Brown cod and bay stocks - science and fish harvesters' knowledge of colouration in populations of Atlantic cod (Gadus morhua) in Newfoundland and Labrador

Gosse, Karen R. (2002) Brown cod and bay stocks - science and fish harvesters' knowledge of colouration in populations of Atlantic cod (Gadus morhua) in Newfoundland and Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

This research investigated the reliability of colouration as an indicator of inshore populations of Atlantic cod (Gadus morhua) in Newfoundland and Labrador, using a combination offish harvesters' knowledge of cod colouration and scientific experimentation. Thirty-two interviews with fishing experts were conducted in 23 communities throughout the west coast and Northern Peninsula of Newfoundland and the southeastern coast of Labrador to gather fish harvesters' knowledge about cod colouration and ecology. Five main colourations of cod were identified through interviews: brown, yellow-brown, red to reddish-brown, black-backed, and dark/black. Harvesters generally associated these colourations with the environment (shallow water and/or fresh water and/or presence of kelp) and/or diet (cod feeding on crustaceans and/or kelp). Thirteen areas were identified where brown cod were believed to overwinter and remain year-round. -- Brown to reddish-golden-brown cod and countershaded cod (cod that are black on back, white underneath with silvery grey sides) were captured within the bay and on the headlands of Gilbert Bay, Labrador. To determine the stability of colouration, these cod were held in net pens from August to October 2001 and fed a piscivorous diet of capelin and herring. Initial and final colouration of each individual fish were compared using a colouration scale. Results of the pen-holding experiment demonstrated that cod will lose their brown colouration within 2.5 months when fed a diet of fish. The absence of carotenoids in the diet of cod held in net pens was concluded to be the main factor contributing to this loss of brown pigmentation. -- I conclude that colouration cannot be used in isolation to identify inshore stocks of cod. However, colouration acts as a time-dependant dietary index and can be used to deduce recent feeding histories of individuals or groups offish. Thus, colouration acts as a general indicator of inshore groups of cod and probable locations for resident coastal cod stocks. Fish harvesters' observations provide a valuable base line of information on which to plan further scientific studies.

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