48 March 2012 Natural spawning and larviculture of southern flounder Paralichthys lethostigma Jeffery B. Kaiser, Cynthia K. Faulk, Emily A. Williamson and G. Joan Holt Southern flounder Paralichthys lethostigma is an important recreational and commercial flatfish, ranging from North Carolina in the western Atlantic Ocean south (discontinuous around the southern tip of Florida) and along southwestern Florida to south Texas in the Gulf of Mexico (Daniels 2010). It lives in a wide range of salinity from freshwater to full-strength saltwater but is most commonly regarded as an estuarine and bay species within its range. Flounder are harvested by gig and rod and reel, with most fish in Texas taken during the annual spawning run to the Gulf of Mexico from October through December. During this time, Southern flounder are concentrated in great numbers in tidal passes that connect estuaries and bays with the Gulf of Mexico, where they are much easier to catch compared to other times of the year. Larger, gravid female flounder, in particular, are targeted by fishermen and generations of anglers have enjoyed the benefits of the spawning migration. Unfortunately the population of southern flounder along the Texas coast has decreased greatly, according to recent surveys conducted by Texas Parks and Wildlife Department (TPWD) (Riechers 2008, Froeschke et al. 2010). During the past decade, bag limits for flounder have been reduced and the size limit increased for commercial and recreational fisheries in Texas in an effort to reverse the downward trend. Most notable were rule changes implemented in 2009 that cut the daily bag limit in half to five fish, with only two fish in November, and prohibited harvest by gigging altogether during November for commercial and recreational anglers. The TPWD is investigating the feasibility of establishing a stock enhancement program for southern flounder if size and bag limit restrictions fail to result in recovery of fish stocks. Scientists in North Carolina, South Carolina and Florida have been investigating spawning, optimal larval rearing conditions, and general husbandry since the mid-1990s, and have developed an extensive knowledge base about flounder aquaculture (Smith et al. 1999, Watanabe et al. 2006, Daniels et al. 2010). However, there are regional genetic differences between flounder from the western Gulf of Mexico and those from the eastern Gulf of Mexico-Atlantic region (Blandon et al. 2001). As such, optimal culture conditions for flounder from the Atlantic coast may be different for flounder from the western Gulf of Mexico. Despite the interest in flounder culture and fairly wellestablished methods for producing Atlantic coast southern flounder, there has not yet been any major commercial production or large-scale stock enhancement efforts in the USA. Research on southern flounder at the Fisheries and Mariculture Laboratory (FAML) of the University of Texas Marine Science Institute is ongoing, with an emphasis on developing improved spawning and larval rearing methods for western Gulf of Mexico southern flounder. Southern flounder were first spawned at FAML in 1978 using photoperiod and water temperature manipulation (Arnold 1978). More recently, broodstock flounder have been collected at FAML annually since 2002 and maintained in recirculating tank systems to allow natural spawning and egg production. Current research focuses on weaning (Faulk and Holt 2009), sex determination and temperature (Montalvo 2011) and methods to improve the quantity and quality of eggs produced by captive broodstock. Broodstock Collection Typically male flounder begin offshore migration through estuarine passes along the central Texas coast during October, whereas female flounder migrate primarily during November. During late autumn, adults can be collected in large numbers at night using an airboat to access shallow water along channel edges. Water depths less than 60 cm allow personnel to easily and safely enter the channel to collect fish with a dip net. Once captured, fish are placed in a saltwater holding tank provided with diffused pure oxygen. Upon returning to FAML, flounder are dipped in freshwater for 20 minutes and injected with oxytetracycline to help prevent bacterial infections. This is an effective way to collect broodstock flounder with advanced gonadal development inasmuch as collections are made just prior to the natural spawning season. Southern flounder males are identified by milt expression from light abdominal pressure. Females are identified by the presence of ovaries that are visible as a long bulge along the ventral side. Recently collected fish are handled carefully and as little as possible to reduce stress that might affect future spawning. When sorting or moving large numbers of fish to a broodstock tank, flounder are first anesthetized with
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