DEPTH DISTRIBUTION AND SEASONAL MOVEMENTS OF CHIONOECETES-OPILIO (BRACHYURA, MAJIDAE) IN BAIE-SAINTE-MARGUERITE, GULF-OF-SAINT-LAWRENCE

Bimonthly beam-trawling from April 1991 to May 1992 and diver observat ions were used to assess distribution and large-scale movement of snow crabs, Chionoecetes opilio, over depths of 4-140 m in a bay of the no rthern Gulf of Saint Lawrence. Megalopae settled during October 1991. Immature crabs of instars I-IV (i.e., 3.3-9.7 mm mean carapace width, CW) were found predominantly on bottoms approximate to 50-80 m deep an d were cryptic and sedentary. Most immature crabs of instars V-VIII (i .e., 14.1-34.5 mm mean CW) and adolescent males (i.e., producing sperm but not terminally moulted) migrated in winter to subtidal grounds, w here they moulted. Movement to shallow waters was massive and resulted in a mean density of 860 crabs per 1000 m(2) at approximate to 15 m i n December 1991. Adult males (i.e., producing sperm and terminally mou lted) of <70 mm mean CW also moved to the shallow grounds from October to December 1991, where some mated with pubescent-primiparous females (i.e., adult, first brood) from January to April 1992. Adult males of >90 mm mean CW were mainly found at depths >80 m over most of the yea r, but from March to May 1992 an increase in mean CW of adult males at <80 m indicated some upslope movement, probably to mate with multipar ous females (i.e., adult, second or ulterior brood). Adult females wer e more gregarious and sedentary than adult males. We expand on the hyp othesis that interannual variability in recruitment to adulthood tends to reflect differences in year-class strength, and that year-class st rength varies in accordance with megalopal supply and (or) survivorshi p of cryptic instars.