Summaries of reviewed publications references 31 35.

Details are limited to information relevant to the UK marine habitats and species listed in the Habitats Directive and the Birds Directive.

[pr] indicates that the paper is from a peer reviewed journal or report


Natura 2000 Habitats & Species

Fishing Technique




REF: 31

Harbour porpoise

Gill nets

Species and community effects - Study using reports of incidental catch of harbour porpoise. Most are killed in monafilament gill nets set for groundfish or pelagic species.

Estimated total catch for the year in the area (based on notifications by fishermen) was 105+10.8 animals. The animals were entangled while nets on the bottom in water depths of 37-96m. They seem to catch certain size classes and not small or large animals. Factors other than fishing effort may also have effected the incidental catch rate of harbour porpoise. In one area it was disproportionately high, perhaps reflecting the high density of porpoises in the region.

There were no changes in porpoise density in the region between 1980-86, but two significant changes in length frequencies (increase in length of calves and absence of large porpoises in the recent samples). These changes may be attributed to the fishery which has been operating for 10-15 years. The effects of sustained adult mortality in the gill-net fishery appear to have compressed the size and possibly the age structure of the population perhaps reducing the reproductive lifetime of females. Given the slow reproductive rate authors consider that these incidental catches seriously threaten the population as porpoises in Bay of Fundy and Gulf of Maine apparently form a relatively discrete population unit.

South-western Bay of Fundy, Canada

Read A.J. & Gaskin D.E. (1988). Incidental catch of Harbour Porpoises by gill nets. Journal of Wildlife Management 52: 517-523


REF: 32

Shallow inlets and bays

Mussel dredging

Bottom trawling

Habitat effects - Effects of mussel dredging and bottom trawling on particulate material, internal nutrient loads and oxygen balance were investigated. Sampling 0, 30 & 60 mins after fishing. Immediately after mussel dredging suspended particulate material increased significantly but 30 mins after the differences had decreased and, after 60 mins, had returned to the start level. Oxygen decreased significantly after mussel dredging and average ammonia content increased but large horizontal variation in the ammonia content prevented detailed interpretation of these increases. Changes in other nutrients were small. Changes in particulate matter and nutrients were also observed at some stations following low wind. Particulate matter and total phosphorus were markedly higher on windy days.

Most dredging and trawling in the Limfjord takes place in summer when there is little wind, nutrients and oxygen consumption are low and temperature high. During these periods trawling and particularly dredging reduce the water quality by increasing internal nutrient loads, oxygen consumption and possibly phytoplankton primary production. Immediate increase in particulate matter, oxygen consumption and increase in nutrients particularly ammonia and silicate were a further effect of the fishing activities. Physical effects were scraping and pressure of gear the magnitude depending on depth of penetration, frequency of fishing and structure of sediment.

Species and community effects - Trawling and dredging can be expected to cause a number of direct and indirect changes in the ecosystem - direct changes in fished populations and the benthos, but also changes in the nutrient level and oxygen budget in the water column. Phytoplankton primary production may increase if nutrients are the controlling factor. During summer when nutrients are generally low in the fjord mixing of sediments will have important consequences for the nutrient regime. It caused the deterioration of the water quality by increasing oxygen consumption and phytoplankton primary production. It was difficult to demarcate trawling and dredging effects versus wind induced effects at this site.

Limfjord, Denmark

Riemann B & Hoffman E. (1991). Ecological consequences of dredging and bottom trawling in the Limfjord, Denmark. Marine Ecology Progress Series 69: 171-178.


REF: 33

Shallow inlets and bays


Scallop dredging

Observation of standard and spring-loaded dredges.

Habitat effects - Bottom deposits settled about 20 mins after hauling. Short teeth of these dredges dug in up to to of their length and generated a large mound of sediment in front of the toothed bar. Most was deposited around the sides of the dredge and at times completely filled the dredge opening, particularly when large stones or shells blocked some of the gaps between the teeth. Dredge tracks were distinct, ridges of sediment being deposited each side, but path of the spring-loaded dredge less obvious than standard dredge.

Species and community effects - The dredges caused some damage to benthic organisms. Most hauls had a few crabs Cancer pagarus, and starfish eg Marthasterias glacialis broken up by the gear. The teeth also dug out several sub-surface animals including heart urchins Spatangus purpureus and the mollusc Laevicardium crassum. These and other organisms raked up by the teeth appeared to attract several fish and invertebrate predators including juvenile cod adult plaice and dogfish, whelks and hermit crabs.


Chapman C.J., Mason J. & Drinkwater J.A.M. (1977). Diving observations on the efficiency of dredges used in the Scottish fishery for the scallop, Pecten maximus (L). Scottish Fisheries Research No. 10 16pp.

REF: 34

Harbour porpoise

Gill nets

Species effects - Harbour porpoises are taken throughout their range and several populations are in decline, at least partly as a result of gill net entanglement. In the eastern North Atlantic substantial numbers are caught in gill nets in most areas. Highest known takes in Norway, Sweden and Denmark. UK also has substantial takes in gill nets as well as other fisheries.

There are reports of harbour porpoise being caught in cod, salmon and whitefish gill nets off the Scottish coast, and in salmon drift nets and inshore set nets off NE England.

Gill nets (which include set nets, drift nets and trammel nets) are considered to represent the single most important threat to porpoises as a group. Most porpoises have substantial problems with them. Harbour porpoise, for example, are found primarily in shallow waters, mostly nearshore which is the area where this form of fishing is generally practised.

North Atlantic

Jefferson F.A. & Currey B.E. (1994). Global review of porpoise. (Cetacea: Phocoenidae) mortality in gill nets. Biological Conservation 76: 167-183.


REF: 35

Harbour porpoise

Gill nets

Species effects - Harbour porpoise are one of the more vulnerable marine mammals to incidental capture by commercial fishing gear and are particularly prone to entanglement. Nearshore habitats, small size and diet of commercially harvested fish contribute to the magnitude of the incidental and/or directed takes occurring through most of their range.

Global review

Polacheck T. (1989). Harbour porpoises and the gill net fishery. Oceanus 32: 63-70



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