With data assembled from the literature, relations are examined between incubation temperature and rate of development from fertilization to hatching for chinook salmon (Oncorhynchus tshawytscha) eggs. Ten forms of three empirical relations are used, based on the thermal sums hypothesis, Bělehrádek's equation, and a form of the logistic curve. In each case comparisons are made using constant, ambient, and combined (constant + ambient) incubation temperatures. In most cases the rules of minimum variance curve fitting are somewhat violated, although results using the log-inverse form of Bělehrádek's equation are superior to those from the other models. Early imposition of low, constant incubation temperatures (below 6–7 °C) appears to slow egg development below those rates occurring at ambient temperatures having the same mean values. Time–temperature relations based on the unmodified thermal sums hypothesis and its associated "degree-days" are compared with predicted values obtained using the log-inverse Bělehrádek model as a standard. The former model tends to underestimate the length of the incubation period by an average of about 5 days at incubation temperatures above 4 °C; below 4 °C the former model increasingly overestimates the incubation period. Based on the log-inverse Bělehrádek model, a table is provided of predicted daily rates of development in relation to temperature. The table may be used to predict hatching time in a manner similar to that employing degree-days. In general, the enquiry points to a lack of data on development time at temperatures below 5 °C; to be meaningful, further analyses will require such data. Key words: prediction, modeling, development rates, hatching time
Pacific herring (Clupea pallasi) eggs were incubated in the laboratory at 13 different salinity–temperature combinations (ranges: 5–45‰ S, 4–14 C). Data were obtained on rate of development, larval length at hatching, percent total hatch, and percent viable hatch. Subsamples of larvae, held in their incubation media, provided data on posthatching survival and larval size at yolk sac absorption. Maximum percent viable hatch, larval size at hatching, and maximum survival to yolk sac absorption occurred in a trial at 16.41‰ S, 6.85 C. Optimum conditions were calculated by response surface analysis — maximum total hatch: 12.01‰ S, 6.46 C; maximum viable hatch: 16.98‰ S, 8.33 C; maximum length of newly hatched larvae: 20.00‰ S, 5.54 C. Maximum percent viable hatch of largest larvae in the shortest incubation period is estimated to occur at or near 16.98‰ S, 8.7 C. Abnormalities of the lower jaw were present in larvae from eggs incubated at temperatures of 4.0 and 4.71 C. Pacific herring eggs are considered euryhaline and stenothermal. A comparison of laboratory and field observations suggests that occurrence and abundance of herring is related to the availability and extent of spawning salinities between 8 and 28‰. Moreover, catch statistics suggest that population abundance in North American waters is associated with spawning temperatures of 5–9 C, abundance is limited by temperatures of 9–10 C, and the maximum temperature for spawning is about 10 C. From the current study, survival of viable larvae to yolk sac absorption would place the lower limit of thermal tolerance of Pacific herring eggs between 4 and 5 C.
A method is described whereby arrays of samples of Clupea pallasi eggs may be stored during their preparation. The high fertilization potential retained by the eggs during short-term storage allows them to be fertilized synchronously when sample preparation is complete. A variation of the "dry" method of storage retained maximum fertilization potential (80-85 ~ of the eggs for about 1 hr, and of milt dilution (1:8 with 17 %o S sea water), about 7 hr. Following dry storage, eggs fertilized in salinities of 0-45 %0 showed maximum rates of fertilization in salinities of 10-20 %0, and fertilization rates .> 50 0/o in salinities of 4.5-42 %0. Salinities of fertilization influenced egg diameter, median hatching time, and larval length at hatching in egg samples transferred 21/2 hr aflcer fertilization to an incubation salinity of 17 %0 at 7 ~ Fertilization rates were higher (90-95 ~ for eggs stored in 17 % S at 7 ~ prior to fertilization. Under such "wet" storage conditions, maximum fertilization potential was retained for about 2 hr. Highest fertilization rates (95-96 ~ were obtained for eggs stored and fertilized in salinities of 12-15 %0. For the species and the area of origin considered (British Columbia), wet storage of eggs should result in maximum fertilization when the eggs are stored at 4 ~ for a period not greater than 2 hr prior to fertilization in the 12-15 %0 S storage medium.
Changes in total volume and volume of the yolk and perivitelline space of Pacific herring eggs were examined throughout incubation at 5 ~ in relation to salinity of the incubation medium (5, 20, 35 %o S), and al%r exposure to cadmium (0.05-10 ppm Cd) at 20%o S. Alter fertilization and filling of the perivitelline space there was a decline in total egg volume in all salinities until 60-80 hr after fertilization. There followed a period of relative stability of total volume (100-240 hr), then a slow decline until hatching (240-619 hr). There was an inverse relation between egg volume and salinity at all stages of egg development. Eggs transferred from 20 %o to 5 or 35 %o S, 87.4 hr after fertilization (90 ~ blastodermal overgrowth of the yolk), showed only minor changes in total egg volume within the period of relative stability (100-240 hr). Prior to 80 hr, changes in egg volume appeared primarily to be simple adjustments to prevailing osmotic and ionic conditions, modified, however, by presumed irreversible changes induced in the egg in relation to salinity experience at, and shortly after, fertilization. Subsequently, between 80-100 hr, egg volume appears to become regulated, commencing in the interval between late blastodermal overgrowth and blastopore closure. Yolk volume declined aflcer fertilization, reached a minimum 40-60 hr aiter fertilization, increased to 100 hr, then decreased in the period of relative stability of total volume-presumably in relation to rapid growth of the embryo. In the latter period, yolk volume appeared resistant to change when eggs are transferred from 20 ~ to 5 or 35 ~ S, 87.4 hr after fertilization. Volume of the perivitelline space reached a maximum after fertilization, then decreased until about 100 hr; between 100 and 240 hr it increased rapidly and was influenced only in a minor way by salinity changes in the incubation medium 87.4 hr after fertilization. Eggs exposed to cadmium in the interval between 1/~ and 30 hr after fertilization showed major reductions in total egg volume; total volume in the period of relative stability (100-240 hr) was much reduced and normal volume was not recovered after removal of such eggs to uncontaminated water at 30 hr. * Prepared under the auspices of the Canadian-German Scientific and Technical Cooperation Agreement (Contribution No. 7).
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