Field observations suggest early sea mortality of pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon fry is largely due to predation by juvenile coho (O. kisutch) salmon. A series of experiments demonstrates a strong bias toward the smaller individuals of the prey population. This results in an apparent growth rate 0.3–0.5% per day due to the biased mortality alone. With a high innate growth rate [Formula: see text], the prey are shown to "outgrow" the predator [Formula: see text], and hence become unavailable. Chums are shown to have an advantage over pinks through slightly earlier entry into the estuary and a larger initial size. The mechanism of selection used by the predator is not known from this study.
The hypothesis that natural mortality rates of pink salmon during early life are generally much larger than during the later period has been tested for 3 brood-years of the Bella Coola River stock using a two-stage marking technique. Average daily losses to the population during the first 40 days are estimated to vary between 2 and 4%, and for the later 410-day period between 0.4 and 0.8%. These rates produce losses amounting to between 59 and 77% of the initial population during the first 40 days. Of the population surviving at 40 days, further losses of between 78 and 95% occurred. The latter losses are considered to be maximum estimates because of bias introduced by catches of unknown magnitude. Thus, although the early mortality is exceeded by the later, the time period is approximately 10 times as long, and the intensity of the mortality rate is much higher during early sea life.
Sockeye smolts, pink and chum fry, and pink and chum fingerlings (Oncorhynchus) were sequentially weighed and measured when alive and after death in water and after killing and storage in formalin up to 225 days. The fish shrank within 12 hours to 97% and by 30–40 days to 96% of live length. Further changes in length were not significant. These same relative changes were observed from fish of different sizes, and the values were not significantly different among groups preserved in formaldehyde solutions of fresh or sea water. In fresh water, fish gained weight while yet alive but under anaesthesia. In freshwater formalin, fish gained weight rapidly for 1 or 2 days, then lost weight at a decelerating rate to the time of last measurement. Fish killed and stored in salt water formalin lost weight for the first few days, then gained weight at a decelerating rate. Relative magnitudes and rates of change were inversely related to size of specimens contained in the sample. These changes had pronounced effects on relative condition factors which varied from 97 to 135% of live values depending upon size of fish, type of formalin and time in preservative.
The use of size as a basic determinant of growth rate is discussed and compared to analogous situations described in literature on other physiological rates. Ecological opportunity and physiological opportunity are visualized as the two interacting components that determine growth, both of which are entered at "threshold" sizes. The parabolic function,[Formula: see text]is developed into a growth equation for linear dimensions and its application explored and discussed using steelhead trout and chinook salmon as an example. Significant differences in growth rate are found between life history types and sexes. The chinook data are then treated on a lt+1, lt plot and it is shown how an apparent fit of the von Bertalanffy type growth equation can result from selectivity of fishing for the larger fish of any brood year. Two lines of research are indicated. (1) The independent measurement of the exponent x through study of size-specific metabolic rates, and (2) the relationship between total mortality rate and growth rate. Without this knowledge a satisfactory synthesis of growth rate into a yield equation cannot be achieved.
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