FISHERY BULLETIN: VOL. 78, NO. 1 



solids sp. gr. These compartments of the egg obvi- 

 ously would have no capacity for elimination of 

 salts. Egg dry weight declined by only 0.04 mg at 

 hatching as newly hatched 20%o larvae appear to 

 have even more yolk than 10%o larvae (Figure 8). 

 The 20%o curve in Figure 9 suggests that salt 

 elimination began very early and increased as the 

 embryo grew. It is probable that at least the early 

 salt elimination had a cellular rather than organ 

 basis. 



Eggs reared in 30%o salinity also had a water 

 content similar to those reared at 10 and 20%o, but 

 the salt load was high. Water accumulation dur- 

 ing the later developmental stages was low (ca. 0. 1 

 mg). The sp. gr. of egg solids goes down over the 

 first 27 d of development, indicating some elimina- 

 tion of salt. The pattern during the later stages of 

 development is strikingly different from that at 

 20%o in that the solids' sp. gr. again rose to about 

 1.37 at 37 d of incubation at which point the larva 

 hatched. Problems with salt balance and os- 

 moregulation may have led to the deformities and 

 early hatching. The dry weight of 30%o eggs de- 

 creased only slightly during development. 



It has been shown, for the eggs and larvae of 

 some marine organisms, that the salinity in which 

 fertilization and the earliest stage of development 

 occur may influence development and growth of 

 subsequent stages in the life history (Kinne 1962). 

 It is therefore possible that eggs fertilized in water 

 of higher salinity might have responded different- 

 ly to the various experimental salinities. Booth 

 (1967) obtained data suggesting that fertilization 

 could occur in salinities as high as 15%o. It is nota- 

 ble, however, that the eggs of Cyprinodon 

 macularius in Kinne's experiments were allowed 

 to develop 3-6 h in the spawning salinity, and at a 

 higher temperature (27° C) than was the case for 

 the tomcod experiments. It is probable that the 

 eggs of C. macularius had developed further be- 

 fore experimentation. 



The tomcod's early life history seems adapted to 

 the hydrodynamics of streams in which it spawois. 

 Spawning migrations occur in late December to 

 early January while water levels are still high 

 from the fall freshets. The eggs develop through- 

 out midwinter when water levels are low, thus 

 minimizing loss of eggs from the stream, then 

 hatch when water levels are rising coincident with 

 the early snow melt. The higher water levels dur- 

 ing hatching would ensure rapid flushing of larvae 

 into the estuary. Newly hatched larvae probably 

 rise to the stream surface soon after hatching and 



156 



ingest air into the swim bladder, with possibly the 

 positive response to light facilitating surfacing. 

 This behavior of newly hatched larvae would also 

 ensure rapid flushing into the estuary. 



The continuous drift of eggs out of the stream is 

 somewhat puzzling. Most eggs taken in the drift 

 samples were alive and apparently developing 

 normally. These perhaps are eggs which had been 

 deposited where they were likely to be taken up 

 into suspension. In support of this suggestion, egg 

 drift was positively correlated with stream level. 

 Whether these eggs continue to develop would de- 

 pend in part upon the salinity conditions where 

 they finally settle and the ambient salinity during 

 earlier development. Laboratory results indicate 

 that less than full salinities are required for nor- . 

 mal development from fertilization, but the eff"ects 

 of variable salinities on tomcod egg development 

 were not investigated. 



The tomcod egg resembled those of freshwater 

 species (rather than marine species) in regard to 

 salt tolerance, assuming that the responses re- 

 ported here are typical. Eggs of brook trout exhibit 

 increased mortality above 6%o salinity with total 

 mortality at 12%o (Sutterlin et al.^). Species such 

 as Abramis will hatch in salinities up to 20%o, 

 although 2.5-5%o is optimal (Holliday 1969). With 

 the tomcod, between 20 and 30%o salinity appears 

 to be the upper limit for production of normal 

 larvae. By way of contrast, eggs of several marine 

 species have been hatched in salinities up to 60%o 

 (cod, herring, plaice), although optima are usually 

 in the 25-30%o range (Holliday and Blaxter 1960; 

 Holliday 1965). 



Eggs of marine species tend to swell at low 

 salinities (usually <15%o); above this salinity egg 

 diameter is constant (Holliday 1965; Solemdal 

 1967). Tomcod eggs require salinities of <10%o for 

 noticeable swelling to occur. 



Several parameters measured (water content, 

 dry weight, solids' sp. gr., and egg sp. gr. for 10%o 

 incubation) begin to change dramatically at about 

 27 d of incubation. In relation to embryonic de- 

 velopment it seems probable the embryonic mass 

 is beginning to increase dramatically at this point, 

 resulting in the noted physiological changes. 

 Perhaps these changes are linked to the high mor- 

 tality occurring at this stage of development. 



^Sutterlin, A. M.,P. Harmon, andH. Barchard. 1976. The 

 culture of brook trout in salt water. Fish. Mar. Serv. Res. Dev. 

 Tech. Rep. 636, 21 p. Fisheries and Environmental Sciences, 

 Fisheries and Oceans Canada, Biological Station, St. Andrews, 

 NB EOG 2X0. 



