The distribution of cod (Gadus morhua) in relation to temperature and oxygen level in the Gulf of St. Lawrence

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  • DDDenis D'AMours
    Denis D'AMours
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Abstract
Bottom temperature and oxygen level were measured in the northern Gulf of St. Lawrence during a ground‐fish trawl survey held in August and September 1991. Cumulative relative frequencies of temperature and oxygen level and cumulative relative frequencies of temperature x cod and oxygen X cod were computed with consideration of the random stratified sampling scheme. A total of 84‐3% of temperature x cod frequency cumulated from 2oC to 4oC, but only 25% of the frequency of temperature alone cumulated from 2oC to 4oC. At an oxygen level of 3.4 mg/1, only 2.6% of the oxygen X cod frequency had cumulated, while 24.3% of the cumulative frequency of oxygen alone was already observed at the same oxygen level. A comparison of the cumulative relative frequencies of temperature and of temperature X cod indicates that cod choose intermediate temperatures in the available range. A comparison of the cumulative relative frequencies of oxygen and of oxygen x cod indicate that cod avoid water with an oxygen level below 3.4 mg/1. This oxygen level is known to cause physiological stress in captivity. Data from the stations with oxygen levels ± 3.4 mg/1 were ignored, and the cumulative relative frequency of temperature was recomputed. As a result, the avoidance of cold water by cod was emphasized: 23% of the cumulative frequency of temperature alone was below 2oC, and 47.8% was above 4oC, in contrast to 17.2% and 57.5%, respectively, prior to removal of data from hypoxic stations.

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