572 



Fishery Bulletin 89(4), 1991 



[dorsal ray elongation is characteristic of larvae of 

 many pleuronectiform fishes; rays regress during 

 metamorphosis (Ahlstrom et al. 1984, Gadomski et al. 

 1990)], and no eye migration was evident. At experi- 

 mental end, 53-day-old larvae at 16°C had a mean 

 length of 8.75mm (SE 0.16, n 38), while at 20°C, mean 

 larval length was 12.88mm (SE 0.23, n 40). All fish at 

 both temperatures had achieved standard length and 

 had fully migrated eyes. Those at 16°C were somewhat 

 less developed, however, since 8% still had slightly 

 elongated dorsal rays. The remainder at 16°C and all 

 fish at 20°C exhibited normal length dorsal rays 

 characteristic of juveniles. 



Discussion 



Temperature tolerance experiments have been con- 

 ducted on eggs and larvae of many fish species (Houde 

 1974, Laurence and Rogers 1976, Laurence 1978, Fer- 

 raro 1980, Bolla and Holmefjord 1988). The tempera- 

 ture tolerance range of California halibut eggs in the 

 current study was at least 8 degrees (12-20°C), and 

 probably more if the range endpoints fell between 

 8- 12 °C and 20-24 °C. This 8-degree range is typical 

 of other flatfish species such as English sole Parophrys 

 vetulus, sole Solea solea, and winter flounder Pseudo- 

 pleuronectes americanus (Alderdice and Forrester 

 1968, Irvin 1974, Fonds 1979, Buckley 1982). Absolute 

 temperatures tolerated varied among species, however, 

 and were reflective of field habitat temperatures; sole 

 S. solea eggs survived slightly lower temperature ex- 

 posures than halibut (8-16°C), whereas English sole 

 and winter flounder eggs (found in colder waters) 

 tolerated much lower temperatures, 4-12°C and 

 2-10°C, respectively. 



Temperature tolerance ranges may change with 

 ontogeny. Halibut larvae exhibited a higher tempera- 

 ture tolerance range (16-24°C) than halibut eggs, a pat- 

 tern which has also been demonstrated for sole S. solea 

 (Irvin 1974, Fonds 1979) and yellow perch Percafla- 

 vescens (Hokanson and Kleiner 1974). Additionally, sole 

 (Devauchelle et al. 1987) and yellow perch (Hokanson 

 and Kleiner 1974) temperature tolerances were higher 

 for late stage eggs than for eggs at early developmen- 

 tal stages. Similarly in our study, larval halibut ex- 

 periments initiated with late-stage eggs resulted in a 

 higher range of temperature tolerance than experi- 

 ments initiated with recently fertilized eggs. As larval 

 halibut development progressed, temperature toler- 

 ance ranges also increased; soon after hatching, the 

 highest mortality rate occurred at 24°C, whereas mor- 

 tality of older larvae was greater at 12°C and 16°C 

 (Fig. IB). 



At low temperatures, biochemical reactions, and thus 

 metabolic rates and growth, of poikilotherms are re- 

 duced (Laurence 1975). For all halibut early-life-history 

 stages, the lowest tested temperatures adversely af- 

 fected growth and survival. Larval halibut may have 

 a minimum necessary growth rate for survival, as sug- 

 gested by Gadomski and Petersen (1988), and reported 

 for larvae of other fish species (Jones 1973, Beyer and 

 Laurence 1980, Govoni et al. 1986). At 8°C, develop- 

 ment of halibut larvae halted soon after hatching when 

 larvae still had significant amounts of yolk, unpig- 

 mented eyes, and nonfunctional mouths. Ehrlich and 

 Muszynski (1982) found that at low temperatures, yolk- 

 sac California grunion Leuresthes tenuis encountered 

 problems with fat metabolism. At 12 °C, survival of fed 

 halibut larvae initially was high and ingestion was 

 observed, but by 17 days after hatching almost all 

 larvae were dead. This type of survival pattern was also 

 reported by Laurence (1975) for winter flounder 

 Pseudopleuronectes americanus, which survived in the 

 laboratory at 2°C for 5 weeks with a very slow develop- 

 ment rate, and then died before metamorphosis; larvae 

 at higher temperatures of 5 and 8°C developed nor- 

 mally. Similarly, Laurence (1978) found that larval cod 

 Gadus morhua and haddock Melanogrammus aegle- 

 finus could only survive for limited periods after 

 hatching at lower tested temperatures, with variable 

 and elevated oxygen consumptions indicating physio- 

 logical stress. 



The temperature tolerance ranges of halibut eggs 

 and larvae in the laboratory approximate temperatures 

 these stages usually encounter in the field; deviations 

 from normal sea temperatures could affect survival. 

 During winter and spring, nearshore (l-20km from 

 shore) sea surface temperatures off southern Califor- 

 nia usually range between 13 and 17 C C (Petersen et 

 al. 1986). Vertical mixing during the winter season 

 generally produces uniform temperatures throughout 

 the water column. In late-spring and summer, surface 

 waters warm as high as 20-22°C, and a thermocline 

 develops (Petersen et al. 1986). Late-spring and sum- 

 mer upwelling events may disrupt this temperature 

 stratification, however, transporting deep colder 

 waters to the surface (Dorman and Palmer 1981). Our 

 study has shown that halibut larvae can endure 12°C 

 for a short period during the first 3 weeks after hatch- 

 ing, but years with prolonged periods of this temper- 

 ature might result in high larval mortality. Since 1920, 

 there have been only three instances when surface 

 temperatures at the Scripps Pier off San Diego re- 

 mained below 12.5°C for as long as 2 weeks, and these 

 were all in the time period of December-February 

 (E. Stewart, Scripps Inst. Oceanogr., La Jolla, CA). 

 Temperatures considered lethal for halibut eggs and 

 larvae (<8°C) were never recorded at this site. In 



