Penobscot Bay Habitat Watch

Proposal for a Habitat Area of Particular Concern for Juvenile Atlantic Cod (Gadus morhua) in the Nearshore Waters of the Gulf of Maine.
Prepared by: EFH Technical Team (Excerpted from the 1999 Habitat Annual Review Report)

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Juvenile Cod Community & Interactions - Research Results

Pelagic Juvenile Settlement
Post-larval pelagic juveniles are transported by prevailing currents to shallow waters off eastern Newfoundland beginning in May and may continue arriving in periodic pulses as late as December (Methven and Bajdik 1994; Grant and Brown 1998). Their length upon settlement is 25-45 mm (Pinsent and Methven 1997). In southwest Nova Scotia, pelagic juveniles arrive inshore slightly larger (- 40-50 mm) in May (Tupper and Boutilier 1995a and 1995b), whereas influxes of larvae begin earlier in Massachusetts waters two to three months after hatching (Bigelow and Schroeder 1953). On Georges Bank, cod settle out in July at 40-60 mm, and those reaching rough and cobble bottom may experience reduced predation risk. This particular habitat may be an important demographic bottleneck to benthic recruitment on Georges Bank (Lough et al. 1989).

Pelagic juveniles exhibit no preference for habitat types at settlement, and they occupy rock reef, cobble, eelgrass (Zostera marina) beds, and sand bottom (Tupper and Boutilier 1995b). Tupper and Boutilier (1995b) assumed that settling also occurred on macroalgae habitat, as noted by Keats et al. (1987) off eastern Newfoundland, however, algal stands were scarce in the St. Margaret's Bay, Nova Scotia, study area due to subtidal grazing by sea urchins (Strongylocentrotus drobachiensis). Sea urchins commonly leave a partially denuded or "barren" zone along nearshore sections of the maritime provinces and Gulf of Maine.

Age-0 Movements and Diel Feeding
Shallow water depths (<5 m) and a strong attraction to features on most substratums, except sand, afford settled juveniles an environment conducive to growth and survival (Tupper and Boutilier 1995a; Grant and Brown 1998b). The shallowness appears to ecologically segregate the 0-group cod from older age-groups at least during daylight (Tupper and Boutilier 1995a; Fraser et al. 1996; Gotceitas et al. 1997; Grant and Brown 1998a and 1998b). Age-0 cod maintain a strict diurnal foraging cycle, school (or shoal) feeding on zooplankton in a tide-related pattern during the day, and remain near protective bottom habitat which they readily seek when threatened (Gotceitas and Brown 1993; Gotceitas et al. 1995; Grant and Brown 1998a).

The mottled coloring of young juveniles effectively conceals them in a pebble-gravel environmerit (Lough et al. 1989; Gregory and Anderson 1997). In contrast, pelagic juveniles on Georges Bank maintain a nocturnal feeding pattern (Perry and Neilson 1988). Age-0 cod cease feeding in surface waters and disperse to the substratum at night (Grant and Brown 1998b) where they are less active to reduce interactions with potential predators (Grant and Brown 1998a). The diel change in vertical distribution and activity of 0-group cod coincides with a nocturnal shoreward movement and foraging by older (age-1-3) conspecifics (Bosgstad et al. 1994; Gotceitas et al. 1997; Grant and Brown 1998a). Intercohort cannibalism is common. The occurrence of age-0 cod in very shallow water (<1.2 m) at tight (Methven and Bajkik 1994) has also been interpreted as possibly an evasive response to predation risk (Grant and Brown 1998a).

Influence of Habitat Structure and Predation on Age-0 Demography
Tupper and Boutilier (1995b) found that the spatial pattern of settlement was altered by post- settlement mortality in St. Margaret's Bay, Nova Scotia. Age-0 survival was positively correlated with an index of rugosity, a measure of actual bottom surface or complexity. Capture success by fish predators (in this case, three species of Cottidae during a diurnal field study) was

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