By Mark Drawbridge *
Aside from aquaculture research on marine fish in the southwest, which has been reported previously in this column, the majority of stock replenishment activity in the U.S. is currently focused in the southeast, including the Gulf of Mexico (GOM). Given the recent efforts to establish an aquaculture permitting framework in the GOM, the carry-over of species from replenishment to commercialization may once again take root. In South Carolina, the Department of Natural Resources (DNR) has been rearing red drum (Sciaenops ocellatus) for decades, as have culturists from Texas Parks and Wildlife (TP&W). Extensive rearing methods are used primarily, and production levels are in the millions as the offspring come from cooperative breeders that spawn volitionally. Both of these state programs have routine stocking activities that are integrated with species management plans. In Texas, several commercial fish farmers are raising red drum in land-based systems.
Another croaker species, the spotted seatrout (Cynoscion nebulosus), is being cultured by these programs in Texas and South Carolina as well as at the University of Southern Mississippi’s Gulf Coast Research Center (GCRL). The culture history for this species is more recent and production levels more modest among these centers, although Texas is currently releasing eight million fry per year from ponds. Although GCRL does utilize some extensive production methods, most of their seatrout production comes from intensive recirculating aquaculture systems. Cobia (Rachycentron canadum) is another priority species at South Carolina DNR, which has been working on the species for the past 11 years. Experimental stocking of cobia have yielded great success with contributions as high as 50% to the recruiting class. This species is being farmed in the southeast region and internationally as well.
The red snapper (Lutjanus campechanus) is a top candidate species at GCRL and one that they have investigated consistently since 2006. This species, which is far more challenging than the Sciaenids we have discussed, is currently spawned using hormones followed by stripping and often yields inconsistent results. Adequate larval survival remains a barrier to mass production, primarily due to the requirements for sufficient quantities of small prey items like copepods.
The southern flounder (Paralichthys lethostigma) has been cultured by TPW since 2006 and represents their most challenging species to culture, particularly through the early life stages. Several other species of Paralichthys are cultured experimentally on the east and west coasts of the United States, while globally they have been successfully commercialized.
In addition to the jump-start that replenishment R&D efforts can ultimately provide to the aquaculture sector relative to culture methods, there are often other important synergies. First, the genetic structure of candidate species is often well studied in the early stages of replenishment programs. This information is then coupled with appropriate breeding and release strategies designed to maintain genetic diversity of the wild stocks. With appropriate quantitative data, it is possible to model not only the consequences of intentional releases of replenishment programs, but also those of periodic escapement events from offshore farms should they occur – i.e. as part of a risk assessment.
Similarly, in cultured populations disease processes invariably become well studied over time, including development of diagnostic tools that can subsequently be used to better understand exposure histories of specific pathogens, including exposure of wild stocks. This information is typically used to develop policies for pre-release health inspections and release certifications. In commercial offshore farming operations, this baseline information will be valuable in helping to understand disease interactions that may occur among cultured and wild fishes, including appropriate responses during outbreak investigations.
In summary, stock replenishment efforts can provide excellent synergies toward the development of commercial farming – on land or at sea. Both aquaculture outlets (replenishment and farming) represent conservation tools that can help balance resource utilization as seafood demand continues to increase. It remains to be seen how or if these synergies will be capitalized on in the United States until the regulatory bottleneck for offshore farming is overcome. For those interested in learning more about the status of fisheries replenishment in the U.S. and abroad, the Fifth International Symposium on Stock Enhancement and Sea Ranching (ISSESR) is being held in Sydney Australia later this year (http://www.searanching.org/symposium5/program5.html). This symposium continues an already rich tradition of highlighting and encouraging further development of the science underlying effective marine stock enhancement, restocking and sea ranching to replenish or augment fisheries and wild stocks.
Acknowledgements: I would like to thank Dr. Robert Vega from TP&W, Dr. Reginald Blaylock from GCRL, and Dr. Mike Denson from SCDNR for providing recent background information on their stock replenishment research programs.
Mark Drawbridge has a B.S. degree in biology and a Master’s degree in Marine Ecology. He’s currently a Senior Research Scientist at Hubbs-SeaWorld Research Institute in San Diego, where he also serves as the Director of the aquaculture program.