Cease or prohibit shellfish dredging
Overall effectiveness category Awaiting assessment
Number of studies: 1
Background information and definitions
Dredging is done to harvest edible shellfish species (e.g. mussels, clams, scallops, crabs) and is undertaken globally and involves towing a dredge along the seabed. Towed shellfish dredges are usually constructed from a heavy metal frame covered with chain mesh and they vary in size and design depending on the target species. Because of their heavy construction and deployment on the seabed, dredges can cause considerable disturbance, reducing water quality and damaging the seabed, and this has been blamed for decreases in catches of fish (Hoffmann & Dolmer, 2000). They may also capture or damage small unwanted fish. Inside areas where dredging is prohibited, its indirect (disturbance) and direct (fishing mortality) effects on fish are removed, although fishing using other methods may still impact the fish species and populations.
For a related intervention, see ‘Cease or prohibit mobile fishing gears that catch bottom (demersal) species and are dragged across the seafloor’.
Hoffmann E. & Dolmer P. (2000) Effect of closed areas on distribution of fish and epibenthos. ICES Journal of Marine Science, 57, 1310–1314.
Supporting evidence from individual studies
A before-and-after, site comparison study in 1981–1998 of a fjord in the North Sea, Denmark (Hoffmann & Dolmer 2000) reported that prohibiting all towed fishing gears (mainly mussel dredges) in an area had no effect on the abundance and species richness of bottom-dwelling fish in the following 10 years, and compared to open areas. Data were not statistically tested. In trawl surveys, fish abundance (closed: 0–13 kg/30 min, open: 0–31 kg/30 min) and number of species (closed: 4–11, open: 1–9) varied between years but no effect of the closure was detected in either area. In set net and trap samples, catch rates were higher in the fished area (closed: 37–486 g/fishing unit, fished: 132–915 g/fishing unit) but there was no difference in the number of species (closed: 3–8, fished: 4–8). In 1988, a 40 km2 mussel Mytilus edulis fishing ground in the Limfjord was closed to all towed fishing gears (to prohibit mussel dredging as the only towed gears in use) and only static fishing gears allowed. Fish data was collected by two methods: annual trawl surveys from 1981–1998 in August/September at two stations inside and two just outside the closed area; and in 1995, 1996 and 1997, experimental fishing with fixed set nets (48 deployments) and eel traps (38 deployments) at three locations inside and three outside the closed area. Catch rates and number of species were recorded. No fish species groups (other than demersal) or individual species were specified.Study and other actions tested