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dc.contributor.authorComeau, Luc A.en_US
dc.date.accessioned2014-10-21T12:36:24Z
dc.date.available2014-10-21T12:36:24Z
dc.date.issued2001en_US
dc.identifier.otherAAINQ66647en_US
dc.identifier.urihttp://hdl.handle.net/10222/55775
dc.descriptionSouthern Gulf of St. Lawrence cod (Gadus morhua L.) perform extended (>500 km) seasonal migrations, moving from coastal to offshore areas in autumn, overwintering in deep slope waters, and returning to more inshore areas in the spring. However, little is understood about the underlying mechanism controlling these migrations, and the year-to-year variability in the timing of migrations. The objective of this thesis was to identify the physiological and environmental factors implicated in the regulation of cod migration. The approach was based on determining whether the onset of migration is at a fixed level (or phase) within the naturally occurring physiological and environmental cycles. A migration-timing index was computed across several years using fishers' private log records (1970--1992) and sentinel survey data (1995--1999). Between 1994 and 1998, nearly 2,000 cod serum samples were collected and assayed for the determination of various endocrine (gonadal and thyroidal) hormones. Food intake, energy stores, environmental temperature and dissolved oxygen were also closely monitored.en_US
dc.descriptionI found little evidence supporting the hypothesis that gonadal hormones (testosterone, estradiol-17beta) are functionally tied to migratory behaviour. On the other hand, I could not reject the hypothesis that thyroid hormones are involved in migration. L-thyroxine (T4) and 3,5,3' -triiodo-L-thyronine (T3) levels were significantly higher at the onset of the autumn migration than during the summer residency period. Moreover, during the return migration in spring, a larger fraction of the metabolically potent T3 hormone was released from plasma binding proteins, and thus made available for cell uptake. These observations are consistent with the documented role of thyroid hormones in increasing swimming capabilities in the laboratory, and reinforce the view that thyroid hormones facilitate seasonal migrations. With respect to experimental work, this thesis has demonstrated that sustained (1 month) experimental hyper- and hypothyroidism is feasible in wild cod by osmotic pump delivery of thyroid or anti-thyroid agents. Tracking thyroid-manipulated subjects in their natural environment would represent a straightforward approach for verifying the postulated link between the thyroid and migration.en_US
dc.descriptionCod initiated the autumn migration progressively earlier starting in the mid-1980s. At the onset of the autumn migration in the 1990s, dissolved oxygen concentrations were above levels known to induce avoidance behaviour in this species, and food resources were considered to be relatively abundant. Autumn migration onset occurred neither at a specific temperature nor following a particular temperature regime during the previous summer. However, cross correlation function analysis indicated that a widespread cooling of near bottom waters preceded the mid-1980s shift in migration timing by 1 to 2 years, suggesting that the cooling event was linked in some way to the change in migration dates. Other correlations showed that the shift in timing coincided with a reduction in growth rates and also with a rise in the abundance of fish prey inside the southern Gulf. I propose that a reduction in metabolic requirements coupled to an increase in the consumption of fish prey led to a rapid build-up of energy reserves on summer grounds, and to an earlier readiness to engage in the overwintering migration. I conclude by discussing the potential impact of earlier departures on stock productivity.en_US
dc.descriptionThesis (Ph.D.)--Dalhousie University (Canada), 2001.en_US
dc.languageengen_US
dc.publisherDalhousie Universityen_US
dc.publisheren_US
dc.subjectBiology, Ecology.en_US
dc.subjectBiology, Zoology.en_US
dc.titleEndocrinological and environmental cues underlying the seasonal migrations of Atlantic cod (Gadus morhua L.).en_US
dc.typetexten_US
dc.contributor.degreePh.D.en_US
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