Temperature preference and activity of juvenile cod in Broad Cove, Conception Bay, Newfoundland

Clark, Donald Stuart (1989) Temperature preference and activity of juvenile cod in Broad Cove, Conception Bay, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Field tracking studies conducted in Conception Bay, Newfoundland employing ultrasonic telemetry, determined the activity patterns of juvenile (3 year old) cod (Gadus morhua L.) on a daily and seasonal basis. In conjunction with the tracking study, the temperature preference of juvenile cod throughout the year was determined in the laboratory, and the influence of holding temperature on temperature preference was assessed. The results of the temperature preference studies were compared with water temperatures inhabited by juvenile cod in the field to assess the role that water temperature might play in mediating movement and activity. -- Juvenile cod exhibited seasonal variation in both temperature preference, and activity patterns. Temperature preference was highly correlated with seasonal changes in the temperature of the inshore waters inhabited by juvenile cod, both for fish acclimated to ambient temperature water, and those maintained at 10°C overwinter. This indicated that seasonal variation in temperature preference was initiated by factors other than changes in ambient water temperature. However, the temperature preference of these two groups differed significantly, indicating that thermal history also influenced temperature preference. -- The tracking studies showed that during summer (May - September) juvenile cod were wide ranging, conducting a diel migration from deep cold water where they remained inactive diurnally, to shallow warm water where they actively fed nocturnally. In autumn (September - December) these fish were diurnally active within a restricted home range in shallow water over a sandy substrate. At night they were inactive and remained in a consistent night-resting site over a rocky substrate in close proximity to their diurnal home range. -- The summer diel migration may result in increased growth since the movement of juvenile cod into deep cool water during non-foraging hours would effect a lower metabolic rate, and hence lower maintenance costs. In September when the water column becomes isothermal their switch to a non-migratory diel activity pattern is consistent with what would be predicted on the basis of energetic efficiency. It is unclear what selective factors may be involved in the switch in September from nocturnal to diurnal activity. However the most parsimonious hypothesis is that juvenile cod time their activity to reduce predation risk.

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