Snow Crab Trap Survey to Ensure Sustainability

In 2008, nearly 8,000 tonnes of snow crab were caught in the estuary and the Gulf of St. Lawrence, up approximately 2,000 tonnes from 2004 and down nearly 2,500 tonnes from 2002, reports http://www.megafishnet.com/ with reference to Fisheries and Oceans Canada.

Catches have fluctuated by about 10% from one season to the next. These are not random fluctuations: the amount of crab caught corresponds to the authorized catch level for the region. Just how are these annual catch levels established? That is one of the tasks handled by Jean Lambert, a biologist with the Maurice Lamontagne Institute in Mont-Joli (Québec), who is in charge of data analysis in the snow crab stock assessment process for the estuary and the northern Gulf of St. Lawrence.

To ensure the long-term sustainability of the resource, Fisheries and Oceans Canada (DFO) uses a number of measures to manage the snow crab fishery. The total allowable catch is the most important measure. Other management measures include the minimum legal size, limits on the number of licences and traps, the length of the fishing season and the closure of the fishery based on certain criteria. To establish the allowable catch rates for the coming season, DFO estimates the quantities of snow crab that will be available the following year. The available biomass is estimated partly on the basis of commercial fishery data. However, the main source of data is the trap survey, which is independent of the fishery. In each of the nine fishing areas covering the estuary and the northern Gulf of St. Lawrence, crab traps are installed on the sea bottom at different depths. The traps are hauled up after 24 hours to check them. By averaging the catches for each category of crab and comparing the results with the data from previous years, the Department estimates the amount of crab that can be fished the following season.

These trap surveys are an effective method for determining snow crab abundance, particularly the abundance of legal size crab (95 mm carapace width or larger). This method can also be used to calculate an abundance index for adolescent crab (carapace width of 78 to 95 mm), which will reach minimum legal size after their next moult. For the smallest crabs-those under 78 millimetres-less precise data are obtained. The large crabs that crawl into the traps scare away the smaller ones. As a result, small crabs are under-represented in traps relative to their natural abundance in the ocean. Trap surveys nonetheless make it possible to determine the medium-term trend for adolescent crabs-those that will reach minimum legal size in subsequent fishing seasons. Fishers are required to return all sub-legal size crabs to the water.

It can take nine years for a snow crab to reach the legal size. Snow crabs moult, or shed their exoskeleton (carapace) several times a year when young but only once per year from the adolescent stage to the adult stage. When fully grown, a crab goes through its terminal moult and its carapace does not get any larger. Some individuals may not reach the minimum legal size by the time they stop growing. They can count themselves lucky, since they will never be fished!