Effect of environmental conditions on the natural activity rhythms and bottom trawl catchability of Atlantic cod (Gadus morhua)

Winger, Paul D. (2004) Effect of environmental conditions on the natural activity rhythms and bottom trawl catchability of Atlantic cod (Gadus morhua). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Environmental conditions such as water temperature and ambient light level are expected to exert a strong influence on the natural behaviour and bottom trawl catchability of marine groundfish species such as Atlantic cod (Gadus morhua). In this thesis, I use acoustic telemetry to investigate this prediction for cod inhabiting coastal inshore waters in the North Atlantic ocean. -- Laboratory and field experiments were first conducted to examine two intragastric methods for attaching acoustic transmitters to cod. I found that cod which voluntarily swallowed transmitters wrapped in bait exhibited a delayed onset of initial regurgitation, longer periods of retention, higher food consumption, and lower mortality in comparison to fish tagged using the method of forced insertion into the gut. Under field conditions, cod in shallow coastal waters were successfully tagged by lowering a tagging frame near the seafloor containing baited acoustic transmitters. The technique did not appear to affect natural behaviour and was found to be ideal for short-term telemetry studies. -- Temporal patterns in the natural activity level of acoustically tagged cod were investigated using a stationary positioning system. Time-of-day was found to have a significant affect on the activity level of cod inhabiting two study areas, the coastal waters of Norway and Newfoundland. High resolution tracking of juvenile cod revealed significant between-individual variation, suggesting that juvenile cod may make different trade-offs in balancing the demands of foraging and predator avoidance. Adult cod in Smith Sound on the northeast coast of Newfoundland exhibited seasonal variation in diel activity rhythm. During summer, cod were active throughout the diel cycle with a pronounced increase in activity during daylight hours. During winter, cod occupied deeper water and were significantly less active. Water temperature and ambient light intensity are thought to be the key proximate factors responsible for this seasonal difference in behaviour. -- Finally, the behaviour of cod was examined in response to an approaching vessel and bottom trawl at different times of year. Using acoustic telemetry, the swimming speed of cod was estimated for the periods before, during, and after passage of a vessel travelling at different speeds as well as in response to this vessel towing a bottom trawl. The likelihood of reaction, reaction distance, and the duration of disturbance produced by the encounter were examined. The results showed that cod are capable of reacting to an approaching vessel from considerable distances and that the disturbance produced by the encounter can persist for periods up to 56 min. -- In summary, the findings from this thesis indicate that variation in environmental conditions play an important role in modifying the behaviour of cod. Diel variation in ambient light intensity and seasonal variation in water temperature appear to be important proximate factors. Both are predicted to affect the availability and vulnerability of inshore cod populations to acoustic and bottom trawl sampling tools used in resource assessment surveys.

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