FISHERY BULLETIN: VOL. 73, NO. 2 



The response of these larvae to elevated tempera- 

 tures is therefore not a true reflection of thermal 

 "acclimation," as the term is generally used, but is 

 more a reflection of thermal stress during sensi- 

 tive periods of morphogenesis. In an analogous 

 way, salinity stress on embryos during acclimation 

 at 4(y/f» probably accounts for the observation 

 that the larvae have a reduced upper TLm for 

 salinity when compared with larvae acclimated to 

 30 and SBVoo. 



Thermal acclimation has also been shown to af- 

 fect the thermal tolerance of larval herring 

 (Blaxter 1960), menhaden (Lewis 1965), and sal- 

 monids (Bishai 1960; Iwai 1962), although only 

 Blaxter's study utilized larvae which hatched from 

 eggs maintained at the acclimation temperature. 

 The mechanisms involved in thermal acclimation 

 during early development have never been inves- 

 tigated, but the present results for bairdiella sug- 

 gest that they must be activated quite rapidly, 

 within a day or two at most. A similarly rapid rate 

 of acclimation to warm temperatures has been 

 found in older fish (Brett 1970; Allen and Strawn 

 1971), so that a similar mechanism may be operat- 

 ing in both cases. Factors involved in setting 

 thermal tolerance limits in fishes are little under- 

 stood (Fry 1967), but thermal inactivation of en- 

 zymes has been suggested as a possible mechanism 

 (Hochachka and Somero 1971). 



HoUiday and Blaxter (1960) found that the 

 salinity prior to hatching had a limited effect on 

 the salinity tolerance of larval herring. This effect 

 was more pronounced in the present experiments 

 with larval bairdiella, but there was a delay in the 

 appearance of the acclimation response to high 

 salinities. The upper TLm (salinity) was similar 

 for all acclimation salinities 24 h after initial ex- 

 posure to the test conditions, but at 48 h the larvae 

 acclimated to high salinities had a higher TLm 

 than those from low salinities (a very slight in- 

 dication of the same phenomenon can be discerned 

 in the results of Holliday and Blaxter 1960). This 

 observation is difficult to explain, especially in 

 view of the rudimentary state of our knowledge of 

 larval osmoregulatory mechanisms; perhaps it is 

 related to the opening of the mouth between 35 

 and 45 h after hatching (May 1974), which could 

 expose the internal larval tissues more directly to 

 the ambient salinity. Incubation at low salinities 

 enables larvae to tolerate much lower salinities 

 than larvae incubated in more saline water. Again, 

 it is difficult to speculate on how this effect might 

 be mediated. 



Early larvae of Bairdiella icistia are more 

 tolerant than the embryonic stages and less 

 tolerant than adults to extremes of temperature 

 and salinity. Very few bairdiella eggs develop 

 normally at 30°C (May 1975), and 15 to i(P/(x> is the 

 approximate salinity range for normal fertiliza- 

 tion and embryonic development. In contrast to 

 the eggs, 50% of the newly hatched larvae are 

 capable of withstanding temperatures between 

 30° and 33°C for 24 h or longer, except at the lowest 

 acclimation temperature and highest salinity; and 

 with proper acclimation, larvae can tolerate 

 salinities ranging from about 4 to iS'^/oo for 24 h, or 

 5 to 45^/00 for 72 h. Juvenile and adult bairdiella 

 must tolerate temperatures ranging from 10° to 

 34° or 35°C in the Salton Sea (Carpelan 1961). 

 These fish have been found in freshwater (R. G. 

 Hulquist, California Department of Fish and 

 Game, pers. commun.) and can tolerate Salton Sea 

 water with a salinity of 52.5''/oo for 96 h after 

 direct transfer from ordinary Salton Sea water 

 (approximately SS'^/oo), and 5S^/oo for over a week 

 after gradual acclimation (Hanson 1970). The 

 early larvae of some other species have also been 

 shown to be more tolerant of temperature and 

 salinity than their eggs. McCauley (1963) reports 

 that prolarvae of the sea lamprey, Petromyzon 

 marinus, are considerably more tolerant of high 

 temperatures than are the eggs, and data 

 presented by Holliday (1965) show that newly 

 hatched herring, Clupea harengus; plaice, 

 Pleuronectes platessa; and Atlantic cod, Gadus 

 morhua, larvae are more tolerant of both high and 

 low salinities than are their respective eggs. 

 However, in the case of the herring and plaice, 

 further larval development and metamorphosis 

 are accompanied by a decrease in salinity 

 tolerance (Holliday 1965), a pattern quite 

 different from that found in bairdiella. 



ACKNOWLEDGMENTS 



I thank Reuben Lasker for his advice and 

 material aid during this work. The University of 

 California Institute of Marine Resources and the 

 Southwest Fisheries Center, National Marine 

 Fisheries Service, NOAA provided financial sup- 

 port. 



LITERATURE CITED 



Allen, K. 0., and K. Strawn. 



1971. Rate of acclimation of juvenile channel catfish, Ic- 



254 



