Dedicated To Tribal Aquaculture Programs
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March 1998 ~ Volume 23 | |
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Topics of Interest:
Pond Fertilization Methods and Walleye Rearing Techniques
Pond Fertilization Methods and Walleye Rearing Techniques
Contributed By Greg Fischer: Tribal Hatchery Manager for the Red Cliff Band of Lake Superior Chippewa Indians
The Red Cliff Tribal Fish Hatchery (RCTFH) is raising walleye in one acre outside rearing ponds. The walleyes are raised as extended-growth fish which are between 6 -10 inches long by fall. These fish are converted from zooplankton to fathead minnows in the fall for the final growout phase. We begin each spring by partially filling the ponds about 3 to 2 full and adding approximately 200 pounds of alfalfa meal to each pond. We have also added some Torula yeast in the spring to help jumpstart the ponds, but have not noticed any drastic changes when it was not added. Once the ponds begin to warm up and plankton blooms begin occurring, we monitor daily. Ponds are monitored for temperature, dissolved oxygen, pH, turbidity, and plankton levels (both zooplankton and phytoplankton). Nitrogen and phosphorous levels are monitored weekly. Alfalfa meal, usually 200 pounds a week, is added as we notice nutrient levels or plankton levels dropping in the ponds. We have also been experimenting with adding inorganic fertilizers in addition to alfalfa meal to help boost the nutrient load in the ponds, especially in the later stages of zooplankton production. The Key to the success of our pond fertilization schedule is actively monitoring the ponds. The organisms in the pond will tell you when it is time to fertilize.
One note of caution when fertilizing is to watch the weather. Do not fertilize if the weather forecast calls for cloudy or rainy days. Lots of sunlight for heat and photosynthesis is optimal for fertilizing. Because the RCTFH walleye ponds are fairly new (4 years old), we have not ruled out any certain type of fertilizer or fertilizing schemes and will continue to experiment until one is found that works best for us.
Contributed By Don Taylor: Natural Resources Department Field Supervisor for the St. Croix Natural Resources Department
We usually allow walleye to grow to at least 2 inches before seining. This minimizes gilling. In 1997, we had two ponds. In the 5-ac pond, we began harvesting at less than 2-in to reduce the total number of fish in the pond. In our larger 15-ac pond, the walleye reached 3.5-in before we began harvesting. This may be a little larger than we wished. About 2.5-3.0-in is more ideal.
We have used only two types of fertilizer: Alfalfa meal and yeast culture (Saccharomyces cereiviae). The alfalfa meal is 17% crude protein, 1.5% crude fat, 28% crude fiber, and 35% nitrogen free extract. The yeast culture is 14% crude protein, 2.5% crude fat, and 8% crude fiber. We fertilize in early spring, after ice-out. Frequency has varied from year to year and from pond to pond. Some years no fertilizer was used. We use a fertilizing ratio of 20:1 nitrogen to phosphorous. We measure this weekly with a colorimeter. The ponds that we have now are surrounded by agricultural fields, so as of last year the ratio was naturally satisfactory and no fertilizer was added. In the past, we used the eye-ball approach and/or secchi disk readings. We added fertilizer sometimes when it wasnt needed.
The key to any successful fertilizing program is knowing whether or not to add fertilizer. Last year we didnt add any and had ample plankton counts. In the past, we added fertilizer and ended up with much more pond weed growth than we wished for and needed aquatic herbicides. We will never again just add fertilizer to our ponds without knowing our nitrogen to phosphorous ratio and without knowing our plankton abundance. As mentioned before, our ponds are natural and surrounded by agricultural fields. A large rain may occur and our ponds could be contaminated with the crop fertilizers. We want ample plankton supply but dont need overgrowth of weeds. Over-fertilizing can be a major problem. Monitoring nitrogen to phosphorous ratios and collecting plankton from nearby sewage treatment plants and stocking them into our ponds has been successful.
Contributed By Gregg Wright: Fisheries Enhancement Coordinator for the Inter-Tribal Fisheries Assessment Program, Sault Ste. Marie
We had a brief experience with fertilizing our 150+ acre walleye pond. We can grow about 100,000 walleye to 7-8". We have only used 46-0-0 urea as fertilizer. However we do rely on nutrients being added from the pasturing of draft horses on the water shed during the fall months. We tried applying every week, by mixing and spraying with a trash pump. Then we switched to laying perforated bags in floating cribs.
We were sampling to try to match the 20:1 ratio of nitrogen to phosphorus but because we had low levels of phosphorus, the ratio bounced around too much to be practical. We really depend on natural productivity and lately we have paid the price in low numbers and small size.
Contributed By Larry J. Wawronowicz and Willis G. Allen: Lac du Flambeau Tribal Natural Resource Department
The Lac du Flambeau Band of Lake Superior Chippewa Indian' s Fish Culture Program has been producing walleye fry since 1936 and fingerlings since 1980. Walleye are raised to stock reservation waters to help sustain a viable subsistence and sport fishery.
Pond management techniques such as fertilization, pond monitoring, harvest and aquatic insect and weed control are discussed. By using the techniques described, 2.0 inch walleye fingerlings (417-450 fish/pound) can be produced in 40 days (0.05 inches/day) by using 10,000 pounds of alfalfa meal and 1,000 pounds of Torulas yeast in 8.5 acres of water. Water temperatures during the production seasons range from 40 to 79 degrees Fahrenheit. Since 1980, survival to 2.0 inches ranged from 30% to 75%.
Walleye fingerling production depends on many biological, physical, chemical and environmental factors which need to be considered during the rearing season. This makes walleye production a continuous learning process. The purpose of the paper is to provide the reader with rearing techniques used by the Tribal Fish Culture Program when raising 1.75-2.25 inch walleye fingerlings in north central Wisconsin in drainable ponds. The ponds are 0.25 to 0.75 acres in size and are either lined with clay or HDPE plastic for a total of 8.5 acres of water. Average depth is 4.0 feet with 34 acre-feet of water available for walleye production. Pond levee slopes are either 2 to 1 or 4 to 1 with 20 feet levee crowns. The water supply is ground or lake water. The ground water can be supplied to the pond at a rate of 1000 gallons per minute and lake water at 2600 gallons per minute. Lake water temperature ranges between 40 to 79 degrees Fahrenheit during the production season and the ground water is constantly 50 degrees. Both the ground and lake water are very soft with calcium carbonate concentrations of 20 and 70 ppm, respectively. Soils associated with the watershed are considered infertile.
The basic principle for producing walleye fingerlings in ponds is to drive the food chain by utilizing the sun's energy and converting it to fish biomass by manipulating physical, chemical and biological parameters associated with a geographical area. Since 1980, many different fertilization programs were tried before it was determined organic fertilization was to be the best alternative. Alfalfa meal and Torulas yeast, a yeast used in the paper industry, is applied at an average rate of 1,175 and 118 pounds, respectively, per acre. This seems to be the best combination when trying to convert the sun's energy into phytoplankton, zooplankton and eventually, 1.5 to 2.25 inch walleye fingerlings. The amount of fertilizer used largely depends on pond fertility, weather conditions, number and size of the fish and the quality and quantity of the zooplankton population. Generally, the fertilization program begins by applying approximately 2 the total amount of fertilizer required when the pond is filled. Subsequent applications are weekly and approximately 1/8 of the total is required. These fertilizers supply the nutrients necessary to produce the preferred cyclopoid copepods and daphnid cladocerans.
Pond preparation begins in the fall for the following year's production season. All ponds are allowed to drain and dry. This consolidates the bottom, aides in disease control and eliminates unwanted fish and plants (i.e. algae, some macrophytes). The earthen ponds are allowed to seep dry, rather than being rapidly drained, in order to simulate drought conditions. We feel this triggers the cladocerans and copepods to produce resting eggs. The eggs survive the winter and contribute to the initial zooplankton present in the spring when the ponds are filled. Levees are repaired along with any plumbing problems and pond bottoms are planted with rye grass, excluding the plastic lined ponds. The rye grass aides in erosion control and adds to the fertilization regime.
In mid-April the earthen ponds are disked to break down the rye grass and to aerate the soil. The ponds are partially filled approximately 4 weeks before 3-day post hatched walleye fry are stocked. Initially, the ponds are fertilized with 600 and 50 pounds per acre of alfalfa meal and Torulas yeast, respectively, and filled to approximately one third the pond volume. This allows the ponds to "cook" or warm more quickly which assists in establishing a zooplankton population by utilizing the resting eggs in the soil. Surface water from Pokegama Lake is used to fill the ponds which is the same water used to hatch the walleye eggs. This is beneficial in two ways. First, the walleye are reared in the same water they were hatched in and second, we are able to observe the zooplankton in the fry tank. When the zooplankton are first observed, water from Pokegama Lake is again pumped to the ponds to capitalize on the lake produced plankton. An additional one third of the pond is filled when we first see the plankton in the fry tank and the final one third is added when the walleye eggs start to hatch. The staggered pond filling regime has been called the "puddle method". The alfalfa meal is broadcasted evenly around the pond by using a scoop made from a one gallon milk container. Because Torulas yeast is a very fine powdery substance, it is applied by mixing it in 50 gallons of water and distributed evenly around the pond. It is important the fertilizer is evenly distributed and the rye grass has been disked because if there are clumps of fertilizers or rye grass, it has been our experience the clumps become starting sites for unwanted algae.
The ponds are monitored for zooplankton production before and after the fry are stocked. Zooplankton are collected by using an 80 micron mesh net with a mouth diameter of 8 inches and a length of 20 inches. The net is extended from a fiberglass pole and pulled through the pond for approximately 20 feet. Sampling areas of the pond will vary depending on the wind direction. The windward side of the pond is usually sampled. The pond manager is responsible for determining the quantity and quality of the zooplankton populations before stocking and during the rearing season. Subsequently, the hatchery and pond managers make the decision when fry are stocked based on the age of the fry and the condition of the zooplankton population. The pond manager is in constant contact with the hatchery manager because the time of stocking is of utmost importance. We feel three day old fry or fry strong enough to swim to a light located at the head end of a fry tank provides us with the best survival at the time of stocking. Three day old walleye fry are stocked at a rate of 120,000 fry per acre.
The question becomes what constitutes a desirable zooplankton population? Being a production facility, we do not have the time to determine the exact number of cyclopoid copepods or daphnid cladocerans we have per fluid ounce. The condition of the population is an intuitive feeling of the pond manager based on relative abundance, size and species composition of the zooplankton determined by daily monitoring of the ponds and years of experience. In general, at the time of stocking there should be copepods and rotifers of various sizes in abundance and only a moderate amount of daphnids. As the rearing season progresses there should be more daphnids than copepods and a variety of benthic invertebrates (i.e. chironomid larvae) as well. The condition of the zooplankton is also noted. Are egg sacs present, are there oil globules in the gut, are there varying sizes and adults present to provide forage? These observations provide the pond manager with the information required to know if more fertilizer is necessary to maintain the zooplankton population. If, for example, it is determined the relative abundance of copepods is decreasing compared to the sample collected the day before and there are no daphnids present, the pond manager would fertilize if first light oxygen concentrations are 5 ppm or better. The amount of fertilizer to be used depends on numerous factors as described above but during a 40 day production season, a total of 10,000 and 1,000 pounds of alfalfa meal and Torulas yeast will be applied, respectively. The cost for alfalfa meal is $1.33 per pound and Torulas yeast, $1.34 per pound for a total annual cost of $2,670.00.
Also during the rearing season other parameters are monitored. Oxygen concentrations of each pond are determined using a YSI 57 Oxygen Meter with a submersible stirrer. Oxygen samples are collected at first light to determine oxygen concentrations before photosynthesis occurs to obtain the lowest daily readings. Samples are usually collected at the deepest area of the pond at mid depth. On occasion all ponds will be sampled to determine if there is oxygen and temperature stratification. We are most concerned about stratification in June to when the walleye are harvested in July. If oxygen stratification is occurring with concentrations of 1-2 ppm at the pond bottom and mid depth, the rearing area is effectively reduced by half. Also, knowing the lowest oxygen readings will determine if the pond manager will fertilize. If oxygen concentrations are below 4 ppm and the weather is predicted to be cloudy, hot and humid with no wind for the next 3 days, the pond manager will not fertilize. Additional fertilizer will add to the oxygen deficit when it begins to decompose. When or if low oxygen and stratification occurs, the bottom water can be drawn off and be replaced with aerated cooler lake water or cold ground water.
Since the ground and lake water have low calcium carbonate concentrations, the pH is monitored to determine if major pH changes are occurring over a 24 hour period. Samples are collected in the morning, afternoon and night to obtain information on any pH changes due to photosynthesis and respiration. Changes in pH were a preliminary concern when we lined the ponds with plastic liners. It was originally thought that if the soil was covered, the soils buffering capacity would be lost. During the first production season using the lined ponds, pH readings between the lined and earthen ponds were compared. An Orion Model 250 A was used to determine pH of the water collected at mid depth and at the surface, the results showed no significant differences in pH.
We do not actively sample the walleye by collecting them to determine growth rates or condition factors during the production season. Growth, condition and density are observed and noted daily. Walleye fingerlings generally can be observed during the daylight hours, 14 days after stocking. At 14 days, the fingerlings are approximately .75 inches and can be easily observed near flowing water. The attraction to flowing water is a characteristic we take advantage of when monitoring the condition and growth of the walleye. Typically we can raise a 2 inch walleye in 40 days which is an approximate growth rate of 0.05 inches per day. The fingerlings range between 417 to 450 fish to the pound. Annual survival to 2.0 inches is variable and ranges from 30% to 75%.
Standard piscivorous aquatic insect control has been used when needed. A portion of a mixture of 3 gallons of fuel oil and 1 quart of 30 weight motor oil is applied to the pond. Only enough oil is used for the wind to spread an oil film over the surface of the water. Because it is thought walleye fry inflate their gas bladders at 5-7 days post hatch by swimming to the surface, it is recommended to treat the pond 10 days after stocking, otherwise the oil film could interfere with gas bladder inflation.
Aquatic vegetation control was conducted by hand removal and was very labor intensive. Approved herbicides for aquatic use are not very effective in our water. Specialized rakes with wheels were designed to remove the algae mats before seining. It was determined this method was hard on both the fish and the seining crew and subsequently discontinued. A combination of fyke nets and a seine is used to harvest the walleye. This effectively reduced the time spent on removing unwanted vegetation before harvest.
Five fyke nets and a seine are used to harvest walleye fingerlings. The fyke nets are 1/8 inch mesh with 4 foot gates, 3 foot diameter hoops and 25 foot leads. The seine is 300 foot long, 8 feet deep and has 1/8 inch mesh. The harvest technique used is to crowd the fingerlings by seining 2 to : of the pond toward the water supply line. Fyke nets are set in the un-seined portion of the pond. The pond is slowly drained while fresh water is being added. Seining and setting of the nets occur in the morning and the nets are checked every three hours. Two floating wood boxes are used to place the walleye fingerlings in when removed from the nets. The boxes are approximately 3'x4'x1' and are constructed so the bottom and the 3 foot sides are screened to allow for water exchange. Water is exchanged by simply moving the boxes through the water. When all nets are harvested, the boxes are floated to a pond side measuring and weighing station where the length, number of fish per pound and total weight of the fish harvested are determined. The fish are transferred to a hauling truck and transported to a designated lake in a 1.0% saline solution. This method of harvest has proven to be very easy on the fish, even in warm weather. The above method is continued until all the walleye have been harvested.
The Lac du Flambeau Band of Lake Superior Chippewa Indians have been raising walleye since 1936. Through the years many rearing techniques have been learned and passed down to current fish culture personnel. Walleye fingerling production is variable based on many biological, physical, chemical and environmental factors and should be considered more of an art than a science. The information written above is only one way to raise walleye fingerlings and some of the techniques used may or may not work in other parts of the United States or Canada. The above is only an exchange of information which will hopefully provide the reader with some useful hints when applied at other production facilities. Remember, walleye production is a continuous learning process.
Submitted By Randy Zortman: Fisheries Manager for the White Earth Conservation Department
Prestock Fertilizing Schedule:
Week 1: 22 lbs. yeast and 100 lbs. alfalfa. Week 2: 22 lbs. yeast and 100 lbs. alfalfa.
Poststock Fertilizing Schedule:
Week 3 through Week 9: Each week apply 5 lbs. yeast and 50 lbs. alfalfa.
White Earth will never again use soy bean to fertilize rearing ponds. The material takes far too long to break down and promotes excessive weed growth, which makes it difficult to harvest the fish.
Submitted By John Ringle: Fish and Wildlife Program Administration for the Leech Lake Division of Resource Management
1000 lbs. of alfalfa meal and 100 lbs. of yeast per acre. These amounts are added in several parts, 2 as the ponds are filling and the rest in weekly amounts (50 lbs. alfalfa, 5 lbs. yeast) over a ten week period. The rearing program at the Leech Lake Reservation found that inorganic fertilizers did not work well.
Growth Performance of Juvenile Lake Sturgeon as Affected by Rearing Density (Draft-12-22,1997)
By: Steve Fajfer, Lee Meyers, Guy Willman and Terry Carpenter, Wisconsin Department of Natural Resources, Wild Rose State Fish Hatchery
Abstract. - Juvenile lake sturgeon (Acipenser fulvescens), 106 d, were reared at three different tank densities, 150(1.35), 300(2.5) and 450(3.75) fish(kg) /m2, for a 35 d trial period. The purpose of the study was to determine if higher rearing densities negatively affected growth. During the trial, diet consisted of 72% chironomid larvae, 8% brine shrimp and 20% krill with a food conversion rate of 7.2 to 1. Feeding rates ranged from 15 to 34% body weight per day with rates dependent upon water temperature and diet. Fingerling lake sturgeon had significant increases in mean length from 126.9 to 169.3 mm (33% increase) and in weight from 8.3 to 20.4 g (146% increase). However, there was no significant difference in growth between the three densities tested, indicating that the higher tank rearing densities of 450 fish (3.75 kg)/m2 are acceptable to produce lake sturgeon fingerling for distribution.
Since the early 1980's, the Wisconsin Department of Natural Resources has began collecting, hatching and rearing lake sturgeon for research and population restoration purposes within the historic range.
The increased demand for fingerling lake sturgeon production at the Wild Rose Hatchery has prompted the staff to conduct tank rearing density/feed trial experiments. A trial was set up to test three rearing densities to see if fingerling growth parameters were negatively affected at higher densities. Czeskleba (1985) indicated that juvenile lake sturgeon preferred live food (Tubifex spp and earthworms) over formulated foods. Also, that what larval sturgeon are first fed may be important to future diets they will accept.
The State of Missouri rears juvenile lake sturgeon (127mm) at average densities of 215 fish/m2. Few rearing trials have been reported on sturgeon of North America; feeding trials have been reported for white sturgeon and Atlantic sturgeon, and the effects of temperature on growth of lake sturgeon.
This paper presents information that will help fish culturists produce quality fingerling lake sturgeon with minimum available hatchery space.
Methods:
A rearing density trial was initiated at the Wild Rose State Fish Hatchery on August 23, 1993, when the fingerling lake sturgeon were
106 d. These juvenile lake sturgeon were products of eggs collected from spawning adults in the Wolf River, Winnebago system. A majority of the lake sturgeon hatched on May 9 and were initially fed a diet of newly hatched brine shrimp, zooplankton and Tubifex spp.
Juvenile lake sturgeon were randomly divided into 9 lots of 300, 600 and 900 per tank for final rearing prior to stocking. Rearing tanks were of equal size, 3.66 m long by 0.56 m wide by 0.61 m deep, that held 1,136 L with a flow through rate of 38 L/min. Bottom area of the tank is considered the important space measurement for rearing lake sturgeon due to their benthic orientation. The low density tanks were started with 150 fish (1.35 kg)/m2, medium density tanks had 300 fish (2.50 kg)/m2 and the high density tanks were started with 450 fish (3.75 kg)/m2.
Fish were hand fed various combinations of frozen chironomid larvae, brine shrimp and or krill two times per day. Feeding occurred over a half hour period as small amounts of food were provided until the lake sturgeon stopped feeding (cessation). The amount of food provided was adjusted daily. Lake sturgeon were consider full when they stopped swimming and began to rest on the bottom.
Results:
The low density fish gained 42.2 mm (13.2 g), medium density 42.6 mm (12.5 g) and high density lots gained 42.4 mm (12.0 g) with no significant difference between the groupings. The low density lots were slightly larger at the start and maintained that edge over the 35 d trial.
There was no significant difference in the mean lengths of the sturgeon between any of the three densities each week throughout the trial, although there were significant increases within each lot from start to finish of the trial. Weight gain continued to increase the last week of the trial even though length increase had slowed.
The best weight gain (4.9 % BW/d) occurred when water temperatures averaged 200 C. Decline in weight gain correlated with decline in water temperature, except in Week 3. We believe that this was related to diet changes during the trial and resulted in the higher variation (lower correlation) between water temperature and growth during the trial.
Daily amount of food eaten by lake sturgeon ranged 15-34% body weight per day. At the start of the trial, frozen food composed of b chironomid larvae and a brine shrimp was provided at approximately 34% body weight daily. This was adjusted the second week as frozen krill was added to the diet during Weeks 2, 3 and 4 and krill was eaten at 15 to 18% body weight per day. During the final week, krill was not available, so 100% chironomid larvae were provided at 22% BW/ d.
The overall conversion rate during the trial period was 7.2 to 1. The low density tanks conversion averaged 7.3 (range 7.0 to 8.3), medium density tank averaged 7.1 (range 6.7 to 7.5) and high density tank conversion averaged 7.2 (range 6.9 to 7.4). The lack of difference between the three densities indicates that the high density lots are capable of growing comparable fingerling lake sturgeon.
Mortality during the trial was not considered a significant factor. The low density tanks had a total mortality of 5 fish during the trial, medium density tanks no mortality and the high density tanks had only 3 fish die during the trial period. Water quality appeared to remain high during the trial as did overall dissolved oxygen readings.
Discussion:
There was no significant difference in the growth of lake sturgeon reared at three densities tested. This suggests that juvenile (100+ days old, 8-22 g) lake sturgeon may be reared at densities of 450 fish (3.75 kg) /m2 of tank bottom without sacrificing condition or the health of the fish. We believe that bottom area (m2) to be the critical measure due to the benthic nature of lake sturgeon.
Warmer temperature correlated with higher feeding rates in most fish rearing situations. In our study, feeding rates were reduced as water temperatures dropped. We believe that diet changes during the trial resulted in some variation.
Wehrly (1995) reported that lake sturgeon growth was positively correlated to increasing water temperature from 7EC to 23EC, however he observed excessive mortality of 45% at 23EC, and concluded that this may be the upper temperature tolerance limits for rearing juvenile lake sturgeon. Wehrly observed growth increases from 0.71 % BW/d at 10EC to 1.52 % BW/d at 23EC. We observed much better growth rates from 1.9 % BW/d at 15EC increasing to 4.9 % BW/d at 20EC.
Cui (1997) suggested that continuous feeding (24 h) of small amounts by automatic feeders provided for the best growth of juvenile white sturgeon. We essentially hand feed our fish to essation twice a day. This allowed for good observations of the feeding and rearing conditions.
In the future, a similar trial should be conducted but with higher rearing densities to better determine the upper level of density at which growth and condition are affected. We also do not know if rearing sturgeon at a higher density will reduce their survival once they are released.
WITH THE ARRIVAL OF SPRING COMES BGD!
By: Terry Ott, US Fish and Wildlife Service, La Cross Fish Health Center, La Cross, Wisconsin
As our feathered friends return from the south, bears move out from their dens, and the skunk cabbage begins its journey through the decayed organic matter on the forest floor. The arrival of BGD or bacterial gill disease begins to make its appearance around the hatchery grounds. This phenomenon which causes BGD is not animal or plant, but a group of opportunistic bacteria that live in the water supply to your hatchery. True, spring is not the only time of year BGD can be found at a hatchery, if at all. It does occur throughout the year, but appears to be more prevalent in the spring. The reason for this is not the arrival of warm weather, nor the return of the swallows, but the arrival of sac fry and fry fish at your hatchery. This life stage of the fish is the most vulnerable to BGD, because fish at this age have not yet developed a complete immune system to fight off invasion by microbial organisms. Their small size coupled with tender gill tissue and the constant bombardment of gill irritants in the water column, like dusty starter feeds, is the perfect environment for the beginnings of BGD.
BGD develops when gill irritants are present in the water column at concentrations high enough to alter gill epithelium. Overcrowding of fry, especially when accompanied by reduced water flows, excessive food, fines or small particles of food, ammoniacal gill excretions, and heavy loads of organic detritus in the water, are the most important in the development of BGD. Irritated gills usually result in a condition fish health biologists describe as hyperplasia.
Hyperplasia is a condition where epithelial cells on the lamellar surface increase in numbers in response to the irritant in the water column. The excess cells are not lost as rapidly as they are formed, so a thickening of the gill surface results. Like a callus you get on your hand, after you have cleaned too many raceways in one day! This thickening obstructs flow of water between the lamellae so that blood cannot be oxygenated, or metabolic gases cannot be exchanged. The severity of the swollen gills determines the ability of the fry to survive. The swollen gills have no direct blood supply and are a target for the invasion of opportunistic bacteria that live in the water supply. Flexibacteria, pseudomonads and flavobacteria are the most commonly found gram-negative bacteria causing BGD.
Fry developing BGD show sudden lack of appetite, orientate against the water current, ride high in the water column, and spread out equidistant from each other. Gill tissue may protrude from under the gill covers or remain in sight when covers are supposed to be closed. The posterior part of the head may appear thickened, because of the swollen gills. These clinical signs can be seen from a distance, especially as the fry swim and turn through the water.
Therapy of BGD should begin as soon as possible after you observe one or several of the clinical signs above. BGD is very controllable in the early stages of the disease, if allowed to progress high losses of fry will occur. BGD is usually best controlled by use of an external disinfectant and the elimination of the gill irritant in the water column. Common table salt prescribed at a rate of 2.0 to 3.0% as a bath or dip is the most economical. Salt helps flush mucus from the gills and with the mucus so goes the bacteria causing BGD. Salt is also helpful to the gills by replacing lost ions from the gills caused by the irritant. There is also another chemical compound Chloramine-T which has shown to be very effective against the bacteria which cause BGD. However, the chemical is still not registered by the Food and Drug Administration (FDA) as an aquatic bactericide. Only those hatcheries which possess a special permit from the FDA are allowed at this time to use Chloramine-T against BGD. Hydrogen peroxide has also shown its effect on fish having BGD, but like Chloramine-T is not allowed for use as a bactericide. It can be only used to treat fungus on fish and fish eggs.
BGD is usually the result of mismanagement of fry at your hatchery. Water supplies should be kept free of feral fish, silt, and mud. Cleaning of waste feed and fecal material in the raceways should be done daily. Water chemistries like ammonia, pH, and oxygen should be measured weekly. Correct management of the biomass in each raceway, and the elimination of gill irritants in the water supply will go along way in reducing your chances of seeing BGD arrive at your hatchery in the spring!
Aquaculture Web Sites on the Internet
The MTAN has been doing some web surfing and has located several aquaculture related sites that you may want to bookmark. Keep in mind that the web is a dynamic information network that is continually changing. Additional locations to your questions are being offered/eliminated every day.
Topic |
Location |
Aquaculture |
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MTAN home page |
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UCDavis |
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Aquaculture News |
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Aquaculture Magazine |
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AquaNic |
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Calif. Aquaculture |
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Fish Information Service (FINS) |
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Fish Vet |
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NetVet |
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NE Regional Aqua. Center |
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Product and company names mentioned in this publication are for informational purposes only. It does not imply endorsement by the MTAN or the U.S. Government. |
