Predator-mediated habitat use by juvenile white hake (Urophycis tenuis) and Greenland cod (Gadus ogac)

Lewis, Christopher W. (2007) Predator-mediated habitat use by juvenile white hake (Urophycis tenuis) and Greenland cod (Gadus ogac). Masters thesis, Memorial University of Newfoundland.

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Abstract

Juvenile white hake (Urophycis tenuis) abundance has been increasing on the northeast coast of Newfoundland over the past two decades. In these coastal areas juvenile white hake and Greenland cod (Gadus ogac) overlap spatially and temporally in eelgrass (Zostera marina) habitat, an important nursery for young of the year gadids. Concern has arisen that the recent increase in white hake abundance may be negatively impacting other gadids, such as Greenland cod. The purpose of my study was, therefore, to investigate the habitat use and behavioural interactions of juvenile white hake and Greenland cod and how this may vary in response to predator threat (an age 3+ Atlantic cod; G. morhua) and interspecific competition. A series of laboratory experiments were undertaken to determine habitat use under these different scenarios by dividing the bottom of two replicate arenas (2 x 2 x 0.5 m) into thirds and covering these with artificial eelgrass, cobble, or sand. My study documented overlapping habitat use by juvenile white hake and Greenland cod and supported the contention that eelgrass serves as an important refuge habitat for both species, with Greenland cod showing the highest affinity. Furthermore, interspecific competition experiments showed evidence of white hake excluding Greenland cod from preferred habitat as a result of competition for refuges. The size advantage that white hake gain over Greenland cod early during demersal life may be the main mechanism that influences the outcome of this habitat competition.

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