Ontogeny of the corticosteroid stress response and effect of differentially enriched live feed on growth, lipid composition and acute stress tolerance of larval Atlantic cod, Gadus morhua

Westelmajer, Sarah Kate Matheson (2008) Ontogeny of the corticosteroid stress response and effect of differentially enriched live feed on growth, lipid composition and acute stress tolerance of larval Atlantic cod, Gadus morhua. Masters thesis, Memorial University of Newfoundland.

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Abstract

The ontogeny of the corticosteroid stress response was examined in hatchery-reared Atlantic cod (Gadus morhua). Pooled whole-body samples of larvae at rest and following a 30 second (s) air exposure were taken at multiple intervals throughout development from 2 days pre-hatch up to metamorphosis at 59 days post-hatch (dph). Extracted samples were analysed using a commercially available enzyme-linked immunoassay kit, which was subsequently validated for use with whole-body samples. A significant increase in whole-body immunoreative corticosteroid (IRC) concentration in response to the air-exposure stress was observed in larvae at hatch (0 dph), which likely indicates the functioning of the hypothalamus-pituitary-interrenal axis (HPI) and thus the endogenous production of Cortisol by the larvae at this time. Two stress hyporesponsive periods (SHRP) were detected, between 0-30 dph and at 50 dph, and likely correspond with two critical periods of neurological and immunological development, during which the production and release of Cortisol is restricted. Potentially stressful aquaculture procedures should be avoided during these SHRPs. -- The effect of differentially enriched live-food organisms, rotifers (Brachionus plicatilis), and brine shrimp (Artemia sp.), on the lipid content, growth and stress tolerance of larval Atlantic cod was also examined. The lipid and fatty acid content of larval tissue was directly related to the lipid and fatty acid content of enriched live feed. Larvae fed rotifers and Artemia enriched with elevated proportions of the highly unsaturated fatty acids, DHA (docosahexaenoic acid; 22:6ω3), AA (arachidonic acid; 20:4ω6), and ω6DPA (docosapentaenoic acid; 22:5ω6) showed best overall growth and survival following an acute 15 s air-exposure and transfer stress. Differences in post-stress survival between treatments were most likely attributable to variations in larval EPA (eicosapentaenoic acid; 20:5ω3)/AA and DHA/DPA content. Corticosteroid levels remained elevated in larvae with maximal post-stress survivorship, and may indicate the requirement of larval cod for a prolonged rise in Cortisol following a stress event. Suggested DHA/EPA/AA ratios for rotifer and Artemia enrichments that yielded best growth and stress tolerance in Atlantic cod larvae were 7/2/1 and 5/2/1 respectively.

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