By Chris Young
Adult red drum purse-seined from the Gulf in September 2016 were moved into the hatchery building at SERF in January 2017. Maintaining these fish in ponds at SERF for several months provided ideal conditions for external parasites to find a reliable and healthy host. Two common parasites with direct life cycles (one host species is used for all life history stages), Amyloodinium ocellatum (dinoflagellate) and Caligus sp. (copepod), can cause mortality of marine fish, including red drum, maintained in recirculating aquaculture systems.
Before introducing the red drum to the hatchery maturation and spawning tanks the red drum were bathed in freshwater for 20 minutes to remove these parasites in particular.
Scrapes of gill and skin from a representative number of fish were examined with the use of a compound microscope before the freshwater bath to provide a historical record of the parasite assemblage. The historical record informed staff to be aware of the signs of specific-parasite infestations and to be prepared for specific therapeutic measures to consider in the event that signs of parasite infestation appeared after the fish were put into maturation and spawning tanks. During the freshwater bath, the red drum were sedated to minimize harm to fish and staff when data was collected. Length, weight and sex were determined for each fish, a passive integrated transponder (PIT) tag was implanted to uniquely identify each fish, and a small tissue sample (fin clip) was obtained for genetic identification of each fish during the freshwater bath.
Genetic identifiers are used to assess parentage of hatchery offspring for hatchery and field studies. The latest group of red drum adults brought into the hatchery are being used for a second of three planned replicates to assess the effects of tank and population size on the spawning contribution of red drum – the aim is to improve stock enhancement operations efficiency and increase genetic diversity by using one 20-foot diameter tank for 30 red drum adults rather than five 12-foot tanks with six red drum each.