Ecology and Growth of Postlarval and Juvenile Lobster, Homarus americanus, off Îles de la Madeleine (Quebec)

Lobsters, Homarus americanus were studied in various habitats off Îles de la Madeleine, Quebec, from June to October 1985. Density, biomass, and size structure were estimated quantitatively using underwater corrals (40 m2) on rocky bottoms or transects (200 m2) on sandy bottoms and eelgrass beds. On the basis of behavioural observations during capture, lobsters were divided into postlarvae (<25 mm carapace length, CL), juveniles (25–76 mm CL), and adults (>76 mm CL). Density, biomass, and size structure were closely related to the coarseness of the bottom. Density and biomass were greater and mean size was larger on algae-covered boulders than on bare stones. Postlarval and juvenile lobsters were most numerous on shallow rocky bottoms where high densities (0.8–3.8 ind∙m−2) of small lobsters (mean size from 23.8 to 35.5 mm CL) were captured. Low density of juveniles and adults occurred on sand covered with eelgrass. No resident lobsters were found on bare sand. Temporal variation of postlarval density was synchronous at the three stations investigated. Postlarval density decreased during the summer until the settlement of planktonic postlarvae in mid-August. Moulting activity occurred throughout the summer, with a peak in early July. Mean size values for stages V–XIV were fitted to the cumulated size distribution and were used to calculate the percent increment for each moult stage (growth factor) and the expected duration of each stage. The 2-mo duration of the stage IV settlement period could result in a size range of 6–21 mm CL (mean size 14.5 mm CL, stage VIII) for a cohort by the end of its first season of benthic growth. The transition from planktonic to benthic life was characterized by a decrease of the growth factor at stage V, which was subsequently compensated at stages VI and VII by high growth factor values. In the first three growth seasons, spring and summer moults exhibit higher growth factor values than fall moults. These seasonal variations result in regular oscillations of the growth factor dampening progressively between stages V and XIV, instead of the smooth decline hypothesized in previous studies. A combination of factors, such as cryptic behaviour, high substrate selectivity, gradual settlement over the late summer months, and variability in growth factor, reflect the very fine tuning of the lobster's early benthic life stages to the constraints of its physical environment.