Swimming endurance of American plaice (Hippoglossoides platessoides) and Atlantic cod (Gadus morhua) : implications for catchability by otter trawls

Winger, Paul D. (1998) Swimming endurance of American plaice (Hippoglossoides platessoides) and Atlantic cod (Gadus morhua) : implications for catchability by otter trawls. Masters thesis, Memorial University of Newfoundland.

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Abstract

Identifying and measuring the factors that affect the catchability of otter trawl fishing gear has been the subject of much research in recent years. Variation in catching efficiency is known to be associated with changes in the vulnerability offish to trawl capture. In particular, the swimming capability offish in response to an approaching trawl plays an important role in detennining the likelihood of successful capture. In order to quantify the efficiency of the capture process, knowledge of the swimming endurance of marine groundfish species is necessary. The purpose of this study was to investigate factors affecting the swimming endurance of American plaice (Hippoglossoides platessoides) and Atlantic cod (Gadus morhua) under laboratory conditions. The effects offish length, water temperature and swimming speed were examined using a swimming flume. The data were analyzed using failure time (survival) analysis, a more robust statistical technique for endurance-type data when censored observations require consideration. -- Swimming trials for plaice were conducted across a range offish lengths (14 to 44 cm) and water temperatures (-0.2 to 9.7 C) at a swimming speed of 0.30 m/s. The probability of plaice achieving a given endurance was found to significantly improve with increasing fish length and water temperature. The findings indicate that the herding of plaice by otter trawl sweeps may be size- and temperature-dependent. At low temperatures, small plaice are expected to have a lower probability of sufficient endurance required to reach the path of an approaching trawl, thereby experience a greater likelihood of escaping capture. -- Swimming trials for cod were conducted across a range offish lengths (41 to 86 cm), water temperatures (0.0 to 9.8 C) and swimming speeds (0.60 to 1.30 m/s). Swimming speed was the only factor found to significantly affect the endurance of cod. The probability of achieving a given endurance was found to decrease rapidly with increasing swimming speed. The findings indicate that even small changes in the speed of a trawl through the water could dramatically affect the rate of exhaustion, i.e. turn-over rate, of cod swimming in the mouth of a trawl.

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