Substrate selection and use of protective cover by juvenile Atlantic cod Gadus morhua in inshore waters of Newfoundland

Abstract

We investigated the habitat preferences and use of cover of 1 to 4 yr old juvenile cod G a d u s rnorhua in the inshore waters (18 to 150 m depth) of Placentia Bay, Newfoundland, Canada. using deep sea submersibles (PISCES IV & SDL-1) in April 1995. We analysed a total of 32 h of 'onbottom' videotape, audiotape, and ivritten records from 9 daylight dives and l nlght dive. Habitat types were characterised by depth, substrate particle size, bathymetric relief, and the presence or absence of macroalgae. Juvenile cod found throughout the dive area were identified as either 'young' (age 1, 10 to 12 cm total length, and mottled in colour) or 'old' (age 2 to 4. > l 5 cm total length, and relatively uniform in colour). Of old juveniles, 80% were found to be associated with areas of coarse substrate and high bathymetric relief (i.e. submarine cliffs). In contrast, 59% of young juveniles were found primarily in areas with a gravel substrate and low relief. Juvenile cod did not exhibit selection for substrates with macroalgae cover. We did not identify any difference between day and night observations. Old juveniles were often associated with ind~vidual substrate features ( e g . a single rock, boulder, or crevice), and exhibited a significant increase In activity (oriented swimming speed) with increasing distance from such features. Young juveniles exhibited no such association with specific substrate features, although they exhibited greater variation in activity (non-oriented swimming speed). The observed patterns in activity between the age groups suggest a difference in predator avoidance behaviour. Young mottled individuals appeared to be relying on crypsis, whereas older uniform-coloured juvenile cod associated with a specific physical feature which represented cover. Our results corroborate the findings of previous laboratory and shallow water field studies on the behaviour of this species. In addition, these results demonstrated that substrate selection by juvenile cod is age specific.