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Despite efforts to protect the resident Atlantic cod (Gadus morhua) population in Gilbert Bay, Labrador, its abundance has decreased since the area was designated a Marine Protected Area (MPA). As part of an effort to understand the reasons for the decline, tracking of cod tagged with surgically implanted ultrasonic transmitters was conducted to determine the migratory behavior of cod in relation to MPA boundaries. Acoustic tracking used a network of data-logging receivers deployed in Gilbert Bay and contiguous marine waters, permitting year-round monitoring of fish movements over several years. Acoustic tracking indicated that 93% of mature migratory cod >55 cm total length homed to the overwintering area where they were initially tagged. During summer months some individuals moved to the mouth of the bay and marine waters outside the MPA. Collectively, the home range of the Gilbert Bay cod encompassed an area within about 40 km of their overwintering and spawning site, but included a large area (∼270 km2) outside the MPA. Eleven percent of tagged cod released outside their home range returned to the capture site within 1 year after tagging. In contrast, fish transplanted 15 or 25 km from the capture site, but still within their home range, exhibited 100% success in homing, suggesting the importance of prior experience with the migration route for successful homing. This study documents the importance of knowledge concerning the migratory and homing behavior of resident fish populations for their conservation and management.

Predicting dispersal paths of marine larvae with long pelagic durations, such as American lobster (Homarus americanus), requires understanding the cues to which larvae respond, and how that response reflects changes in larval behaviour. If larvae respond to conspecific presence by varying their movement, this behaviour can bias laboratory estimates of environmental responses. We tested whether larvae actively decreased their local intraspecific density by measuring how the vertical distribution of larvae changed under high versus low concentrations of conspecifics. We observed weak increases in vertical dispersion at higher concentrations in both newly-hatched larvae and in post-larvae, but not in intermediate larval stages. Further, we found that larvae from different mothers consistently differed in vertical distribution, which may indicate maternal effects on dispersal behavior. We also tested for differences in horizontal swimming behaviour in high and low concentrations, by fitting a novel random walk model that allowed us to model both larval interactions and persistent turning behaviours. We showed substantial reduction in diffusive behaviour under high concentration conditions resulting from more frequent turns by each larva, but no evidence for consistent avoidance of conspecifics. Our study is the first to demonstrate concentration-dependent behaviours in lobster larvae.

The surfclam, Spisula solidissima (Dillwyn), is a common species in many sandy locales off the east coast of North America from Labrador to North Carolina, yet it rarely occurs in muddy areas. In order to determine if this distribution may result, at least in part, from larval habitat selection, 24-h laboratory still-water and flume-flow experiments were conducted using a sediment-filled array. Sediment treatments included low-organic sand typical of adult habitat and a contrasting organic-rich mud. In flow experiments, larvae consistently chose sand over mud. In still water, however, results were variable, with higher settlement sometimes observed in mud. Measurements of settled individuals from four pairs of still-water/flow experiments indicated that mean shell lengths of clams in sand treatments were generally similar to those in mud for flow experiments, but in three of the corresponding still-water experiments clams in mud treatments were significantly smaller than those in sand. This difference may be a result of a deleterious effect of the mud on the larvae, resulting in a disproportionate representation of small, precompetent individuals in the mud treatments. These flow and still-water experiments indicate that larval habitat selection may contribute to the sediment-specific adult field distributions of the surfclam.