50 March 2012 and applying a conversion factor of 1300 eggs/mL. On a few occasions, FAML personnel have observed flounder during spawning, which typically occurs from noon until early afternoon. One female swims around the tank at or near the surface, typically with one male following her closely. Eggs are released while fish are swimming together and collected from the filter box a few hours later. Several combinations of broodfish have been used for spawning—all new wild-caught, a mix of new and previously spawned fish, and all previously spawned fish— with variable results from year to year. The 2010-2011 season at FAML was the most productive to date as indicated by number of spawns, quantity of eggs, and egg viability. Two spawning raceways were used, one with flounder spawned during the previous year and the other with new wild-caught females and previously spawned males. More than 300 natural spawns were collected from these tanks from November 2010 to March 2011, with an average spawn size of 222,000 eggs and viability rate of 18percent (Table 1). Results were similar between the two combinations of fish in terms of quality (i.e., viability), although the system with larger, previously spawned female broodfish produced more eggs. Flounder typically spawn batches of approximately 100,000 eggs per kg of body weight (Daniels et al. 2010) so it is advantageous to have large females in broodstock systems to increase overall production. After collection, viable flounder eggs were used in various studies conducted on larvae and juveniles at FAML. Excess eggs were given to the Coastal Fisheries Division of the Texas Parks and Wildlife Department to support efforts to develop a stock enhancement program for flounder, similar to their successful program for red drum. During the 2010-2011 spawning season, over three million eggs on 30 occasions were donated to the Coastal Fisheries Division, allowing their staff to gain experience with grow-out of flounder in outdoor ponds. The Effects of Temperature on Larval Rearing Larval rearing is often a significant bottleneck to the establishment of stock enhancement and aquaculture programs for marine fishes. For successful larviculture, a thorough knowledge of optimal rearing parameters— such as temperature, salinity, density, and feeding regimes—is necessary. As indicated previously, considerable research has been dedicated to optimizing larval rearing conditions for southern flounder inhabiting Atlantic waters. However, because of potential genetic differences among populations, current rearing practices for Atlantic flounder may be different for fish from the western Gulf of Mexico. For larvae spawned by wild-caught fish in North Carolina and held in small, static systems, the optimal rearing temperature is 17 °C from first feeding through the onset of metamorphosis and 21 °C from the onset to the Fig. 3. Gravid broodstock flounder showing advanced gonadal development. (Photo by Jeffery B. Kaiser). Fig. 4. Southern flounder eggs during development a) 4-cell, b) morula, c) early embryo d) tail bud. (Photos by Cynthia K. Faulk). (Continued on page 52) ly 21-22 °C. At two-week intervals, day length is gradually reduced to 10 hours of light and temperature to 19-20 °C, which are considered conditions for flounder spawning in the area. This manipulation results in continued gonadal maturation (Fig. 3) with spawning usually beginning in mid-December and continuing for approximately three months, if spawning conditions are maintained. Flounder eggs (Fig. 4) are collected nearly every day during this period by placing a 500-µm nylon screen bag into a filter box that receives surface water from a broodstock tank. Spawns vary in terms of quantity and egg viability, ranging from a few thousand to several hundred thousand eggs, depending on the number of females that spawn on a particular day. Eggs are enumerated by measuring floating egg volume with a graduated cylinder
RkJQdWJsaXNoZXIy MjExNDY=