Natural mortality and movement of juvenile cod (Gadus spp.) inhabiting eelgrass (Zostera marina) in coastal Newfoundland waters

Sheppard, G. L. (2005) Natural mortality and movement of juvenile cod (Gadus spp.) inhabiting eelgrass (Zostera marina) in coastal Newfoundland waters. Masters thesis, Memorial University of Newfoundland.

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Abstract

The natural mortality of teleost populations is age-dependent - young, small-bodied individuals experience comparatively higher mortality rates than older, larger individuals. The use of structurally complex habitat, such as eelgrass (Zostera marina) and macroalgae, is believed to increase survival of early stages by providing protective cover from predators and increased food availability. However, few studies have actually quantified natural mortality rates for juvenile cod associated with nearshore habitat. I determined mortality rates for juvenile Greenland cod (Gadus ogac) and Atlantic cod (G. morhua) associated with eelgrass in the coastal zone of Newman Sound, Newfoundland. During Oct-Nov 2002, I fin-clipped 818 age-0 Greenland cod; an additional 1442 age-1 Atlantic cod were fin-clipped during June-July 2003. During Oct-Nov 2003, a further 765 age-0 Greenland cod were individually marked with visible implant alphanumeric (VIalpha™) tags. Re-sampling revealed that juvenile cod remained near the site of release, rarely venturing beyond a distance of a few hundred metres. For age-1 Atlantic cod in eelgrass habitat, the mortality rate was M = 0.068day⁻¹ (estimated using the Jolly-Seber method), which was significantly higher than that predicted (M = 0.010day⁻¹) using an age-dependent mortality model based on published cod mortality rates. High mortality rates may be attributed to high predation and other biotic and abiotic sources of mortality (i.e., starvation, environmental extremes). Juvenile cod mortality rates were high despite the availability of areas with complex structure; however, consistently low cod abundances in structurally simple habitats suggest such areas may be too risky to frequent for extended periods.

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