No-Trawl Zones Benefit Great Scallops in the Isle of Man


Photo copyright Ian Woolner

The great scallop is important to commercial fishermen in the Isle of Man; a rugged outcrop in the Irish Sea bordered by Ireland, Scotland, England, and Wales. It was here that a small, 2 square kilometer inshore area was closed to destructive trawling in 1989. Because scallops stay in one place for much of their relatively long lives and grow rapidly, it was expected that populations would rebound easily with protection. Given that great scallops reproduce by releasing vast numbers of larvae into the water column, which can drift for up to 40 days before settling and maturing on the sea floor, recovery was anticipated both inside and outside the protected areas.

A group of Irish scientists jumped into the cold waters to count scallops, categorizing them by size and age to determine the health of populations inside and outside of the no-trawling area. In addition, scientists trawled a small subset of the protected region to directly measure the effects of dredging on great scallop populations.

The no-trawl area clearly benefits great scallops. This study shows that after 14 years of protection, scallops were more abundant and much larger inside protected areas. In fact, the overall density of great scallops was nearly 5 times higher than in the fished area, and the density of legal-sized scallops (> 110 mm in diameter) was nearly 10 times higher. Inside protected areas older, more reproductively mature scallops thrived; the reproductive potential was 12.5 times higher inside the no-trawl zone. This reproduction reservoir suggests that some of the larvae produced inside the protected area will settle in fishable waters and help rebuild nearby exploited populations. This “spillover” effect is supported by the increase in commercial catch-per-unit-effort, a metric used to measure target species populations, in the fishable waters near the protected area.

For the great scallop in the waters off the Isle of Man, this small no-trawl zone is a classic win-win. By protecting a small area of habitat, not only have the populations within the no-trawl zone recovered, the larvae of these older, larger scallops will help rebuild populations that will provide a livelihood to trawlers in the future.

To read more:
Beaukers-Stewart, B.D., B.J. Vause, M.W.J. Mosley, H.L. Rossetti, A.R. Brand. 2005. Benefits of closed area protection for a population of scallops. 298:189-204. (422K PDF file)