MAY: EFFECTS ON BAIRDIELLA ICISTIA 



lowest salinities (0.3-0.5%o), as was true for bair- 

 diella; however, in herring the duration of sper- 

 matozoan activity was much longer (4-8 days at 

 6-7°C) and some fertilization occurred even in 

 fresh water. Spermatozoan activity and its re- 

 sponse to salinity appear to be extremely variable 

 among fish species, which is hardly surprising in 

 view of the diversity of habitats and modes of 

 reproduction of fishes. 



The large proportion of postmature unhatched 

 eggs at low salinities (Table 6) reflects a high 

 incidence of malformations under these condi- 

 tions, the embryos being physically unable to 

 break from the chorion. Edema seen among larvae 

 in low salinities suggests that deformities and the 

 inability to hatch may be related to osmotic prob- 

 lems. Battle (1929) noted a similar difficulty in 

 hatching among embryos of fourbeard rockling, 

 Enchelyopus cimbrius, in low salinities and attrib- 

 uted it to abnormally developed musculature, 

 which prevented movements required to free the 

 embryo from the egg case. An inability to complete 

 hatching at low salinities has been reported for 

 other species as well (Ford 1929; McMynn and 

 Hoar 1953; Alderdice and Forrester 1967; Dush- 

 kina 1973). The generalization that gastrulation 

 and hatching are the two developmental stages 

 most sensitive to physical disturbance (e.g., Holli- 

 day 1969) seems valid in the case of bairdiella. 



The finding that unfed bairdiella survive 

 longest in low salinities and low temperatures is 

 not unique. Nakai ( 1962) and Hempel and Blaxter 

 (1963) likewise found that starving larvae of 

 Sardinops melanosticta and Clupea harengus 

 survived longer at lower salinities, and more rapid 

 mortality among unfed larvae at higher tempera- 

 tures has been observed on a number of occasions 

 (e.g., Qasim 1959; Bishai 1960; Hempel and Blax- 

 ter 1963; Alderdice and Velsen 1971; Hamai et al. 

 1971). High temperatures increase metabolic 

 rate, accelerate yolk absorption (May 1972), and 

 no doubt hasten death from starvation. The effect 

 of high salinities on larval physiology is less cer- 

 tain: a salinity effect on embryonic or larval ox- 

 ygen consumption has not been demonstrated ex- 

 cept after abrupt transfer (HoUiday 1969), and 

 salinity has only a small effect on the rate of yolk 

 absorption in bairdiella (May 1972). High 

 salinities may increase larval mortality by caus- 

 ing osmotic or ionic changes in the interior milieu, 

 although the larvae of some species have proved 

 capable of osmoregulating over rather wide 

 ranges of salinities (Holliday 1969). Lower levels 



of activity have been observed among larvae of 

 some species in low salinities (Hempel and Blax- 

 ter 1963; Holliday 1965), and may reduce their 

 metabolic demand and thus extend their survival 

 time (Holliday 1965). 



The salinity tolerance of bairdiella eggs is not 

 significantly affected by acclimation of the parent 

 fish to low salinity (15%o). This might suggest 

 that the enhanced survival at low salinities which 

 Solemdal ( 1967) observed in eggs from the Finnish 

 population of flounder, Pleuronectes flesus, has a 

 genetic basis. If acclimation of spawning fish to 

 low salinities does not cause an increase in em- 

 bryonic tolerance to low salinities, one might ex- 

 pect that high-salinity acclimation would be simi- 

 larly ineffectual in aiding embryonic survival at 

 high salinities. This supposition should be verified 

 experimentally; but, if valid, it implies that salin- 

 ity responses determined on eggs from fish living 

 in ordinary seawater should be accurate predic- 

 tors of reactions to different salinities in nature, 

 except where genetic adaptation has occurred. 

 This could be a significant advantage in cases 

 where it is important to estimate the effects of 

 rising salinities in specified habitats, such as the 

 Salton Sea or the Gulf of California, where high 

 salinities may in the future pose a threat to exist- 

 ing stocks of fish. 



Because of the unusual chemical nature of the 

 Salton Sea, it is impossible to estimate the salinity 

 tolerance of bairdiella eggs in Salton Sea water 

 from the present data concerning their responses 

 in ordinary seawater. There is evidence that the 

 ionic composition of Salton Sea water has a del- 

 eterious effect on the survival of eggs and larvae 

 (Lasker et al. 1972; May 1972), so that the upper 

 salinity limits defined in the present study are 

 probably higher than those which hold for bair- 

 diella in the Salton Sea. 



The spawning season of bairdiella occurs dur- 

 ing a period of rapidly rising temperatures. In the 

 Salton Sea this species spawns mainly in April 

 and May, with a peak of spawning probably in 

 mid-May (Whitney 1961; Haydock 1971). Max- 

 imum surface temperatures in the Salton Sea are 

 plotted in Figure 15, where the spawning time of 

 bairdiella is also shown. It is clear that some bair- 

 diella may spawn in water of 30°C or higher, al- 

 though most spawning is probably finished before 

 temperatures reach this level. Whitney (1961) re- 

 ports finding bairdiella eggs in 1955 as late as 1 

 August, which means they could have been ex- 

 posed to the undoubtedly lethal temperature of 



19 



