Hormone Induced Spawning of Summer Flounder Paralichthys dentatus

-During their first year in captivity, summer flounder Parulicthys denratus were induced to spawn with gonadotropin releasing hormone analogue (GnRHa) implants, injected carp pituitary extract (CPE) or human chorionic gonadotropin (hCG) injections. The percentage of fertile eggs was greatest (-69%) in CPEtreated females. CPE, but not GnRHa or hCG, was capable of stimulating oocyte growth (increased follicle diameter during vitellogenesis) followed by ovulation. Fish with maximum ovarian follicle diameters between 180 and 435 p m at the initiation of CPE injections produced the greatest percentage of fertile eggs. For most females, fertilization rate was greatest for the first batch of eggs ovulated. The mean fertilization rate for the first spawn of CPE-treated fish was 42% compared with 14% for the second spawn from the same fish. Fish with maximum initial follicle diameters of 5 8 5 4 0 p m that were implanted with GnRHa ovulated the greatest number of eggs, but fertility was low and variable. Approximately 35% of females injected with hCG ovulated a limited number of eggs, but only one hCG-treated female produced fertile eggs. Only a limited number of spermiating males were available for spawning trials. Hormone treatments used on females were ineffective for inducing or maintaining spermiation in male summer flounder. The summer flounder Paralicthys dentatus is an important species in commercial and recreational fisheries inhabiting continental shelf and estuarine waters from Nova Scotia to Florida (Gutherz 1967). The center of abundance lies between Cape Cod, Massachusetts and Cape Hatteras, North Carolina (Bigelow and Schroeder 1953; Grosslein and Azarovitz 1982). During the fall and winter, adults and older juveniles ’ Present address: Department of Fisheries, Animal and Veterinary Science, University of Rhode Island, Kingston, Rhode Island 0288 I USA. ’ Corresponding author. undertake annual, offshore migrations from coastal and estuarine water. Adults spawn at water temperatures that range from 12 to 19 C, during the offshore migration, beginning in September in the northern end of their range through February in waters off the North Carolina coast (Smith 1973; Morse 1981). Summer flounder have been over exploited in recent years. State fishery quotas for this species were implemented in 1993 after commercial and recreational landings in 1992 were 40% and 25% of total landings in 1979 and 1983 respectively when the fishery landings peaked (Terceiro 1993). In addition, the population structure has been compressed with regard to age, such that relatively few individuals are older than 3 yr. As a result, spawning flounder are likely younger and less fecund than in the past (Terceiro 1993). Poor recruitment has been evident throughout much of the range of summer flounder suggesting that factors other than fishing pressure may be impacting the population (Mid-Atlantic Fishery Management Council 1990, 199 1 a, 199 1 b). As a result of this population decline, interest in development of captive broodstocks and production of fingerling flatfish for aquaculture has increased. Development of reliable spawning and fingerling production methods must be accomplished before commercial culture of summer flounder can be undertaken. The method routinely used to induce ovulation and spawning in this species involves injections of carp pituitary extract (CPE) (Smigielski 1975). Based on their success0 Copyright by the World Aquaculture Society 1997