Niche partitioning in sympatric Greenland cod (Gadus ogac) and Atlantic cod (Gadus morhua) in coastal Newfoundland

Knickle, David Craig (2013) Niche partitioning in sympatric Greenland cod (Gadus ogac) and Atlantic cod (Gadus morhua) in coastal Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Niche partitioning, the process by which evolutionary pressures divert competing species into different patterns of resource use, is often used to explain the coexistence of closely related species. In this thesis, I test the theory that niche partitioning facilitates the coexistence of two sympatric gadid species (Atlantic cod and Greenland cod) in a coastal region of Newfoundland using a multi-method approach. Dietary and isotopic analyses were used to examine the degree of trophic niche overlap between species. Results revealed a general partitioning of food resources; G. morhua consumed more pelagic prey and showed a more pelagic (more negative) δ¹³C signature while G. ogac consumed more benthic prey and had a more benthic (more positive) δ¹³C signature. It was concluded that interspecific competition for prey resources and dietary overlap is low and that trophic niche partitioning is likely a key mechanism enabling coexistence. Acoustic-radio telemetry was used to examine the spatial and temporal movement patterns of juveniles during summer. G. morhua were wider ranging, moved at faster rates and were active throughout the diel cycle compared to G. ogac of the same size suggesting spatial and temporal niche partitioning occurs between species during the summer season. Fine-scale habitat use and vertical distributions were investigated using radio-acoustic positioning and habitat mapping. Both species preferred coarse substrates with moderate or dense vegetation, areas of low bathymetric relief and shallow (<10 m) water. G. ogac remained closer to the seafloor while G. morhua was generally distributed more pelagically and showed greater variation in vertical positioning. Results indicated high overlap in microhabitat use but differential use of vertical habitat which may reduce competitive interference between species. Size-at-age and maturity data were used to compare growth rates and maturity patterns between species. Age and size at maturity were lower for G. ogac than for G. morhua despite similar growth rates. Length-weight relationships suggested slightly positive and negative allometric growth, for G. ogac and G. morhua respectively. It was concluded that differences in maturity patterns may promote coexistence by decreasing resource competition between species. In summary, the findings from this thesis suggest that niche partitioning occurs along several niche dimensions which facilitate coexistence between ecologically similar Atlantic and Greenland cod in coastal waters of Newfoundland.

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