Page 5

Among-Habitat Variation in Age-0 Growth
Growth of settled age-0 cod appears to be temperature dependent (Tupper and Boutilier I995a). Growth was most rapid in eelgrass beds, which may positively effect overwinter survival of demersal0-group juveniles. Growth was slowest on sand bottoms; differences in growth between young inhabiting reef and cobble bottoms were not significant (Tupper and Boutilier 1995b). The growth advantage conferred by seagrass is related to the variety of microhabitats therein that support a diverse community of invertebrates (Orth et al 1984; Heck and Crowder 1991; Heck et al. 1995; Grant and Brown 1998a). Planktonic organisms may be passively concentrated by water currents and effectively retained within the eelgrass canopy. Also, invertebrates and fish may actively seek its confines even crossing predation-risky sand to reach isolated patches (Sogard 1992).

Small planktonic crustaceans, but mostly copepods, are preyed upon by young cod (Keats and Steele 1992; Grant and Brown 1998a). When mouth gape size is large enough, at a length of 6 to 10 cm, cod transition to predominately benthic prey (Keats et al. 1987; Lomond et al. 1998) which they then consume at dusk and dawn (Grant and Brown 1998a).

Among-Habitat Variation in Age-0 Survival
Eelgrass provides age-0 cod protection from predators (Tupper and Boutilier 1995; Gotceitas et al. 1997; Grant and Brown 1998b). In a laboratory experiment, eelgrass significantly increased the time required for an age-3 cod to capture 0-group cod and decreased the number captured. With a predator present, young cod either hid in cobble or in eelgrass when stem density was >720 stems/m2. Time to capture was highest and total prey taken was lowest in combinations with cobble or vegetation of 1,000 stem/m2 (Gotceitas et al. 1997). Results demonstrated that high plant density and/or biomass, whether eelgrass or macrophytic algae (Isaksson et al. 1994), means reduced predation risk just as does use of certain substrates. Moreover, there may be a trade-off between nutritional gain and enhanced predation risk for age-0 cod utilizing eelgrass habitat (Tupper and Boutilier 1995).

Mark-recapture experiments indicate age-0 cod remain very localized, not moving more than several hundred meters in both eelgrass and no-eelgrass habitats (Grant and Brown 1998b). Those that settled earliest and were largest at settlement grew faster and defended a larger _ territory than later/ smaller settlers (Tupper and Boutilier 1995a), thus a competitive advantage in growth and survival may exist for the earliest pulse of post-larval juveniles over those settling later when temperatures and day length are reduced (Tupper and Boutilier 1995b).

Abundance in the seagrass sites of St. Margaret's Bay, Nova Scotia, was noted to decline after early June. This was attributed to predation rather than emigration because young were strongly site-attached and defended territory as they grew. Marked individuals were not found in areas surrounding the study site (Tupper and Boutilier 1995b). As the summer season advanced, a greater decline in abundance occurred in eelgrass beds and on sand than in structurally more complex reef and cobble habitat. Observing in situ young-of-the-year seeking shelter in rock crevices, empty scallop shells, and other debris within dense grass beds, Tupper and Boutilier (1995b) believed that cod out-grew eelgrass blades as suitable refuge.

.

NEXT PAGE