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.

Behavioural consequences of density-dependent habitat use in juvenile cod Gadus morhua and G. ogac: the role of movement and aggregation

“…Juvenile cod have also been shown to use the interstitial spaces of less complex mineral substrates such as cobble and gravel for predator avoidance (e.g. Lough et al 1989, Gregory & Anderson 1997. However, the substrate particle size at the sand sites was ca.…”

“…Grant & Brown (1998a) report limited movement of juvenile cod between sites following settlement in nearshore areas, suggesting there were large sections of unfavourable habitat between sampling sites. Age 0 juvenile cod are restricted to shallow, nearshore habitat such as eelgrass, possibly due to increased predation with depth (Linehan et al 2001) and reduced habitat complexity beyond the littoral zone (Gregory & Anderson 1997). Mark-recapture studies conducted between areas of varying habitat and depth may therefore misrepresent the capacity of movement in juvenile cod.…”

Colonisation of fish and crabs of wave energy foundations and the effects of manufactured holes – A field experiment

“…Nakamura, 1985). A number of fish species, including codfishes, wrasses, and flatfishes have been shown to associate with high relief structures of comparably low complexity during parts or most of their life cycles (Gregory and Anderson, 1997;Light and Jones, 1997;Stanley et al, 2002;Johnson et al, 2003;Cote et al, 2004;Wilhelmsson et al, 2006b). …”

Habitat structure in soft-sediment environments and abundance of juvenile snapper Pagrus auratus

“…Modifying the risk of predation for an individual involves searching out areas that reduce a predator's ability to locate and capture prey (Giske et al 1998). A number of studies have demonstrated that survivorship of juvenile Atlantic cod Gadus morhua is related to habitat structure, which may at least partly explain the slow recovery of the stock after its collapse (Lough et al 1989, Gotceitas & Brown 1993, Tupper & Boutilier 1995, Gregory & Anderson 1997, Lindholm et al 1999). This implies that we need to understand the behaviour and ecology of fish over various space and time scales, as well as their recruitment dynamics, to sustainably manage populations (Bailey 1994).…”