Research Document 2019/007

Spatial distribution and demography of the green sea urchin, Strongylocentrotus droebachiensis, around Île Blanche and the eastern tip of Île aux Lièvres (Quebec) in 2011

By Sainte-Marie, B. and Paille, N.

Abstract

A benthic imagery survey was conducted in August 2011 between 2 and 18 m depth around Île Blanche and the eastern tip of Île aux Lièvres in the Upper St. Lawrence Estuary. The objective was to characterize the bottom and measure the abundance and size structure of the green sea urchin, Strongylocentrotus droebachiensis, in order to estimate the legal biomass in the area on the south side of these islands where commercial fishing occurs (Area 8). The conversion of the numbers and sizes of urchins measured by imagery into biomass was based on a weight-length relationship derived from green sea urchin samples collected using scuba diving on both sides of the islands. Bottoms less than 4–5 m deep including the passage between the two islands, supported seagrass beds with abundant kelp. The density and mean size of legal-size green sea urchins (≥ 50 mm) were higher on the north side than on the south side of the islands. Between 2008 and 2011, fishing on the south side was concentrated in a territory of 0.794 km2, in which the survey showed a fairly high mean abundance of legal-size sea urchins (about 15 individuals per m2). The maintenance of high densities of sea urchins in this territory could be explained by a more favourable environment for growth due to a fairly regular influx of macroalgae from the passage between the two islands. It is also possible that this territory is supplied by a sporadic influx of large sea urchins from the north side via the passage between the two islands as a result of storm surges combined with strong tidal currents. Sea urchins of non-legal size, quite abundant near this fishing territory, represented a potential recruitment. The biomass of legal-size green sea urchins in the fishing territory on the south side was estimated at 1,233 ± 311 t.

Research Document 2019/007 (PDF, 2,321 KB)