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 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
Shallow inlets and bays
- 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
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.
Riemann B & Hoffman E. (1991). Ecological
consequences of dredging and bottom trawling
in the Limfjord, Denmark. Marine Ecology Progress
Series 69: 171-178.
Shallow inlets and bays
Observation of standard and spring-loaded
Habitat effects - Bottom deposits
settled about 20 mins after hauling. Short
teeth of these dredges dug in up to ½ to ¾
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.
- 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.
Jefferson F.A. & Currey B.E. (1994).
Global review of porpoise. (Cetacea: Phocoenidae)
mortality in gill nets. Biological Conservation
- 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
Polacheck T. (1989). Harbour porpoises and
the gill net fishery. Oceanus 32: 63-70