Dedicated To The Tribal Aquaculture Program
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September 2002-Volume 41 |
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Administrative
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Edited
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Topics Of Interest:
Intensive Culture of Walleye Fry
The following information was obtained from the Walleye Culture Manual (NCRAC). What follows is a slightly shortened version of the actual article. If you need additional information or the list of references, please refer to the manual or contact Dr. Summerfelt.
Intensive Culture of Walleye Fry
By: Robert C. Summerfelt, Department of Animal Ecology, Iowa State University, Ames, IA.
Walleye may be raised from newly hatched fry to phase I fingerlings in extensive or in intensive culture, that is in tanks and raceways. Currently, the predominant method for raising phase I fingerlings is in drainable or undrainable ponds. Walleye fingerlings may be raised from first-feeding fry in intensive culture systems, hereafter called "intensive culture of fry." This can be done because major constraints to intensive culture of fry (noninflation of the gas bladder, nonfeeding, and maladaptive clinging behavior) have been resolved, and production-scale systems for intensive culture have been developed.
Advantages of Intensive Culture
Not many years ago, the prospects for intensive culture of walleye fry seemed remote because of seemingly insurmountable biological and technical difficulties. It can now be stated that intensive fry culture is a viable production technology to produce walleye fingerlings. Pond culture is economically effective for the production of large numbers of phase I fingerlings, but it is far less so for the production of large, phase II fingerlings ( 6 inches), or for the commercial grow out from fry to food fish. Pond culture of phase II fingerlings requires relatively low initial pond stocking densities, or partial harvest and restocking, followed by provision of substantial quantities of minnows. Because of the high cost and problems with the availability of minnows, tandem pond-tank (i.e., extensive-intensive) culture methods have gained popularity as a method to raise fingerlings.
The extensive-intensive method involves the transfer of pond-raised phase I fingerlings to intensive culture systems where the fish are habituated to formulated feed. In extensive-intensive culture, substantial mortality typically occurs at two intervals. First in the pond, and secondly in the intensive culture system during the first 21 days when fish are habituated to formulated feed. Pond culture techniques must be contrived to manipulate complex ecosystems within the constraints of variable weather: fertilizers are added with the expectation that food webs will provide the right kind and size of zooplankton for first feeding walleye, but without over fertilizing, causing an oxygen depletion, or growing aquatic weeds. When zooplankton populations are lacking, cannibalism may occur in the pond. Also, insect predation may contribute to fry mortality.
Intensive culture may be a better alternative for hatcheries that lack the pond facilities or sites for cage culture. In intensive culture of walleye, a single interval of high mortality occurs during the critical period when fish switch from yolk-sac nutrition (endogenous feeding) to active feeding (exogenous feeding), which coincides with the interval of gas bladder inflation. The critical interval occurs before the fish reach 21 days posthatch; thereafter, with suitable feeds and appropriate husbandry, fingerlings may be raised to whatever target size is needed, or to the limits of the cultural system, whichever occurs first.
Techniques for intensive culture of fry on formulated feed have developed sufficiently to make the intensive culture of fry to fingerling a practical alternative to the extensive-intensive system. Will intensive culture of fry replace pond culture of fingerlings? Intensive culture has many factors in its favor: It is not subjected to variable environmental conditions, so cultural conditions are fairly stable, temperature can be controlled to lengthen the growing season, nuisance aquatic organisms are eliminated, and the quality and quantity of the feed is controlled. Technological advances are more likely applicable in an intensive culture system than in pond culture. In spite of high stocking density, cannibalism may be less of a problem in intensive culture than pond culture because it is easier to monitor, and the environment (temperature, light, turbidity) or feeding rates can be adjusted to reduce the intensity of the problem. In intensive culture, growth rates can be stepped up or slowed by temperature manipulation to meet production schedules.
Intensive culture is the only technology that can be used to raise fry produced by out-of-season spawning. The advantage of out-of-season culture is that it increases the growing season. Ambient water temperature during the natural culture season is usually too short to raise a phase II fingerling to a target size of 6 to 8 in by fall in either ponds or cages.
Would intensive fry culture with heated water be economical for commercial producers? As a rule, heating water for production aquaculture is uneconomical. The larger operators typically have a source of steam or warm water from coal-fueled electric generating plants or facilities that distill alcohol.
The prolarval stage is also called a free embryo or eleutheroembryo, or finfold stage. Fish culturists call them sac fry because it is the first period of life post-hatch when the yolk sac is present. At a mean temperature of 16.4 C, the yolk sac disappears in the 5th day posthatch, about 68 temperature units (TU), but it may persist up to about 13 days at a temperature of 13.2C.
The prolarvae are weak swimmers, so water currents in culture tanks should be low, because larvae are quickly exhausted. Their weak swimming ability also causes problems in shipping larvae in plastic bags. Prolarvae often are shipped in plastic bags at 100,000-150,000 per 15 L of water, and transported for up to 18 hours, but unless the bag is moved to and fro by the motion of the vehicle, the larvae pile up and suffocate.
Hatchery personnel frequently measure the volume of a known number of 2-3 day-old fry as a basis for estimating number of fry for stocking. Information on number of fry/mL has not been published. In the course of our research, many volumetric measurements of fry for stocking experimental tanks have been made but the values are variable. Variability may be related to the diversity of stocks that were used, the care or lack of it in removing excess water, but especially the age of the fish at the time of stocking. Although the fish have not begun to feed, the number/mL decreases with age from 1-4 days; measurements in our laboratory are usually made at the time of stocking, about 2-3 days posthatch.
Postlarva I is the stage following absorption of the yolk sac but before disappearance of the oil globule. After disappearance of the yolk, or slightly before then, larvae begin feeding (exogenous feeding), and gas bladder inflation begins. Krise and Meade (1986) indicated that first feeding begins at 100-122 TU. Our measurements indicate that the TU to yolk sac disappearance (68 TU) is much earlier than first feeding (132 TU). Before feeding, the energy requirements of the larvae for swimming and searching for food must be provided by the oil globule. First feeding walleye (8.5-9.0 mm) have a mouth width of 0.7 mm and a gape of 1.5 mm, which is sufficiently large to cannibalize similar-sized siblings or to consume the standard size starter feed. Balon (1984) considered the transition of the embryo (prolarva) to exogenous feeding to be the decisive event in development, because failure to accomplish first feeding results in mortality. Because persistence of the oil globule aides buoyancy and it serves the basic energy needs of the larvae, the peak period for mortality does not occur until the 16 to 19 days posthatch or at the end of postlarval II.
Postlarva II begins about the time the oil globule disappears, which is about 226 TU, in the 14th day posthatch at a mean temperature of 61.5F (16.4C). After the yolk sac and oil globule disappear, nutrient reserves are exhausted and exogenous feeding must be effective or the larvae quickly starve. Starving fish exhibit a stress syndrome behavior, or they revert to prolarval behavior, swimming to the surface and drifting back to the bottom. These behavioral patterns may be related to noninflation of the gas bladder.
The end of the postlarval II stage marks the end of gas bladder inflation. Reports that show progressive increases in gas bladder inflation rates after this period are the result of mortality of fish that have not inflated the gas bladder, not because fish are continuing to inflate the gas bladder. The end of postlarval II stage and beginning of the juvenile (fingerling) stage is not marked by conspicuous and convenient developmental milestones. The swim bladder has inflated and fish are feeding successfully or the fish dies. Other than mortality related to transportation stress, the period of greatest mortality is 16-19 days posthatch, that is the beginning of the juvenile stage.
Juvenile stage begins about 15-18 days posthatch at a length of about 20 mm. At the end of the postlarval II stage, fin rays are limited to the lower lobe of the caudal fin, but as the juvenile stage progresses the fin rays develop in the rest of the caudal and other paired fins, and spinous rays develop. Further maturation of the gastrointestinal tract takes place, gill rakers differentiate, and pigmentation increases. Scale development begins at 24 mm but it is not completed until they are 45 mm.
Bulkowski and Meade (1983) described a behavioral change in juvenile walleye from positive phototaxis to negative phototaxis behavior when they are 32 to 40 mm. In intensive culture, this change begins in juveniles as early as 25 mm. The changing response of juveniles to light is important in both extensive and intensive culture. In intensive culture, relatively high light intensity (680 lux) is used until gas bladder inflation is first detected (8-10 days posthatch) as a means to attract fry to the surface for gas bladder inflation. After gas bladder inflation begins, reduce light intensity to 140 lux for the duration of the culture interval to disperse the fry vertically in the water column. Before they change from positive to negative phototaxis, fish in ponds can be attracted to light for night harvest. The change in behavior is also important in habituating pond-raised fingerlings to formulated feed, because after 40 mm walleye fingerlings are cultured at low (<16 lx) light intensity or intank lighting is used to reduce stress and enhance feeding and growth.
Critically important cultural problems
The most critical problems affecting success of walleye culture are:
1. Noninflation of the Gas Bladder (NGB)
3. Nonfeeding
4. Cannibalism
These constraints had to be overcome before intensive culture became possible, but it seems from present perspective, that NGB and clinging behavior have had a greater impact on nonfeeding than the suitability of fry feeds, and both problems influence incidence of cannibalism.
Noninflation of the gas bladder (NGB)
NGB is important because it contributes to mass mortality, larval deformities (lordosis), susceptibility to stress-induced mortality, diseases and toxicant, it slows growth, and increases susceptibility to cannibalism or predation. Fry without an inflated gas bladder struggle to maintain position and they eventually starve because of the high energy cost of swimming and difficulty in capturing food. NGB makes fry vulnerable to cannibalism because fry without an inflated gas bladder have erratic behavior and poor swimming ability. It was essential to develop effective strategies to overcome this imposing problem.
In most fish, initial filling of the gas bladder is accomplished by gulping and swallowing air, then mechanically forcing air bubbles from the foregut through the lumen of the pneumatic duct to the gas bladder. For walleye, the interval of gas bladder inflation after hatch is said to be 214 TU. The window of opportunity for initial gas bladder inflation is relatively short. It is assumed that failure of the gas bladder to inflate during this specific period of development is irreversible.
The inflation process is a two step procedure: The first step is to initiate inflation; the second is to fully extend the gas bladder. In the first step, air gulped at the water surface is ingested, but bubbles must be small enough for passage through the pneumatic duct. The relatively large bubbles ingested at the surface are fragmented by surfactant-like secretions in the foregut. Immediately following initiation of gas bladder inflation, the second stage begins when the vascular rate of the gas gland (located on the ventral surface of the gas bladder) functions to continue the inflation process, stepping up pressure within the gas bladder until the gas bladder assumes an elongated-shape.
An oily layer on the water surface will prevent the larvae from breaking the water surface, and the primary stage of gas bladder inflation will not occur. Thus NGB is most often the result of contamination of the water surface by oil. This may be from compressors or submersible pumps, feed or even the fry.
Interference with the second stage of inflation, which takes place 12 to 19 days posthatch, results from bacterial aerocystitis an inflammation of the gas bladder epithelium. With or without oil, the surface tension is usually sufficient to support small particles of feed as well as bacteria and fungi. If the fry ingest surface film or partially decomposed feed that is heavily contaminated with microbes, feed and microbes may be aspirated into the gas bladder on the microfilm surrounding the air bubble. Although gas bladder inflation advances to the spherical stage, inflammation of the epithelial lining of the gas bladder will lead to dysfunction of the gas gland, which prevents full extension and can cause deflation of the small, gas bladder. However, in an intensive culture environment, where formulated food is abundant, some fish without inflated gas bladders may survive.
The effectiveness of the culture system to produce high percentage of fish with inflated gas bladders should be determined for a new system, but also routinely for quality control. Fish lacking an inflated gas bladder should not be stocked, and pond-raised fish that are habituated to formulated feed in the extensive-intensive culture procedure or stocked in cages should be checked for gas bladder inflation. In my research on intensive culture of walleye, gas bladder inflation is examined in fish samples from all culture tanks at weekly intervals from 7 to 28 days. At least 30 days, and 1.1 in, walleye are quite transparent and the gas bladder easy to observe. Fry are observed in a petri dish on a dissecting microscope, using transmitted light with 6-7 x magnification.
In intensive culture of walleye in relatively clear water, a large portion of fry cling to the walls of the tank. This seems to be an innate phototactic behavior. They swim toward light, not only direct light, but reflected light, even in tanks painted with a flat black paint. Fry up to 3 weeks of age showed a marked preference for extremely bright light (7,800 lux), by week 9, most juveniles preferred lower (2-4 lux) light intensities.
Turbid water does an exceptional job of dispersing the fry. In tanks with turbid water, fry are concentrated near the surface, but dispersed horizontally, in clear water, fry have a greater vertical distribution, even going to the bottom of tanks with black lateral walls but gray-colored bottom. The horizontal dispersal of fry in turbid water seems to be the consequence of the light-scattering effect of turbidity which reduces the amount of light reflected from the tank walls, thereby eliminating fry attraction to the tank walls. Turbidity also reduces cannibalism, perhaps because cohort encounters are reduced inasmuch as the fry are more evenly distributed throughout the tank.
Early reports of nonfeeding by larval walleye raised in intensive culture were commonplace: Nonfeeding fish and cannibalism were considered the major problems for intensive culture of walleye. The problem was related to color or palatability of the feed, or inadequate particle density, but success with brine shrimp was sometimes poor also. Fry distribution within the rearing tank may also affect the opportunity of fry to encounter and consume feed particles.
It seems that water turbidity improved the ability of larval walleye to distinguish and eat. Perhaps it enhances contrast between the brownish-orange feed and the milky gray color of the turbid water, or because the light-scattering effect of the turbidity may have better illuminated each feed particle (i.e., some light would be reflected on all sides of the feed particle as opposed to only the top half in clear water), thus increasing the ability of the fry to see the feed. Nonfeeding would be expected of larvae that are clinging to the walls of the culture container, or in walleye that had not inflated their gas bladder.
Walleye are predaceous fish, consuming zooplankton, aquatic insects, and fish in that order as they grow from first-feeding to juveniles in ponds. Cohort cannibalism is the name given to sibling cannibalism, where fry of about equal size attack each other. Most mortality from cohort cannibalism occurs from trunk attacks, not the result of successful consumption of the prey, which is from the tail first. Cannibalism begins as soon as the fry begin feeding and it can be a major factor in total mortality when fish are not feeding.
Cannibalism in intensive culture of walleye to 21-days posthatch differs among stocks. Thus, certain wild stocks that exhibit an inherent tendency to cannibalize should be avoided. It is important to avoid stocking fish that differ by more than 1-day of hatching. Fish hatched on separate days and fish from different stocks often have different initial lengths. It has long been known that fish-size differential in walleye populations increases cannibalism. Fish that differ by more than 1-day in age would have a size advantage over younger fish that may be sufficient to induce cannibalism.
Cannibalism can be avoided by feeding frequently with adequate amounts and appropriate sizes of a quality feed. Cannibalism is reduced in turbid water culture, perhaps because fish accept formulated feed sooner than in clear water.
Intensive culture of walleye from hatch to a small fingerling requires provisions for biological characteristics of the species and environmental factors: Culture tanks: size, shape, and color, Screens, Surface sprays, Aeration and pumping, Light and temperature, Turbid water culture, Stocking, Feeds, feeders and feeding, Tank hygiene, Water quantity and quality.
Clinging of fry to the sidewalls of the tank will affect success of the culture system. The clinging behavior is a function of tank size, color of the tank walls, light intensity, and turbidity. Fry are strongly attracted to light, direct or reflected light. They will cling to the sidewalls, screens, and they are even attracted to the bottom in tanks with black sides but light colored, bottom. Their presence on the bottom results in some loss of fry when they are siphoned up in process of cleaning the tanks. Tank size is another factor affecting clinging. A greater percentage of the stock cling to the sides of smaller than larger tanks.
Colesante (1996) reports that darkened sides of the rearing tanks minimizes clinging behavior, and high-intensity lights help achieve uniform distribution of fry, and attracts the fry to the surface to aid in the process of gas bladder inflation. Corazza and Nickumn (1981) observed improved larval dispersal in tanks with gray walls compared with tanks with white, yellow or green walls. A diffuse light source and a flat black or gray tank color is helpful to reduce reflection of the light from the tank walls. Survival in my laboratory was generally poor until turbid water was used. Artificial turbidity was developed by addition of a small volume of a clay slurry. Some hatcheries have naturally turbid water (15-25 NTS) from colloidal clay that is not removed by rapid sand filtration.
With turbid water, clinging behavior is avoided and tank color does not matter.
Whatever form of the standpipe or drain system, it must be equipped with a screen to retain the fry, but in feeding formulated feed, it is important to use the largest mesh size possible to keep the screens from clogging, even if feed is lost through the screen.
Since the 1990 culture season, the drain in our culture tanks has been surrounded by 6-in PVC pipe which has large areas of PVC cut away, the cut-away sections are covered with square-weave, monofilament nylon. After cleaning the PVC with PVC cleaner, the fabric is attached to the PVC pipe with PVC pipe glue. The fabric needs to be held in place for about a minute, but then it bonds tightly. In the first 21days posthatch, 710Fm mesh screens are used; after 21 days posthatch, a second set of screens with 1,000 Fm (1 mm) mesh with 58% open area to improve effluent flow. To prevent accidental use of the 1 mm mesh when fry are first stocked, the top rim of the pipe with 1000 Fm screens is painted red. Fry size at hatching varies with the stock, but the smallest size fry we have encountered are those produced by out-of-season spawning. A few of those fry were small enough to pass through the 710 Fm mesh.
The spray (composed of tank water) removes the oil film and cleans the surface of feed and debris. In circular tanks with a circular flow pattern, the water passes under the spray head with each revolution of the water mass. The critical volume of flow needed for an effective spray has not been determined. Moore (l994a) used a flow rate of 0.3 to 0.5 L/min through the spray head and Bristow and Summerfelt (1994) used 1 L/min. It seems important for the spay to impact the surface with enough pressure to produce a slight depression in the water under the spray. The sprayer is a 180 degree perimeter nozzle and it is directed at a 80~90 degree angle with the water surface from a distance of not more than 20 cm above the water surface. The number of spray-heads needed has not been critically evaluated.
Degassing and aeration of the water supply should be done before the water is delivered to the culture tanks. Compressors should not be used to aerate water destined for use in intensive culture of fry because they often contaminate the air with oil in the compression cycle. Compressed air contaminated with oil that is bubbled through water will transfer the oil to the water. The oil will rise to the surface and interfere with gas bladder inflation. Submersible pumps should also be avoided. They often leak minute quantities of oil, but when a seal breaks, the pump releases a large volume of oil. To avoid contamination of fry culture tanks with even small quantities of oil, water should be pumped with centrifugal, axial flow or peristaltic pumps.
A column of rising air bubbles in a fry culture tanks may cause turbulence, and fast rising air bubbles will throw fry out of the water where they will stick to the side walls above the water line. Barrows (1988) used a curtain of small air bubbles from micropore tubing to prevent fry impingement at the water outlet. In some hatcheries, an air line is placed around the center standpipe to keep fry from being impinged on the screen. A large surface area of the screen reduces current velocity. Also, if screens are painted black, fry attraction to reflected light will be reduced.
Research findings on light intensity are varied. It seems that fluorescent lights, flood lamps, and natural light at intensities of 100 to 700 lux are acceptable. Diffuse lighting is often recommended to distribute fry and to deter fry from clinging to the sides of the tank. However, light is a necessity. Fry do not feed in the dark.
The minimum water temperature should be 55 F. Moore (1996) reported that feed acceptance and survival is greater at 65 F than at 55 F, and an ideal temperature range is 60-65 F, with 65 F as optimum.
Survival, length, and weight of larvae raised in turbid water to 21-30 days were significantly greater than for larvae raised in clear water. Larvae were more evenly distributed in the turbid water than in clear water. The turbid water was milky gray. In turbid water, larvae did not congregate on the tank walls as they did in clear water. The light-scattering effect of turbidity seems to reduce the amount of light reflected from the tank walls. The clinging behavior of walleye to sidewalls was greatly reduced or eliminated in turbid water. When screens are painted black, fry attraction to reflected light will be reduced.
In turbid water, survival is increased, the fish are on feed sooner, and their weight at 21 to 30 d posthatch was 200 to 300% larger than weight of fry raised in clear tanks. In all of the six comparisons of larval performance in clear and turbid culture since 1993, the fish in turbid grew substantial faster than fish in clear water.
The system described by Bristow and Summerfelt (1994) to supply a clay-slurry for six, 40 gallon tanks has been enlarged for use with six, production-scale (264 gal) tanks. The clay slurry (8 g of clay per L) is stirred with a commercial mixer with a 3 hp motor that turns a shaft at 1,725 rpm. The clay slurry is distributed with a 2 hp pump that has a calibrated flow of 38 gpm with 10 ft of head. To attain a turbidity level of 50 NTUs in a 1,000 L tank would require addition of clay slurry every 20-minutes. To avoid running the clay-slurry tank dry, the setup is designed to not pump more than 144 of the 600 L capacity of the clay-slurry tank per day. The mixer runs continuously, and the pump distributes 2 L of stock every 20 minutes, 72 times each day. For a single 1,000 L, it would require about 3.3 lb (1,500 g) of clay per day. To supply more culture tanks with the same system, the concentration of the clay slurry should increase proportionally, but the mixer and pump can handle at least six, 1,000 L tanks.
Populations of yolk sac fry (prolarvae) shipped in plastic bags to the culture site will often experience high mortality after stocking, which is a major contrast to stocking fry that are hatched on site, because substantial mortality in these populations rarely occurs until the critical period in the late postlarval II stage. Although shipment of eggs may result in some egg mortality, it is better that the mortality take place in the hatching jar rather than the culture tanks because dead fry liberate oil from their oil globules. Colesante and Schiavone (1980) attributed mortalities that occurred in the first 36 hours after transporting walleye fry in plastic bags to suffocation of the fry in the bottom layers and corners of the bags, especially after the bags were tempered in holding troughs. The mortalities increased in relation to the length of time the bags are left stationary rather than to fry densities in the bag. We have observed substantial mortalities in the first few days after stocking fry that were transported to our facility in plastic bags, but not with fry that were hatched on site from eggs that were shipped.
Fry for stocking should be from those collected in the fry catch-tank that hatched within a short time interval, preferably within 12 hours, but not more than 24 hours. The fry are removed from the catch tank and placed in another holding tank for 2-3 days with low water flows before stocking the culture tanks.
Enumeration of fry for stocking can be done by hand counting, volumetrically, gravimetrically, or by an electronic counter. The counter can count 500,000 fry per hour with an average error of 3%. Gravimetric methods were more accurate than volumetric estimates, which have been the standard hatchery procedure.
Recommendations for stocking density (number of fry/ L) for large-scale production of walleye fry in intensive culture varies from 21 for fry started on brine shrimp, to 40-56 for fry fed formulated feed. With improvements in survival, stocking densities of 80 to l00/L may result in excessive fry density before 30-days are reached, and problems may develop with poor water quality and disease (bacterial gill disease and columnaris disease) causing unnecessary mortality.
Early efforts in feeding formulated feeds quite naturally included trout and salmon feeds, but all early efforts at feeding formulated feed were unsuccessful. Feed size, color, and texture have been considered the most important factors affecting acceptability. Failure of the fish to feed or digest the feed has been cited frequently as a major factor in the failure of walleye culture on formulated feeds. There is no consensus on the amount of formulated feed to feed walleye fry during the first 30 days.
Typically, fry are started on FFK-B series (B-400), followed by either the B-700 or C-700. Fish feeders found at most hatcheries are for dispensing much larger quantities of feed than used in fry culture system; they are not sufficiency precise for feeding small quantities of fry feed accurately and consistently to prevent food deprivation or excessive feeding and tank fouling.
Feeding rates and feeding frequency are important for successful fry culture. Feeding should be at 3-5 minute intervals at least 22 hours/day. Feeding should be stopped only to clean the tanks. Survival has been poor when feeding was stopped for more than an a 6 hour interval per day.
In feeding formulated feed, tanks must be cleaned at least once per day. During tank cleaning, the inflow is not turned off to avoid forgetting to return the flow. The bottom with food, debris and dead fish is siphoned out. A siphon hose is inserted between the screen and standpipe to lower the water halfway to allow cleaning of the sidewalls; the wiping stroke is upward to facilitate removal of the scum from the walls and to avoid contaminating the culture water. When needed, the sidewalls are wiped to the bottom to remove accumulation of scum. Providing that the water level is still sufficiently below the outflow level of the standpipe, the screen is removed and washed in warmwater with a spray and replaced.
Before walleye fry begin feeding, inflows are regulated to obtain 0.5 exchanges per hour. Once feeding begins, the exchange rate is increased to 0.75 exchanges per hour and eventually to 1.0 exchange by 21-days. Higher flow rates are recommended by others. Although high flow rates may be necessary to obtain desirable water quality, high exchange rates imply higher current velocities which may overtax the swimming ability of the fry and deplete their energy resources.
Poor water quality causes stress and mortality, and reduces growth. The reduction in water quality which is sufficient to stress fish often increases the risk of bacterial gill diseases and catastrophic loss of fish, even if it does not have an immediate impact on production. The water quality parameters of primary concern are dissolved oxygen, ammonia, nitrite, carbon dioxide, and gas supersaturation from either oxygen or nitrogen. Dissolved oxygen should not be less than 5 ppm, preferable near saturation, but oxygen supersaturation should be avoided. Recent observations suggest that walleye fry are especially sensitive to supersaturation.
Upper limits for continuous exposure to the common metabolites (ammonia, carbon dioxide, nitrite), pH, and chlorine should not exceed general standards for fish culture as given by Piper (1982) and Meade (1989):
|
Unionized Ammonia |
<0.0125 to 0.02 ppm |
|
Carbon Dioxide |
<10 ppm |
|
Nitrite |
0.1 ppm in soft water, 0.2 ppm in hard water |
|
pH |
6.5-8.0 |
These values are guidelines, they should not be interpreted to mean that concentrations slightly above will be lethal. Toxicity will change with the age, temperature, pH, alkalinity, turbidity and other factors. Because walleye fry have been too difficult to raise in experimental systems, relatively little information is available on their sensitivity. High pH (>9.8) may be lethal to walleye in the absence of ammonia, but pH as low as 8.3 shifts the equilibrium of ammonia to the unionized form (NH3) that is more toxic to fish.
Tolerance of walleye for highly alkaline waters is not known. Obviously, the water supply should not be contaminated with pesticides, PCB s, heavy metals (mercury, lead, zinc, cadmium), chloramines, or free chlorine, or toxic residues from toxic paints or sealants.
The case studies by Colesante (1996), Moodie and Mathias (1996), and Moore (1996) provide contrasting strategies for production-scale intensive culture of walleye fry. Colesante uses brine shrimp for 30 days, followed by 14 days of mixed feeding brine shrimp and formulated feed; Moodie and Mathias and Moore use only formulated feed. Moodie and Mathias use a unique trough and recycle system for fry culture; Moore uses standard circular tanks with a central drain; Colesante uses rectangular tanks. Colesante does not report a problem with NGB but he describes some precautions undertaken to avoid the problem. Moore uses surface sprays to prevent NGB. Moodie and Mathias reported occurrence of the NGB problem with their system. To reduce the problem, they added much larger volumes of "new" water to the culture system when gas bladder inflation was taking place to remove surface oils during the period. However, they have had gas bladder inflation rates of only 16 to 23% in the production scale tanks.
Lets Get Tanked
The MTAN was very impressed with the new aluminum fish distribution tanks now being manufactured by Aquaneering Inc.. The pictures below show a small unit that was built for the Iron River National Fish Hatchery. The tank can easily fit inside a pickup truck. This live haul tank measures 46" by 48" by 30." Although many options are available, this unit includes two compartments with 6" camlock discharge pipes, oxygen injection system and flowmeters, two Point Four diffusers, Fresh Flow pumps and lifting tugs.
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