The euryplastic Pacific herring (Clupea harengus pallasi) generally encounters temperatures ranging between 0 and 10 °C throughout its distribution during the maturation and spawning of adults, incubation of eggs, and hatching of larvae. For many Asian stocks these events occur in the lower half of the temperature range; with North American stocks they tend to occur in the upper half of the range. In British Columbia waters, salinities associated with these events (range, optimum) are spawning (2.6–28.7‰, 27–28.7‰), [Formula: see text] fertilization of eggs (4.5–42‰, 12–15‰), and maximum total hatch and hatch of viable larvae (4.5–42‰, 12–17‰). A low/low–high/high interaction between salinity and temperature also influences total hatch, hatch of viable larvae, and salinity tolerance of larvae. In addition, the following implications arise regarding aspects of the Pacific herring reproductive cycle, based on previously published and new data, and on speculative inference. The response of Pacific herring to salinity and temperature appears to have a commanding influence on the reproductive cycle and, thereby, on distribution of the species. Survival of eggs on substrate, related to respiratory activity, appears to be influenced by the transport and perfusion velocity of interstitial water in an egg mass. Such transport may involve perivitelline fluid colloid osmotic pressure; natural convection; the surge associated with wave action, beach slope, and depth; and possibly differences in resistance to convective flow of deoxygenated water from an egg mass based on orientation of the substrate. These relations would be modified by variations in deposition intensity (number of egg layers) and packing density (eggs per unit volume), and both factors may affect survival of occluded eggs in an egg mass differentially, depending on the substrate used. A review of data on salinity tolerance of herring larvae indicates that a variety of dosage-mortality techiques has been used, leading to noncomparable estimates of response. An assessment of upper incipient lethal salinities will require standardization of such techniques. Recent studies show that salinity tolerance of larvae is influenced significantly by salinity–temperature conditions during egg incubation. At usual incubation conditions in British Columbia waters, the upper boundary of larval tolerance is estimated as 27.5–31.7‰ S (72-h LC10). depending on incubation history. The fate of Pacific herring larvae carried into the higher salinities of offshore waters has been controversial. In the Strait of Georgia, British Columbia, substantial offshore dispersion of larvae occurs where surface conditions generally are 27–28.6‰ and 9–10 °C in the early larval period. Although these salinities are near the upper boundary of salinity tolerance, larvae sampled in offshore waters (1981) had an apparent mean age of 15 d and were actively feeding and growing. From rates of disappearance of larvae in the offshore waters (9% wk) and inshore waters (45% wk) we conclude that usual surface salinities and food supply in the open waters of the Strait were not a dominant influence on larval survival. Assuming the larvae remain in the upper 10 m, we suspect their disappearance, at least offshore, to be largely the result of predation.
Advection, diffusion and mortality of Pacific herring larvae Clupea harengus pallasi in Bamfield Inlet, British Columbia
