Snow Crab Migration
Little is known about the seasonal migration of Chionoecetes opilio. What is known was discovered by researching for the purpose of the fishery. Female snow crab are illegal to fish, thus less known about females than males. It is known that seasonal migrations occur, meaning that in the winter, snow crab can be found in different locations than spring and summer. Winter months bring ice leading to great obstacles in t studying snow crab at this time of the year, so most information is gathered during warmer months.
Actual distances traveled by snow crab from one year to another appear to be relatively small. A study by Watson and Wells (1970) involved tagging crab at the end of the 1969 season and spring of 1970. Information was gathered from commercial fleet by the end of the 1970 season. The maximum time between tagging and recapturing is less than one year. The return rate was 25.2%. Most of the crabs recaptured were found within ten miles of their release location, and 86.4% moved less than fifteen miles. Broken down, of the crabs tagged the fall of 1969, 80.4% moved less than fifteen miles and 19.6% moved between fifteen and thirty-five miles. Of the snow crab tagged in 1970, 88% moved less than fifteen miles, 12% moved between fifteen and thirty-five miles. The minimum displacement was 2.5 miles, maximum displacement was 35 miles and an average of 10.9 miles. No definitive movement patterns could be found.
Similarly, Lefebvre and Brethes (1991) found an average displacement of 9.6 km for Chaleur Bay and 13.7 km for Iles-de-la-Madeleine (both located in the Gulf of St. Lawrence). The maximum displacements were 47 and 89.5 km respectively. Although averages were similar, maximum displacements were larger in the second study.
More recent literature points to the link between migration and the depth of location. Lovrich, Sainte-Marie and Smith (1995) performed bi-monthly beam trawling and utilized diver observations to study movement during the time period of April, 1991 to May, 1992. Immature crabs were found approximately 50 to 80 meters deep and gradually changed from a sedentary to a more active life-style. Movement of immature crabs occurred only between shallow and intermediate strata, while movement of adult crabs occurred across all three depth strata. According to Lovrich et al. the movement could be related to a number of factors including temperature, substrate type and mating.
Occupation of shallow grounds between the months of October and May are thought to be temperature dependant. The lethal temperature for snow crab is 15 degrees Celsius. The crabs prefer temperatures between –1 and 2 degrees, which coincides with depths of 70-160 m in the Gulf of St. Lawrence. Fishing of snow crab off eastern Cape Breton usually occur at depths of 130 to 250m (dfo 1999). At a temperature of seven degrees Celsius, metabolic costs are greater than energy consumed, thus it is energetically costly for snow crab to live in shallow water during the summer months. Lovrich et al. (1995) state that it is possible that snow crab synchronize their inshore migrations with temperature to obtain a stable thermal environment conductive to sustained motor activity. No snow crab were found in the shallow stratum between June and August while on the contrary there was a density of 861 per 1000m^2 in the month of December. Adult males were found to migrate first, as they were more abundant than adolescent males in December but outnumbered by adolescents in March to May. During this time, virtually no adolescent males were found in intermediate and deep strata. Crabs below the stage of adolescence do not migrate. After molting and mating, the crabs gradually descent to greater depths. Migration to shallow grounds is thought to be a strategy for avoiding predation and cannibalism while molting.
Hooper (1986) reports of diving observations that conclude the similar information. Snow crabs were found only in shallow water for April and May, and were not found at depths less than 40 m during the rest of the year. The large size of the crabs observed support a migratory separation of mature males from the rest of the population It was also noted that all crabs observed were within .8km of depths greater than 100m, and suggested that females occupy a different niche than males when not involved in mating.
Migration of snow crab is also highly related to mating. A study cited in Lovrich et al. (1995) found that C. tanneri adult females are concentrated at depths around 650 m while males are found at 450 m. In winter, the males move toward 650m depths then disperse with the females to shallow grounds. The movement is seen as a competitive exclusion of males form deeper areas.
No concern was shown as to the direction of the correlations shown in the studies mentioned. In each, migration was seen to occur for different reasons. Reality is probably that migration is a combination of temperature and a competitive exclusion purposes. It should be noted that studies on snow crab migration are not abundant and those that exist concentrate on movement of the adult males for fishery purposes. Little is known about females and crabs younger than the adolescent stage, both are thought to be somewhat sedentary, but more work needs to be performed.
References
Hooper, R.G. (1986). A spring migration of the snow crab, Chionoecetes opilio (O.
Gabr.),into shallow water in Newfoundland. Crustaceana 50: 257-264
Lefebvre & Brethes. (1991) Orientations des deplacements de crabe des neiges males (C.
opilio) marque dans le sud-ouest de golfe du Saint-Laurent. Canadian Journal of Fisheries and Aquatic Science 48: 1167-1175.
Lovrich, G.A., Sainte-Marie, B. & Smith, B.C. (1995) Depth distribution and seasonal
movements of C. opilio (Brachyura: Majidae) in Baie Sainte-Margurite, Gulf of
St. Lawrence. Canadian Journal of Zoology 73: 1712-1726.
Watson, J. & Wells, P.G. (1972) Recaptures and movements of tagged snow crabs
(Chionoecetes opilio) in 1970 form the Gulf of St. Lawrence. Fisheries Research Board of Canada Technical Report 349 11p.