INDUCTION OF SPAWNING IN 

 THE WEAKFISH, CYNOSC/ON REGALIS 



Reproductive activity in the weakfish, Cynoscion 

 regalis, is associated with extensive, north-south 

 migrations that result in spawning in the estuar- 

 ies of the Middle Atlantic Bight during the late 

 spring and early summer. Spawning is appar- 

 ently related to increasing water temperature 

 and day length, but there have been no experi- 

 mental investigations of specific factors that con- 

 trol this process in the weakfish. In contrast, gen- 

 eral aspects of the reproductive biology of the 

 species are well known (reviewed by Mercer 

 1983). Both males and females become sexually 

 mature at 1 year of age, and remain sexually 

 active throughout their lifespan (10^ years). 

 Spawning involves external fertilization of eggs 

 by pairs or small aggregations of fish. 



There has been limited study of larval develop- 

 ment in the laboratory, and descriptions of 

 growth and development of larval weakfish come 

 entirely from field investigations (Lippson and 

 Moran 1974). Weakfish larvae resulting from 

 gametes stripped from sexually mature adults 

 captured in the field have been reared for a few 

 days on natural zooplankton diets (Public Service 

 Electric and Gas Company 1984), but no informa- 

 tion is available on mortality or growth rates on 

 prescribed diets and rations. In contrast, Houde 

 and Taniguchi (1981) and Taniguchi (1981, 1982) 

 have conducted extensive investigations of the ef- 

 fects of diet, ration, and temperature on growth 

 and survival of larval spotted seatrout, Cynoscion 

 nebulosus, under laboratory conditions. Similar 

 studies have been conducted with other sciaenids 

 including red drum, Sciaenops ocellatus (Holt and 

 Arnold 1983; Holt et al. 1981), spot, Leiostomus 

 xanthurus (Powell and Gordy 1980), and 

 bairdiella, Bairdiella icistia (May 1974). 



The present paper describes a technique for in- 

 duction of spawning in a laboratory population of 

 weakfish and provides preliminary information 

 on early development and growth of weakfish lar- 

 vae. 



Methods 



Sixteen adult weakfish, C. regalis, (approxi- 

 mately 30-45 cm and 0.5-1.5 kg) were captured 

 in September 1984 in Delaware Bay by hook-and- 

 line. Five of these fish were dissected and found to 

 have regressed gonads. The remaining 11 fish 

 were placed in two large tanks (2,000 L) con- 



nected to a 20,000 L recirculating system that 

 delivered temperature-controlled seawater to 

 each tank at 10 L/minute. Water in the recircu- 

 lating system was replaced approximately 

 monthly. Ordinary white room light was provided 

 by two 1.25 m fluorescent lamps positioned 1 m 

 above the surface of the water in each tank. Ini- 

 tial conditions in the tank were similar to ambi- 

 ent conditions in Delaware Bay in September: 

 18°-19°C, 30%c, and 12 hours light:12 hours dark. 

 Temperature and salinity in the tanks were mea- 

 sured daily and pH approximately weekly. The 

 pH was maintained between 7.0 and 7.6 by 

 additions of new seawater to the system; this was 

 accomplished by replacing 40% of the water in the 

 system with new seawater approximately 

 monthly. 



Fish began to feed 5-7 days after capture. Diet 

 consisted of an ad libitum ration of sliced squid 

 with weekly additions of penaeid shrimp or fresh 

 calf liver. After approximately one month in the 

 laboratory, the fish were subjected to a prescribed 

 regimen of temperature and photophase (Fig. 1). 

 Temperatures were lowered and light phase 

 shortened over a period of three weeks until con- 

 ditions reached 8 hours light, 13°-14°C; this ap- 

 proximated winter conditions on the continental 

 shelf off Cape Hatteras where adult fish are 

 known to overwinter (Merriner 1976). Fish were 

 held under these conditions for 11 weeks after 

 which temperature was gradually raised and 

 light phase increased until spring conditions of 14 

 hours light and 22°-23°C were reached. Fish were 

 held at these conditions throughout a period of 

 extended spawning activity. After spawning ac- 

 tivity ceased, six newly captured fish were placed 

 in the system to replace fish that had died during 

 the previous year, and the process of gradually 

 changing temperature and photoperiod to winter 

 conditions was repeated (Fig. 1). 



Fertilized eggs were buoyant and exited the 

 tanks at the surface via stand pipes that emptied 

 into a sump tank. The drain was located at the 

 bottom of the sump tank allowing eggs to accu- 

 mulate at the water surface. The presence or ab- 

 sence of eggs in the sump was determined daily 

 with a fine-meshed dip net. After collection, eggs 

 from each spawning were allowed to hatch in 20 L 

 plastic aquaria filled with 5.0 ixm filtered sea- 

 water at 30%c and 23°C. Eggs exposed to gentle 

 aeration under these conditions hatched in 24-36 

 hours. 



Larvae were cultured in 2 L beakers filled with 

 filtered seawater (25°C, 30%f, and 5 mg/liter 



168 



FISHERY BULLETIN: VOL 86, NO. 1, 1988. 



