Sustained, natural spawning of southern flounder Paralichthys lethostigma under an extended photothermal regime

Hormone-induced spawning of southern flounder Paralichthys lethostigma has produced substantial numbers of viable eggs, but wide variations in fertili zation and hatch rates have been reported. Recently, sustained natural spaw ning of southern flounder broodstock, without hormone induction, has been a chieved in our laboratory. Adults (average weight = 1.12 kg; N = 25), inclu ding 6 captured as juveniles in 1993 and 19 captured as adults during Septe mber 1998, were stocked in two 4.8-m(3) controlled-environment tanks in Oct ober 1998 and held under natural photothermal conditions until January 1999 , when an artificial winter photo-period of 10 L:14 D was initiated and the n maintained through April 1999. Sex ratio was approximately 13 females:g m ales:7 unknown. Natural spawning was observed in early December 1998 and in creased in frequency to a peak in March 1999, before declining in late Apri l. Water temperature ranged from 13.9 to 24.5 C during the spawning period, Natural spawnings over 142 d produced a total of 18.3 x 10(6) eggs, with a mean fertilization rate of 28.0% (range = 0-100%), yielding 4.94 x 10(6) f ertilized eggs. The mean percentage of eggs that remained buoyant in full-s trength seawater (34 ppt) was 41.3% (0-98%), while hatching rate of buoyant eggs was 37.3% (0-99%) and survival of yolksac larvae to the first-feeding stage was 30.2% (0-100%). Gonadal biopsies in late April identified six fe males from both tanks as probable spawners. A preliminary comparison sugges ts that natural spawning produced much larger numbers of viable eggs per fe male, with higher egg quality (i.e,, fertilization and hatching success) th an hormone-induced spawning. In contrast to natural spawning, hormone-induc ed strip-spawning enabled timing of spawnings to be more precisely controll ed. These results suggest that a combination of both natural and hormone-in duced spawning of photothermally conditioned fish will help produce the lar ge numbers of eggs required to support commercial production.