Length–depth relations of Enchelyopus cimbrius fourbeard rockling (Gadiformes: Phycidae) from the southern Gulf of St Lawrence and Cabot Strait in relation to abiotic factors

Abstract

Research vessel survey data collected by Fisheries and Oceans Canada in the southern Gulf of St Lawrence (1971–2002) and Cabot Strait (1994–1997) were analysed to determine if Enchelyopus cimbrius the fourbeard rockling, distributed itself with larger individuals occurring in deeper water. A positive size–depth relationship was first observed for the European plaice, Pleuronectes platessa in the North Sea and has been reported for other fish. Positive relationships were found between the total length of E. cimbrius and depths where it occurred in this study. However, the biological relationship was not significant since the linear regression slopes were very weak, explaining less than approximately five per cent of the variance observed. Data were analysed further to determine the water depth, temperature and salinity ranges where the fourbeard rockling was most abundant. Enchelyopus cimbrius occurred throughout the depth-ranges sampled with highest catches occurring at 25–50 m (southern Gulf of St Lawrence) and deeper than 200 m (southern Gulf of St Lawrence and Cabot Strait). Low catches at intermediate depths (~50–100 m) in the southern Gulf of St Lawrence may have been due to the lack of suitable mud substrate and colder bottom temperatures in the Magdalen Shallows, a large central region of the southern Gulf. Enchelyopus cimbrius was mostly caught at relatively narrow temperature and salinity ranges of 4–6°C and 34–34.9 ppt in both regions, and only occurred in areas with a mud substratum, predominantly in the eastern Northumberland Strait, Baie des Chaleurs and in the deeper water of the Cape Breton Trough, Laurentian Channel and Cabot Strait. Enchelyopus cimbrius was caught throughout the day and night, contradicting a previous study that characterized it as being nocturnal in shallow coastal waters off Newfoundland.