Feeding and bioenergetics of Chinook Salmon during the first winter at sea

Date

2024-01-02

Authors

Innes, Katie

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Abstract

It is hypothesized that winter is a period of nutritional stress and elevated mortality for juvenile Chinook Salmon (Oncorhynchus tshawytscha). However, little is known about the winter ecology of this species. To address this gap in knowledge, first ocean winter Chinook Salmon were sampled systematically by microtrolling (hook-and-line capture) over three consecutive winters from late September to early April in 2020-2023 in the Strait of Georgia, British Columbia. Sampling regions included the Discovery Islands, Northern Strait of Georgia, and Southern Gulf Islands. Chinook Salmon were weighed and measured, and scales were collected for genetic stock identification. Winter diet samples were collected by gastric lavage and intact prey were preserved for energy density determination. By mass, Chinook Salmon consumed primarily Pacific Herring (Clupea pallasii), euphausiids, squid, and Primno spp. amphipods, and diets differed by region and season with some interannual variability observed. Both diet energy content and body condition declined throughout the winter, although the decline in diet energy content was not significant. The presence of Pacific Herring in the diets had a significant positive effect on diet energy content. A subset of sampled Chinook Salmon was also retained for energy density and organosomatic index analyses, both of which had significant seasonal trends that may be associated with allocating energy to storage in autumn and reduced rations in mid-winter. Using field-derived data as inputs, I developed bioenergetics models to estimate differences in overwinter growth, consumption, and feeding rates over two years and between two regions in the Strait of Georgia. These inputs included diet composition, prey and predator energy density, temperature at depth of capture, and predator weight. Regional and interannual differences in model estimates were observed, and January and February were consistently estimated to be periods of reduced consumption rates. Bioenergetic model estimates also provided possible evidence of the occurrence of size-selective processes, although this result should be interpreted with caution. The models based on our longitudinal sampling framework were then compared to ‘seasonal’ models which mimicked a field sampling design wherein discrete sampling events occurred once prior to winter and once following winter to highlight the temporal variability in fish bioenergetics which may be missed using a seasonal approach. Overall, the data presented in this thesis suggest that some degree of food limitation occurs during winter but does not provide strong evidence that supports the plausibility of winter as a period of nutritional stress for overwintering juvenile Chinook Salmon.

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Keywords

Chinook Salmon, Winter, Ecology, Diet, Strait of Georgia, Bioenergetics, Feeding, Energy density, Juvenile

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