Effects of Chronic Bottom Trawling on the Size Structure of Soft-Bottom Benthic Invertebrates
Robert A. McConnaughey, Stephen E. Syrjala, and C. Braxton Dew
Abstract. Chronic bottom trawling reduces benthic biomass, but it is generally unknown whether this represents a decrease in numbers of individuals or their mean body size. Because this distinction provides insight into the mechanisms of disturbance and recovery, we investigate the matter here. Using comprehensive historical effort data, adjacent untrawled (UT) and heavily trawled (HT) areas were identified along the boundary of a long-standing no-trawl zone in Bristol Bay, a naturally disturbed offshore area of the eastern Bering Sea. The study site was shallow (44–52 m) with a sandy substrate, ubiquitous bottom ripples, and strong tidal currents. A modified research trawl was used to collect 42 HT–UT paired samples of benthic infauna and epifauna. These data were used to compare mean sizes (kg) of 16 invertebrate taxa. On average, 15 of these taxa were smaller in the HT area, and the overall HT–UT difference in body size was statistically significant (P = 0.0001). However, individually, only the whelk Neptunea spp. (P = 0.0001) and Actiniaria (P = 0.002) were significantly smaller in the HT area after correcting for multiple tests. Mean size of red king crabs Paralithodes camtschaticus was 23% greater in the HT area (P = 0.17). Supplemental length–frequency data indicate that substantially fewer small red king crabs, rather than more large individuals, occupy the HT area (P = 0.0001). Finally, a large number of within-year, within-taxon comparisons of mean body size were made using 1982–2001 U.S. National Marine Fisheries Service trawl survey data collected in the same closed area. Overall, these comparisons indicate natural variability of body size in UT areas is large relative to the observed HT–UT differences due to chronic bottom trawling. Since active fishing in the HT area occurred 3 or more years before our field sampling program, our findings reflect conditions associated with an intermediate level of recovery.