| Summary: | Thesis (Ph.D.)--Memorial University of Newfoundland, 2011. Biology Includes bibliographical references. Over the last 30 years, lipids have been used to study patterns of energy flow and food web dynamics in cold and temperate marine ecosystems. I used fatty acids (FAs) and lipid classes to better understand the early survival, nutrition, habitat use and growth of age-0 Atlantic and Pacific gadids, both through laboratory and field approaches. Firstly, I used lipids to examine condition of age-0 juvenile Atlantic cod (Gadus morhua) as they settled in eelgrass (Zostera marina) nursery habitat. Cod reduced lipids per wet weight at the time of settlement, indicating that energy was directed towards rapid growth rather than storage for overwintering. Secondly, I used both FA biomarkers (FABMs) and compound specific carbon isotopes of FAs to show that increased terrestrial carbon entered the diet of juvenile cod during settlement. Higher dietary short chain polyunsaturated FAs (PUFA), coupled with low proportions of dietary essential FAs (EFAs) in the nearshore foodweb, indicated that the functional significance of eelgrass was refuge, and not elevated nutritional food quality. Thirdly, I conducted a laboratory experiment on the nutritional requirements of Pacific cod (Gadus macrocephalus) larvae to compare with literature values for Atlantic species. This comparative approach indicated that trends in larval nutrition could not be generalized across the family Gadidae. Pacific cod larvae grew fastest with diets containing high levels of n-3 PUFA, similarly to Atlantic species. Unlike Atlantic cod, however, Pacific cod larvae did not show elevated growth and survival with higher dietary proportions of DHA (22:6n-3) relative to EPA (20:5n-3). Fourthly, I investigated the rate of uptake of two C18 PUFAs in tissues of two species of Pacific juvenile gadids (Pacific cod, Gadus macrocephalus and walleye Pollock, Theragra chalcogramma). I examined how both biotic and abiotic factors influenced the rate of uptake in liver, flesh and heart tissues. C18 PUFAs showed high temporal sensitivity, and were evident in fish tissues after only one week of feeding. The differential uptake of these FABMs among tissues could represent a new tool to disentangle timing of offshore-inshore nursery migrations in juvenile fish. Throughout, this thesis I have shown that lipid classes, FABMs and EFAs can be successfully used to indicate critical trophic events during the early life history of gadid fish.
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