Optimal timing of initial exogenous feeding, as affected by temperature, in Chinook salmon (Oncorhynchus tshawytscha Walbaum)
Date
1979
Authors
Heming, Thomas Arthur
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Abstract
The relationship between timing of initial food presentation, alevin growth and early survival was examined in chinook salmon (Oncorhynchus tshawytscha Walbaum) at 6, 8, 10, and 12 °C. Feeding of test groups of alevins was begun in serial order from shortly after hatching to complete yolk absorption, in relation to progressive wet weight changes observed in unfed alevins. Experimental work at each temperature was terminated when cumulative mortality of unfed fish surpassed 50%. In addition, growth of chinook salmon reared unfed and in commercial incubation trays from hatching to complete yolk absorption, and emergence of alevins from simulated redds were investigated at each of the four temperatures.
Length and tissue weight of unfed alevins increased with time to maxima following hatching. Maximum tissue weight (MTW) and maximum alevin length were reached simultaneously, at 1125.1 tu after fertilization. Yolk reserves remaining at this time averaged 6% of those present at fertilization (dry weight basis). Thereafter, although yolk remained, unfed alevins began to catabolize tissue and tis sue we ight declined. Alevin length, however, remained unchanged with continued starvation. Over the four temperatures, alevins completed yolk absorption at 1456.1 tu after fertilization. The developmental index (kD) of unfed alevins at complete yolk absorption averaged 1.96. Generally, the efficiency of yolk utilization from fertilization to MTW decreased at higher temperatures, from 67.1% at 6 °C to 58,6% at 12 °C, Thus, more than a third of the yolk absorbed to MTW was utilized for purposes other than growth. Nevertheless, total wet weight (tissue plus yolk) of unfed alevins increased with time to a maximum alevin wet weight (MAWW) . This increase in total wet weight resulted from absorption of water. Total wet weight reached a maximum and began to decline before MTW was attained at each temperature. Thus, measurement of alevin wet weight was considered a poor indication of growth. Temperature influenced t he developmental stage at which MAWW was reached. MAWW occurred earlier in development at higher temperatures, and as a consequence, alevin size decreased and the yolk reserve remaining at MAWW increased from 6 to 12 °C. Yolk reserves remaining at MAWW increased from 9% at 6 °C to 29% at 12 °C (from fertilization, dry weight basis). Over the four temperatures, 50% emergence of alevins from simulated redds occurred within 26.8 tu of MAWW.
Chinook salmon began ingesting external food while retaining 51% of the yolk reserve present at fertilization. Feeding was not dependent on attainment of neutral buoyancy (swim-up); alevins were capable of exogenous feeding before inflating their air bladders. However feeding did not significantly affect growth, when compared with that of unfed alevins, until shortly before MAWW was attained. Alevins presented food initially before attaining MA WW did not exhibit a maximum wet weight; their total wet weight continued to increase while that of unfed fish reached MAWW and then declined. The ultimate size achieved and total survival of feeding alevins were maximized when feeding was initiated before MAWW occurred, regardless of the specific timing of initial food presentation. The ultimate size achieved was reduced when feeding was initiated at or after alevins attained MAWW. Survival was reduced when initial feeding was delayed until alevins attained MTW. Exogenous feeding had no effect on yolk absorption. As well, timing of initial food presentation had no effect on the subsequent growth rate of feeding alevins.
The results establish that chinook salmon are capable of utilizing exogenous food to supplement their endogenous reserves. This "mixed feeding" primarily determined subsequent growth and survival. Natural populations of chinook probably utilize external food prior to emerging from the gravel redd. In fish culture, the optimal time to commence feeding of chinook salmon is before alevins attain MAWW. Occurrence of MAWW can be regarded as a preliminary symptom of starvation, indicating that a greater ultimate size could have been achieved by initiating feeding earlier in development. Estimates of the optimal time (tu after fertilization) to commence feeding at each temperature are given; initiating feeding of chinook salmon at or before these optimal times should maximize subsequent growth and survival. These optimal times were found not to be related to swim-up, but ton-up (disappearance of the visible yolk sac), MAWW, MTW, or complete yolk absorption.