Ontogenetic Habitat Shifts of the Atlantic Tomcod (<Emphasis Type="Italic">Microgadus tomcod</Emphasis>) Across an Estuarine Transition Zone |
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Authors: | Marc Antoine Couillard Gilbert Cabana Jean François Dery Gaétan Daigle Julian John Dodson |
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Institution: | 1.Département de biologie, 1045 avenue de la Médecine,Université Laval,Québec,Canada;2.Département de chimie-biologie, C.P.500,Université du Québec à Trois-Rivières,Trois-Rivières,Canada;3.Département de mathématiques et de statistique, 1045 avenue de la Médecine,Université Laval,Québec,Canada |
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Abstract: | Tomcod (Microgadus tomcod) in the St. Lawrence estuarine transition zone (ETZ) undergo an ontogenetic habitat shift. Older age classes, characterised
by a male-dominated sex ratio, disperse downstream over the summer months to occupy the colder more saline waters of the estuary.
Significant differences in length and mass along the salinity gradient were observed in September with upstream fish of any
given age class generally exhibiting greater growth. These differences were not seen in early summer. Benthic amphipod δ
34S signatures were strongly correlated with salinity and served to demonstrate that tomcod δ
34S signatures were not in isotopic equilibrium in the more saline waters of the ETZ. Seasonal distributional patterns, growth
dynamics and isotopic disequilibrium all indicate that the observed habitat shift may occur on an annual basis, following
winter aggregation in warmer, less saline waters. Tomcod located in the downstream parts of the ETZ, predominantly males,
were significantly more sexually developed than upstream tomcod for a given age. On the other hand, greater growth early in
life is insured by occupying warmer, upstream waters during the summer months. |
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