Depth distribution and seasonal movements ofChionoecetes opilio(Brachyura: Majidae) in Baie Sainte-Marguerite, Gulf of Saint Lawrence

Abstract

Bimonthly beam-trawling from April 1991 to May 1992 and diver observations 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 northern 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 ≈50–80 m deep and 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, where they moulted. Movement to shallow waters was massive and resulted in a mean density of 860 crabs per 1000 m2at ≈15 m in December 1991. Adult males (i.e., producing sperm and terminally moulted) 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 year, 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 multiparous females (i.e., adult, second or ulterior brood). Adult females were more gregarious and sedentary than adult males. We expand on the hypothesis that interannual variability in recruitment to adulthood tends to reflect differences in year-class strength, and that year-class strength varies in accordance with megalopal supply and (or) survivorship of cryptic instars.