Gadomski and Caddell: Temperature effects on Paralichthys caltfomicus 



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contrast, areas north of Point Conception, California 

 commonly have extended periods of temperatures 

 below 12°C (SIO 1955-85, Parrish et al. 1981). Low 

 northern California ocean temperatures may possibly 

 limit the geographic range of larval halibut abundance, 

 although areas off northern California are also char- 

 acterized by strong offshore surface transport (due to 

 upwelling), which in itself limits survival of pelagic fish 

 eggs and larvae (Parrish et al. 1981). 



Growth and development rates of halibut eggs and 

 larvae in the laboratory increased with temperature in- 

 crease. We did not determine the upper temperature 

 tolerance limit of halibut larvae, but the highest tem- 

 perature tested, 24°C, exceeds normal southern Cali- 

 fornia ocean temperatures. Swift development of 

 pelagic eggs and larvae in the field may be advan- 

 tageous, since these stages are particularly vulnerable 

 to predation (Bailey 1981). Conversely, faster develop- 

 ment requires that more food be available, or starva- 

 tion may quickly occur; we found that newly-hatched 

 halibut larvae died sooner from starvation at higher 

 temperatures (Fig. 1A), since yolk absorption was 

 faster. Thus, warm ocean temperatures would increase 

 starvation-related mortality of larval halibut in the field 

 during periods with inadequate available food, while 

 cold temperatures might decrease mortality. 



Juvenile halibut had a higher temperature tolerance 

 range than halibut eggs and larvae; survival of 

 3-month-old halibut was significantly lower at 16°C 

 than at 20-28°C. Juvenile growth rates at 20, 24, and 

 28°C were directly proportional to temperature. Sur- 

 viving juveniles were similarly sized at 16°C and 20 C C 

 (Table 3), but this final size data are biased because fish 

 that died at 16°C were significantly smaller than those 

 that died at 20 °C. The less robust, slower-growing fish 

 at 16°C died, perhaps indicating that young juveniles, 

 like larvae (Laurence 1977, Houde and Schekter 1980), 

 require a minimum growth rate for survival that was 

 unattainable for some individuals at 16°C. 



The higher temperature range required for best sur- 

 vival of juvenile halibut reflects observed spatial and 

 temporal distribution patterns. High densities of newly- 

 settled larval and juvenile halibut have been collected 

 during spring and summer in southern California bays 

 where waters may be as warm as 24 °C (Allen 1988, 

 Kramer 1990); solar heating of very shallow waters 

 could result in even higher temperatures. Juveniles in 

 shallow areas of bays and estuaries may thus have the 

 advantage of enhanced growth and survival due to 

 warmer waters, in addition to other advantages, such 

 as increased food availability and protection. Larval 

 cohorts that settle and remain in open-coast areas do 

 not have these advantages, and could experience low 

 survival. As an example of this, in 1988 Kramer (1990, 

 1991) collected most newly-settled halibut juveniles in 



open-coast areas, whereas bays contained larger sized 

 juveniles; Kramer suggested that individuals that 

 settled on the open coast eventually moved into bays 

 or died. However, Kramer (1991) did not find a signif- 

 icant difference in growth rates of smaller juveniles 

 (<40mm standard length) from bay versus coastal 

 locations. 



The El Nino years of 1982 and 1983 may be an ex- 

 ample of warm ocean temperatures enhancing juvenile 

 halibut survival, both in open-coast areas and during 

 normally colder seasons. Higher densities of juvenile 

 halibut were collected in bays in 1983 (Allen 1988). Ad- 

 ditionally, strong fishery catches of halibut 1982 and 

 1983 year-classes have been reported (Miller 1990, 

 Pattison and McAllister 1990). 



The mechanism of larval or juvenile halibut entry into 

 bays, and the stage at which entry occurs, are un- 

 known. Kramer (1990) found newly-settled halibut 

 almost exclusively in southern California bays in 1987, 

 whereas in 1988 most settled in shallow open coast 

 areas and then possibly moved into bays. English sole 

 Parophrys vetulus have been reported to enter bays 

 both during and after metamorphosis (Misitano 1976, 

 Boehlert and Mundy 1987, Rogers et al. 1988). Plaice 

 Pleuronectes platessa settle in deeper waters and move 

 into shallow areas after metamorphosis (Lockwood 

 1974). For both English sole and plaice, tidal stream 

 transport is an important migration mechanism (van 

 der Veer and Bergman 1986, Boehlert and Mundy 

 1987). Other physical factors that have been suggested 

 to mediate shoreward migration of larval and juvenile 

 fishes are often associated with river discharge (salin- 

 ity; olfactory cues) (Creutzberg 1961, Boehlert and 

 Mundy 1987), and are thus not as applicable in drier 

 southern California, where rainfall is only 25-40 

 cm/year (Petersen et al. 1986). Temperature differ- 

 ences between near and offshore areas might be an im- 

 portant cue (Boehlert and Mundy 1988, Miller 1988), 

 although the ability of halibut larvae and young 

 juveniles to seek preferred temperatures has not been 

 demonstrated. 



High inshore densities of juvenile halibut are likely 

 a combination of passive larval transport to coastal 

 areas, followed by active inshore movement of larvae 

 or juveniles mediated by environmental stimuli. Shore- 

 ward transport of pelagic fish eggs and larvae to 

 coastal nursery grounds has been demonstrated to 

 enhance cohort survival (Nelson et al. 1976, Parrish et 

 al. 1981). Onshore and offshore surface water transport 

 in nearshore southern California areas is sporadic and 

 associated with late-spring and summer downwelling 

 and upwelling events (Winant 1980, Dorman and 

 Palmer 1981). Another possible mechanism of onshore 

 transport is surface slicks generated by tidally forced 

 internal waves (Kingsford and Choat 1986, Shanks 



