankton by the larvae and fry.

These factors may influence the decision when to commence weaning the larvae (fry) onto formulated feed. Under normal circumstances weaning of silver perch fry can commence 2-4 weeks following stocking using a "dust" feed of approximately 0.6-mm in size. During weaning, fry should be fed 3 or 4 times per day broadcasting the feed around the perimeter of the pond.

REFERENCES

Avault, J.W., (1996). Fundamentals of Aquaculture. AVA  Publishing Co. Inc.

Boyd, C.E., (1990). Water Quality in Ponds for Aquaculture. Birmingham Publishing Co.

Sequence of Events when Breeding Silver Perch

DAY 1 – FILLING LARVAL REARING PONDS

The management of the larval-rearing pond will vary from farm to farm, but prior to filling the pond, the pond should be prepared by drying (over winter), screening outlets, harrowing bottom and removing weeds. Commence filling and fertilising with organic and inorganic fertilisers on day 1.

DAY 5 – PREPARING EQUIPMENT

Spawning tanks, incubators and anaesthetic baths are prepared.

DAY 6 – INJECTION OF BROODFISH

Broodfish are carefully harvested from ponds or tanks in the morning; hormone is prepared and injected.

DAY 8 – FERTILISATION RATE and INCUBATION OF EGGS

Fertilisation rates of eggs are determined in the morning; broodstock are moved from spawning tank to quarantine tanks; eggs are counted and stocked into incubators; salt applied (3ppt); incubators are routinely cleaned and water partially exchanged over the following 5 days.

DAY 9 – HATCH RATE OF EGGS and RE-FERTILISATION OF LARVAE POND

Hatch rates are determined in the morning ie. hatching has not commenced however the number of eggs that are likely to hatch can be determined; hatching is completed by late afternoon. The hatch rate enables the number of larvae in each tank to be estimated. Larvae numbers can also be estimated by counting sub-samples of hatched larvae. Fertilisation regimes are more of an art than a science and often depend upon soil fertility, soil pH, and prior management. Weekly application of fertilisers may be necessary ie. day 1, 8, 15 etc. (see "Fertilisation and Plankton Management" extension note).

DAY 14 – STOCKING LARVAE

Once the yolk-sac has been absorbed the larvae are counted (determined from hatch rate or sub-samples) and carefully harvested, packaged, acclimated and stocked into the larval rearing pond.

Hormone Preparation

Prior to Spawning Day

Determine the amount of hormone required based on the number and weight of fish to be injected using an injection rate of 200 IU or hormone per kg of fish.

Purchase the required amount of hormone, preferably in 1500 IU ampoules, well in advance of the spawning day and store in a refrigerator.

Purchase 5 ml and 2 ml syringes and 0.8 x 38 mm (21 gauge) needles from a medical stores supplier.

Spawning Day

It is essential to wear protective clothing (gloves, glasses etc.) when using hormones as manufacturers are unable to guarantee these products free from contamination by diseases transmissible to humans.

Remove hormone and solvent ampoules from refrigerator and warm to room temperature.

Remove syringes from sealed packages and free up the plungers.

Remove the sheath-protected needles from the sealed packages.

Fit a needle to a 5 ml syringe, remove the protective sheath, draw in some air and puncture the lid of the solvent ampoule.

Holding the syringe below the solvent ampoule, draw all the solvent into the syringe. Holding the needle in an upright position work the plunger to the 3 ml graduation removing all air bubbles.

Using this syringe, puncture the crystalline hormone ampoule and holding the syringe above the ampoule mix the solvent with the crystallised hormone until it dissolves. The plunger can be inserted and withdrawn to aid mixing.

Holding the syringe below the dissolved hormone, draw the dissolved hormone back into the syringe and set the plunger to the 3 ml gradation being careful to remove all air bubbles. This is the "stock" solution. Each of the 15 gradations marked on the syringe now represent 100IU of hormone (1500 IU ¸ 15).

Once a fish has been weighed, draw the required amount of hormone (200 IU/kg) from the prepared "stock" solutions into a 2 ml syringe eg. a broodfish weighting 1.5 kg requires a total does of 300 IU or 3 gradations of the stock solution; take 4 gradations of the stock solution; expel 1 gradation plus bubbles and inject remainder.

Inject the fish using the 2 ml syringe.

Use each syringe once only and discard after use into a medical "sharps" container.

Note: 1 gradation = 0.2 ml