Impacts of otter trawling on infaunal bivalves living in sandy bottom habitats on the Grand Banks

Abstract

Otter trawling has been the prevalent method of ground fishing in Atlantic Canada. Despite this, little is known about associated impacts to benthic habitat and communities. This thesis examines the impacts of otter trawling on bivalves living in sand, a bottom type occurring widely over the Grand Banks. Bivalves are dominant members of sandy bottoms on continental shelves and are good indicators of physical disturbance. Two broad categories of impacts to benthos from mobile fishing gear are direct harvest and incidental damage. Bivalves have low susceptibility to capture by groundfish otter trawls; trawl capture efficiency is on the order of 10⁻⁵. In order to investigate incidental impacts, a three-year otter trawling experiment was conducted on a fine to medium sand bottom on the northeastern Grand Bank. Each year a total of 12 trawl passes were made along the centre line of two 13 km by 200 m experimental corridors. Sampling was conducted inside experimental and adjacent reference corridors with a 0.5 m² hydraulic grab. No significant effects of trawling were detected on bivalve populations in any year. Shallow burrowing species showed no significant changes in density or biomass and recruitment of juveniles ≤3 mm was apparent inside trawled corridors. The size structure of populations from trawled and reference areas were similar. Mean (± sd) percent major shell damage immediately after trawling was low, ranging from 2.8 ± 6.1% to 13.5 + 9.4%. Trawl doors are the most destructive gear component of otter trawls. A physical trawl door model was towed through an artificial sand testbed, constructed to resemble an offshore seabed. Although bivalves within the scour path were displaced, levels of damage were low (c. 5%), similar to levels of damage from the combined effects of all gear components recorded in the field experiment. The anomaly of displacement, accompanied by few instances of damage, is explained by sediment mechanics associated with scouring and size and life position of infaunal bivalves. Compared to natural sediment-mediated disturbances, otter trawling can be manipulated over a wide range of frequencies. Individual and population-level adaptive traits probably confer considerable stability to sandy bottom bivalve populations exposed to typical patterns of trawling activity on the Grand Banks.