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Pacific herring (Clupea pallasi) eggs fertilized in 20‰S and incubated in 5, 20 or 35‰ S at 5°C, some being cross-transferred between 5 and 35‰ S, 61.8 hr after fertilization, showed variable yolk and perivitelline fluid (PVF) osmolalities until beginning of epiboly. Coincident with blastopore closure and for a period of ca. 150 hr thereafter (period of stability), both yolk and PVF osmoconcentrations were relatively constant. Thereafter osmolalities rose slowly to asymptotic levels prior to hatching. Osmolal values in the period of relative stability (100–250 hr) were approximately (a) yolk: 285–310 (5‰ S), 350 (20‰ S), and 390 mOsm (35‰ S); (b) perivitelline fluid: 105–118 (5‰ S), 370 (20‰ S), and 530–670 mOsm (35‰ S). Prior to hatching, these were (a) yolk: 330–350 (5‰ S), 400 (20‰ S), and 460–480 mOsm (35‰ S); (b) perivitelline fluid: 175–210 (5‰ S), 530 (20‰ S), and 850–860 mOsm (35‰ S). Yolk osmolalities decreased, after hatching of the larvae, to approximate those levels attained between 100 and 250 hr. An hypothesis is presented whereby minimum osmotic work is defined on the basis of isosmotic relations existing between yolk, perivitelline fluid, and incubation medium. This leads to the description of a range of salinities defined in terms of osmoregulatory scope. At the 5°C test temperature, incubation salinities associated with maximum osmoregulatory scope (13.2–19‰) correspond closely to the range of salinities providing maximum or near-maximum hatches of viable larvae. The presence of a salinity-temperature interaction and its inlfuence on the association between osmoregulatory scope and production of viable larvae is suggested as a mechanism underlying and providing the recognized plasticity of Pacific herring egg development. In a companion study involving cadmium as a contaminant, eggs incubated at 20‰ S were exposed to 0, 1 or 10 ppm Cd in 5, 20 or 35‰ S for 48-hr periods at six stages of development between fertilization and hatching. Cd exposure resulted in a reduction in osmolality of perivitelline fluid. Reduction was greater in higher Cd concentrations and in eggs transferred to higher salinities. An hypothesis is presented to explain observed effects of Cd exposure on some physical properties of teleost eggs, including reductions in perivitelline fluid osmolality and egg volume. Finally, newly hatched (0-, 3-, 6- and 9-day old) larvae from eggs incubated at 20‰ S (8.5°C) were exposed to salinities from 0 to 50‰ for a period of 72 hr. Median tolerance limits (72-hr) were 2.8–5.2‰ for low salinities and 33–35.8‰ for high salinities. No trends in salinity tolerance were noted amongst the four ages of larvae. The data suggest that natural larvae entrained in sea water would be susceptible to salinity-induced mortality in salinities greater than about 20‰ However, this conclusion is subject to confirmation in view of the low/low-high/high salinity-temperature interaction previously noted in relation to egg development, and its possible continuing influence in larval stages.

Osmotic responses of eggs and larvae of the Pacific herring to salinity and cadmium