Timing of Atlantic cod (Gadus morhua L.) seasonal migrations in the southern Gulf of St Lawrence: interannual variability and proximate control

Abstract

The objective of this study was to identify likely proximate cues to the seasonal migrations of cod, Gadus morhua L., in the southern Gulf of St Lawrence. We computed a migration timing index across several years using commercial fishery and sentinel survey (1995)(1996)(1997)(1998)(1999) catch rate data, and closely monitored the seasonal changes in temperature, dissolved oxygen, food intake, and energy stores between 1995 and 1998. The inter-year variability in the timing of the seasonal migrations was relatively low (s.d.=8 d for spring migration, 10 d for autumn migration), suggesting that photoperiod was involved in the control mechanism of migration. However, other factors were also implicated in the control mechanism, given that cod initiated the autumn migration progressively earlier in the late 1980s and 1990s. At the onset of the autumn migration, dissolved oxygen concentrations were above levels known to induce avoidance behaviour in this species, and food resources were considered to be relatively abundant. Similarly, cod did not initiate the autumn migration at a specific temperature or following a particular temperature regime experienced during summer. However, cross correlation function analysis indicated that a widespread cooling of near bottom waters preceded the mid-1980s shift in migration timing by one to two years, suggesting that the cooling event was linked in some way to the change in migration dates. Other correlations showed that the earlier migrations coincided with older cod becoming more abundant and smaller in size, and also with their principal fish prey (herring and capelin) increasing in number. These results suggest that older cod played a key role in launching the early autumn migrations. One plausible interpretation is that their decline in body size and lipid-rich diets led to a rapid build up of energy reserves over summer grounds, and thus to an earlier readiness to engage in the autumn migration. We discuss the potential impact of earlier departures on stock productivity.

Diel vertical interactions between Atlantic cod Gadus morhua and sprat Sprattus sprattus in a stratified water column

“…It furthermore fits in with the general tendency that planktivorous fishes in the temperate zone are food-limited during overwintering (Byström et al 2006). Previous laboratory work has shown that a number of fishes on low ration, or starved, select colder temperatures compared to well-fed individuals (Comeau et al 2002). A generally accepted Q 10 value of ~2.2 for metabolic rates (Schmidt-Nielsen 1983, Willmer et al 2000 provides a reduction in metabolic rate of 33% when sprat are located above the thermocline at 3°C, compared to the bottom at 8°C in the present study.…”

Development of foraging strategies with age in a long-lived marine predator

“…For ex ample, fish condition strongly follows a sinusoidal an nual cycle (see Fig. 9 in Comeau et al 2002). The cycle of fish condition, however, is offset from primary production and water temperature, as well as the reproductive cycle of grey seals.…”

“…They forage primarily on the Scotian Shelf, a broad, shallow ocean margin off Canada's east coast characterised by complex glacially altered bathymetry and an ecosystem that has been highly disturbed by overfishing (Frank et al 2005). This region is temperate at 44°N and, typical of temperate oceans, has strong annual cycles of ocean productivity and temperature, major annual fish migrations (which affects prey availability) and strong seasonal changes in fish condition (which affects prey quality) (Perry & Smith 1994, Swain et al 1998, Comeau et al 2002. Seasonality in foraging behaviour has been detected in adults from this population (Beck et al 2003a, Austin et al 2006, Breed et al 2006.…”

“…Prey species also show pronounced annual cycles in energy density, becoming lipid rich in the summer prior to spawning and losing condition through the winter. Thus, the same prey items in winter will have less energy than they do in the summer (Smigielski et al 1984, Comeau et al 2002, Beck et al 2007). In addition to changes in energy density and depth distribution, some prey such as sandlance, a major component of the diet, spend more time hidden in sand (Smigielski et al 1984), requiring predators to switch foraging tactics.…”

Season and site fidelity determine home range of dispersing and resident juvenile Greenland cod Gadus ogac in a Newfoundland fjord

“…Specific populations of adult Atlantic cod have been characterised as resident, homing, or dispersing (Robichaud & Rose 2004, Neat et al 2006. Site fidelity or homing patterns can vary across life stages in response to increased opportunity for spatial or social learning, depending on habitat stability (Dodson 1988, Odling-Smee & Braithwaite 2003 or body condition (Comeau et al 2002). These varying patterns affect the contributions of movements of individual fishes to population connectivity.…”