Sea Scallop Research - Lifehistory Parameters:
Growth, Mortality, and Movement Estimates

Growth and Movement

Graduate student Brad Harris is examining the growth and movement of sea scallops in the open and closed areas of Georges Bank. The sea scallop resource is presently managed using a yield-per-recruit model which predicts how much harvestable meat a scallop can produce in its life-time. The scallop resource on Georges Bank is considered a single stock and therefore has a single growth equation (32nd SAW, January 2001). The 2001 National Marine Fisheries Service (NMFS) Stock Assessment Review Committee stated that predicted scallop growth failed to describe observed shell height frequencies on Georges Bank.

Following individuals through time provides the most precise measure of sea scallop growth. Therefore, we tagged approximately 11,700 scallops and released them in the Great South Channel (GSC) just north of the Nantucket Lightship Closed Area (NLSA) during May-June 2001. To date 1236 tags have been returned from the 2001 experiment. We conducted a second experiment tagging 20,000 scallops in September 2002 and released them 50 nautical miles to the north of the 2001 tagging experiment. To date 790 tags have been returned from the 2002 experiment. The preliminary information from these experiments suggest site specific growth occurs on Georges Bank with a variation in the yield per recruit equation of as much as 20%. This difference in growth rate for just the scallop in the NLSA could mean an increase in the biomass estimate of 4.16 million lbs equal to $20 million in harvest for one year.

Photograph: Brad Harris (front), students and fishermen tagging scallops on board the F/V Liberty in September 2002.

Mortality

Sea scallops have many natural predators including, lobsters, crabs, and fishes, but their primary predator is the sea star. Scallop fishing is also considered a form of predation of sea scallops. Therefore, fishing and natural mortality must both be considered in determining the total mortality of a fishery to avoid overexploitation. However, a change in the management of a fishery, such as area closures, eliminates the fishing mortality from these areas, but natural mortality continues. The effects of these closures on these predator – prey interactions is unclear. However, it is clear that 80% of the Georges Bank sea scallop resource is in the closed areas. Are natural predator densities, such as sea stars, increasing in these closed areas?

Graduate student Mike Marino is studying sea star–sea scallop predator-prey interactions on open and closed areas of Georges Bank. He uses the information collected by SMAST scientists and the local scallop industry during video surveys conducted between 1999-2002. This research will try to determine: 1) if sea stars are attracted to these closed areas, 2) if groups of sea stars are feeding and migrating between clumps of sea scallops, and 3) if sea stars are larger in areas where high numbers of sea scallops are found. Additionally, scallop fishermen have indicated that higher levels of natural mortality occur in specific areas of Georges Bank, where large numbers of clappers (dead scallops from natural mortality) are found. Therefore, this research will also try to determine if predation of sea scallops is site-specific, causing high levels of localized natural mortality.

Photographs: left, sea stars show their technique for prying open a hard shell; right, sea stars and scallops on the sea floor in Closed Area II.

Recruitment

The three closed areas on Georges Bank present an opportunity to study what happens to a shellfish population in the absence of fishing. We know that the abundance of scallops in these areas has increased dramatically. We also know that the natural environment imposes a limit on the maximum size that a population can obtain (this is called the carrying capacity). What we don’t know is what causes this. Is it predation or resource limitation? Graduate student Jake Nogueira is examining the effects of these very high densities on recruitment success, which is a measure of how many young scallops are coming into the fishery. It may be that high densities of scallops are limiting recruitment by utilizing resources, or by some other mechanism. It is important to understand the recruitment process because it is the future of the population and, hence, the fishery. With some standardization the video survey can be used as an alternative measure of recruitment.

In our video surveys few small scallops (<90 mm) were observed in the Nantucket Lightship Area. However, the density in this area is increasing each year (From our video surveys: 0.38 scallops m-2 in 1999, 0.40 scallops m-2 in 2000, and 0.58 scallops m-2 in 2001). Applying the von Bertalanffy growth equation to the shell height frequencies we observed in 1999, it appears that there was a 19% increase in recruiting scallops (50-90 mm) in 2000 over 1999. Where have these recruits come from? Small scallops of this size range were observed 50 nautical miles to the North of the NLSA in the Great South Channel, which is open to fishing. In some scallop populations, juveniles are thought to settle in one area and then migrate into the adult bed. Previous tagging studies on Georges Bank show that scallops can move as much as 15 km in 5 years primarily as a result of the water current direction. The sea scallop is one of the strongest swimmers of all the scallop species. Could recruitment into the Nantucket Lightship Area be the result of migration from the Great South Channel? We are examining this question and also the relationship of adult densities to juvenile recruitment ratios using the video surveys and tagging studies.