Background information and definitions
Trawling is a method of fishing that involves pulling a fishing net (trawl) through the water behind one or more boats. A beam trawl is a type of trawl where the mouth of the net is held open by a wooden or metal beam, which can be up to 14 m long. Beam trawls can negatively impact subtidal benthic invertebrates through direct physical damage, bycatch, and alterations to the seabed (Bergman & Van Santbrink 2000). A smaller beam could be used, which may potentially limit impact on subtidal benthic invertebrates through reduced damage to the seabed. Evidence related to the use of other fishing gear instead of a beam trawl is summarised under “Threat: Biological resource use – Use a pulse trawl instead of a beam trawl” and “Use an otter trawl instead of a beam trawl”.
Bergman M.J.N. & Van Santbrink J.W. (2000) Mortality in megafaunal benthic populations caused by trawl fisheries on the Dutch continental shelf in the North Sea in 1994. ICES Journal of Marine Science, 57, 1321–1331.
Supporting evidence from individual studies
A replicated, paired, controlled study in 1992-1995 in four areas of silty or sandy seabed in the south-eastern North Sea, Netherlands and Germany (Bergman & Van Santbrink 2000) found that using a smaller beam trawl caused similar mortality of invertebrates in the trawl tracks compared to using a larger beam trawl. Mortality using a 4-m beam trawl varied between 2 to 80% depending on species, similar to a 12-m beam trawl (1–82% mortality). Mortality did not differ across sediment type (sandy or silty). In spring-summer 1992–1995, parallel strips (2,000 x 60 m, 300 m apart, number unspecified) in one sandy location and three silty locations were fished with either a 12-m (commercially used) or 4-m beam trawl (both with tickler chains). Prior to trawling, mega-invertebrates (>1 cm) and macro-invertebrates (>1 mm) were counted from samples taken from each strip using a dredge and a sediment grab. After 24–48 h following trawling, all strips were sampled again using the same methods. Mortality (from trawling) of invertebrates present in the trawl tracks was calculated using the difference between the before and after-trawling abundances (assuming all animals killed by trawling had been eaten by predators).Study and other actions tested