Snow Crab Population Research Articles

Life history schedule and periodic recruitment of female snow crab (Chionoecetes opilio) in the eastern Bering Sea

Snow crab ( Chionoecetes opilio ) populations have fluctuated cyclically in eastern Canada and the eastern Bering Sea, where recruitment to the mature female pool has occurred over a period of three decades (1978–2007) in pulses with a mean period of 7 years. It has been hypothesized that this was the result of a parent–offspring relation between sequentially linked strong cohorts of mature primiparous females, which requires that periodicity matches the time lapsed between egg extrusion by the maternal broodstock and the offspring reaching maturity. We show that female age at maturity (post-settlement) varies between 4.5 and 7.5 years, with most females maturing at 5.5–6.5 years (7–8 years after egg extrusion). Pulses of female recruitment to the mature population do not show a latitudinal trend, consistent with uniformity in age-at-maturity. Results of tracking crab abundance and size–frequency distributions in cod stomach and trawl samples between successive pulses of the cycle are consistent with the hypothesis of serial linkage among pulses. Periodicity is reflected in trends of clutch fullness and average shell condition and in the negative correlation between the strength of primiparous female cohorts and the mean size of their members.

Population genetic structure of the snow crab (Chionoecetes opilio) at the Northwest Atlantic scale

Marine species with planktonic larval durations of several months (teleplanic larvae) can potentially maintain demographic connectivity across large geographical distances. This perspective has important fundamental and applied implications, notably for the understanding of evolutionary and ecological processes in the marine realm, the implementation of marine protected areas, and fisheries management. Here we present, at the scale of the Northwest Atlantic, a spatial analysis of snow crab (Chionoecetes opilio, Majoidea) population genetic structure, a species that has a planktonic larval phase of 3 to 5 months. Eight microsatellite markers analysed on 847 C. opilio samples from 13 locations revealed an absence of significant genetic structure along the west coast of Greenland and within Atlantic Canada from southern Labrador to Nova Scotia. These results are consistent with a scenario of extensive demographic connectivity among C. opilio populations and have implications for the management of this species, which supports one of the most important Canadian and Greenlandic fisheries in terms of economic value. A genetic break is nevertheless identified between Greenland and Atlantic Canada, showing that genetic structure can develop within seas (the Labrador Sea in this case) despite the occurrence of very long planktonic larval stages.

Mating of Snow Crab Chionoecetes Opilio (Ο. Fabricius, 1788) (Decapoda, Majidae) in the Fjord of Bonne Bay, Newfoundland

Pairs of male and female snow crabs, Chionoecetes opilio, in mating courtship embrace were collected by SCUBA diving between 1984 and 1991, and with tangle nets between 1987 and 1993 in the Bonne Bay fjord, Newfoundland. A Nephrops trawl was also used to study the snow crab population size structure between 1988 and 1993. Males in mating courtship embrace were all identified as hard-shell morphometrically mature. Although large hard-shell juvenile males were present in the population, they did not participate in the reproduction. Comparison of the size structure of males in mating pairs and in the population also show that small mature male crabs can be excluded from mating by larger dominant mature males. The size structure between females in mating pairs and in the population was not significantly different. A depth segregation by size on the very steep slope of the fjord was observed for male snow crabs in mating pairs, as smaller males were found above 50 m. There was no correlation between male and female size in mating couples, but males were always larger than females. We conclude that there is a strong intra-specific competition between males during mating in nature.

Observations on the Occurrence of Hematodinium sp. (Dinoflagellata: Syndinidae), the Causative Agent of Bitter Crab Disease in Newfoundland Snow Crab (Chionoecetes opilio)

We describe the first occurrence in Atlantic Canada waters of "bitter crab disease" in the snow crab, Chionoecetes opilio. The condition is caused by a parasitic dinoflagellate, Hematodinium sp. Male crabs held in captivity harbored vegetative, prespore, and dinospore stages. Incidence of the disease was low (= <0.11%) off the eastern and northeastern coasts of Newfoundland in 1992-1993, except in Conception Bay, where 3.7% of 135 crabs examined were infected. The significance of Hematodinium sp. to snow crab populations in Newfoundland and to the commercial fishery is not yet clear, but it is prudent to monitor this parasite which has caused local depletion of Tanner crab populations in Alaska.

MOLT INDICATORS AND GROWTH PER MOLT FOR MALE SNOW CRABS (CHIONOECETES OPILIO)

ABSTRACT Examination of mouth parts is a standard method for determining the position in the molt cycle of individual male snow crabs. We showed that the color of the carapace can also be used to determine whether a crab is about to molt. A third, destructive, method is to tear off the dorsal carapace and look for the presence of a second carapace. During a bottom trawl survey in April 1991 in Conception Bay, Newfoundland, we found that green carapace color and a well-developed second carapace were strongly correlated and each was strongly associated with mouthpart classification D, or above. During a subsequent trap survey in May 1991, we found crabs with the normal reddish coloration that had seemingly partially formed second carapaces and mouthparts in the C or D0 stage. However, further work is necessary to describe and validate the early stages of carapace formation. We concluded that the collection of crabs with a green carapace is a viable way to obtain crabs for growth studies, because they will molt in the near future, thus minimizing effects of captivity. We collected 79 green crabs and maintained them in the laboratory until they molted. All but one molted within 24 days of capture. Growth per molt was similar to that estimated for another Newfoundland snow crab population, based on the same approach, but was greater than earlier estimates derived from longer-term laboratory studies.

Changes in the Composition of Snow Crab (Chionoecetes opilio) Participating in the Annual Breeding Migration in Bonne Bay, Newfoundland

Sexually-paired snow crabs (Chionoecetes opilio) participating in an annual spring breeding migration to shallow water in Bonne Bay, Newfoundland were collected by SCUBA divers from 1984 to 1989. In 1984 the breeding pairs were composed mainly of relatively large, old-shell animals of both sexes, but by 1988 much smaller animals, most of which were new-shell, dominated the migration. Evidence from research fishing indicates that these changes are related to a rapid decline in what was characteristically a virgin population in 1984 and a coinciding strong pulse of recruitment into the breeding component of the population. A very high incidence of small, subcommercial males participating in the 1988 and 1989 breeding migrations demonstrates a capacity for maintaining a high level of reproductive potential in a snow crab population despite high exploitation rates on larger animals in the male-only fishery for this species. The strong recruitment pulse which occurred in Bonne Bay as the virgin population declined after 1984 also indicates a capacity for enhanced annual production within a snow crab population when exploitation rates are high.

Functional Maturity in Small Male Snow Crabs (Chionoecetes opilio)

The mean size of male snow crabs (Chionoecetes opilio) in sexual pairs during the annual spring breeding migration to shallow water in Bonne Bay, Newfoundland, decreased from 118.6 mm carapace width (CW) in 1983 to 100.3 mm in 1987. This decrease is due to an increase in males &lt;95 mm CW participating from 1.5% in 1983 to 32.3% in 1987. This change appears to have resulted from a reduced abundance of commercial size [Formula: see text] males due to a rapid development of an illegal fishery on this previously unfished population and, as a consequence, less competition between males for possession of females. The percentages of spermathecae containing new spermatophores for females paired with males &lt;95 mm CW (67%) and those with males [Formula: see text] (79%) were not significantly different. We assumed that each female with new spermatophores had recently mated with the male with which it was paired. Observations on selected pairs in captivity showed that males &lt;95 mm CW are capable of mating with both primiparous and muitiparous females. Our results indicate that small mature males can replace large males in breeding activity in a snow crab population. We conclude that in the male-only snow crab fishery in Atlantic Canada with a minimum legal size of 95 mm CW, population reproductive potential is maintained at a high level despite high exploitation rates on males [Formula: see text].

Changes in Size Distribution of Snow Crabs (Chionoecetes opilio) in the Southwestern Gulf of St. Lawrence

Size distributions of the exploited snow crab (Chionoecetes opilio) population in the southwestern Gulf of St. Lawrence were obtained from commercial landings in New Brunswick ports during 1977-82. The unexploited part of the stock was sampled during research vessel surveys in 1980 and 1981. The size distributions were analyzed by factorial analysis of correspondence. The trend in mean size was negative from 1977 to 1981 and positive thereafter. The analysis identified two periods of recruitment to the standing stock within each year: the first at the beginning of summer and the second in late summer and autumn. Growth of the previouslyrecruited crabs occurred between these two periods. Analysis of research vessel data indicated that many prerecruits inhabited areas outside the usual fishing grounds. The hypothesis is proposed of recruitment by platoons, with a part being due possibly to migration of crabs from shallow unexploited areas to deeper water where the commercial fishery occurs.

Morphological, Electrophoretic and Fecundity Characteristics of Atlantic Snow Crab, Chionoecetes opilio, and Implications for Fisheries Management

For adult snow crabs (Chionoecetes opilio), from the western Gulf of St. Lawrence, eastern and western Cape Breton Island, and eastern Newfoundland, we compared morphometric, meristic, electrophoretic, and fecundity characteristics. Our morphometric, meristic, and fecundity data indicated that snow crabs from the four areas are morphologically and biologically distinct; therefore, they represent four "phenotypic" or "biological" stocks. We propose that the differences in morphology are due largely to environmental effects on growth during juvenile stages. The electrophoretic data indicate that Newfoundland and western Gulf of St. Lawrence snow crabs differ genetically from each other and from the Cape Breton Island snow crabs; therefore, they represent two different "genetic" stocks. Eastern and western Cape Breton Island snow crabs did not exhibit electrophoretic differences and, thus, they represent a single genetic stock. Genetic exchange between Atlantic Canadian snow crab populations appears possible through larval dispersal. There is a widely different degree of resilience to exploitation and response to the same management strategy between eastern and western Cape Breton Island snow crab populations; hence, a phenotypically and/or genotypically defined stock is not necessarily a useful management tool. Stocks may be subdivided into more meaningful management units that reflect intra-stock factors such as growth and recruitment patterns.