Marine mammal bycatch
Marine mammals can be divided into two groups in UK waters:
whales, dolphins and porpoises (collectively known as cetaceans)
and seals (pinnipeds). Although generally not considered to be
marine mammals, otters also occur in estuarine and coastal waters.
Otters have been caught and drowned in fyke nets set for eels and
in creels/pots set for crustaceans. All fyke nets must now be
fitted with otter guards which are provided by the Environment
Agency. The introduction of mandatory otter guards in fyke nets has
reduced the otter bycatch, but illegal nets still catch otters on
an annual basis.
Very little data exists on the bycatch of seals in UK waters,
but bycatch deaths are thought to be in the low hundreds per year.
These are mainly grey seals associated with the herring and
mackerel fisheries. In a recent study, 2% of tagged seals were
killed in fishing gear, mainly gill and tangle nets. The impact of
this bycatch level on the seal population is not thought to be
significant. However, deaths from shooting and those associated
with fishfarms have not been taken into account and may be
significant in some areas.
In contrast, cetaceans are one of the more high profile
victims of accidental capture in fishing nets. It was estimated
that almost 60,000 cetaceans are killed globally each year through
entanglement in fishing gear. In European waters, there have been a
number of studies of such incidental bycatch. These, though not
complete or comprehensive, indicate that the bycatch of cetaceans
is widespread, and may threaten the integrity of dolphin and
porpoise populations. In particular, the harbour porpoise bycatch
from bottom-set gill nets is estimated to be over 7000 animals
annually in the North Sea. This exceeds 2% of the population and is
considered unsustainable. Dolphins tend to be caught more often in
pelagic trawls. For example, large pelagic trawlers targeting horse
mackerel south west of Ireland are known to catch white-sided and
common dolphins and long fin pilot whales, with a catch rate of one
dolphin per 93 towing hours. Whilst observers recorded 91 common
dolphins caught in 313 hauls in the pelagic trawl fisheries for
bass (south west England) between 2001 and 2003. Prior to the
introduction of EU legislation to ban the use of driftnets for
tuna, dolphins, particularly stripped and common, were caught in
large numbers (over 750 individuals in 1420 hauls). In contrast,
larger whales may break free of fishing gear. However, injuries and
entanglement in broken warps etc may cause illness, starvation and
death at a later date.
The UK is party to several agreements with legal obligations
to protect small cetaceans, including the EU habitats directive,
the Agreement of the Conservation of Small Cetaceans of the Baltic,
North East Atlantic, Irish and North Sea (ASCOBANS) and the UN
Convention on the Law of the Sea. Technical, educational and legal
measures can be taken to avoid or reduce cetacean by-catch. Most
fishermen would not want to catch cetaceans, so a number of
projects have been undertaken with their co-operation to test
measures such as the attachment of acoustic devices ('pingers') to
nets to alert cetaceans of the presence of the nets. More recently,
tests have been made on devices to allow dolphins to escape from
pelagic
trawl nets.

Pingers © Simon Northridge/SMRU
In March 2004, the European Commission introduced a new regulation
aimed at reducing the bycatch of harbour porpoises in bottom set
gillnets and entangling nets. From the summer of 2005, pinger use
will become mandatory on bottom-set gillnets or entangling nets in
the North Sea and the Skaggerak & Kattergat region deployed
from vessels greater than 12m in length. Similar rules will apply
to the western English Channel and South Western Approaches from
January 2006, and to the east English Channel from January 2007.
This regulation also made provision for the monitoring of dolphin
bycatch in trawl fisheries from January 2005 in the English
Channel, Irish Sea and off western Britain and Ireland, and from
January 2006 in the North Sea and west Scotland.
Further Reading:
ASCOBANS, 1997. Cetacean by-catch issues in the ASCOBANS area.
Unpublished report of the ASCOBANS advisory committee working
group on by-catch. 23pp.
Barlow, J. & Cameron, G.A., 2003. Field experiments
show that acoustic pingers reduce marine mammal bycatch in the
Californian drift gill net fishery. Marine Mammal Science, 19,
265-283.
Berrow, S.D., O'Neill, M. & Brogan, D.,
1998.Discarding practices and marine mammal by-catch in the
Celtic Sea herring fishery. Biology and Environment
Proceedings of the Royal Irish Academy, 98B, 1-8.
Berrow, S.D., Tregenza, N.J.C. & Hammond, P.S., 1994.
Marine mammal bycatch on the Celtic shelf. DG XIV/C/1
study contract 92/3503.
Berggren, P., Wade, P.R., Carlstrom, J. & Read, A.J.,
2002. Potential limits to anthropogenic mortality for harbour
porpoises in the Baltic region. Biological Conservation, 103,
313-322.
Cox, T.M., Read, A.J., Swanner, D., Urian, K. & Waples,
D., 2004. Behavoural responses of bottlenose dolphins, Tursiops
truncatus, to gillnets and acoustic alarms. Biological
Conservation, 115, 203-212.
Cox, T.M., Read, A.J., Solow, A. & Tregenza, N., 2001.
Will harbour porpoises (Phocoea phocoena) habituate to
pingers? Journal of cetacean Research and Management, 3,
81-86.
Dawson, S.M., ead, A. & Slooten, E., 1998. Pinger,
porpoises and power: uncertainities with using pingers to reduce
bycatch of small cetaceans. Biological Conservation, 84,
141-146.
House of Commons Environment, Food and Rural Affairs
Committee, 2004. Caught in the net: by-catch of dolphins and
porpoises off the UK coast. Third Report of Session
2003–04.48pp.
Kuiken, T., Simpson, V.R., Allchin, C.R., Bennett, P.M., Codd,
G.A., Harris, E.A., Howes, G.J., Kennedy, S., Kirkwood, J.K., Law,
R.J., Merrett, N.R. & Phillips, S., 1994. Mass mortality of
common dolphins (Delphinus delphis) in south west England due to
the incidental capture in fishing gear. The Veterinary Record,
134, 81-89.
Lopez, A., Pierce, G.J., Santos, M.B., Gracia, J. &
Guerra, A., 2003. Fishery by-catches of marine mammals in
Galician waters: results from on-board observations and an
interview survey of fishermen. Biological Conservation, 111,
25-40.
Morizur, Y., Berrow, S.D., Tregenza, N.J.C., Couperus, A.S.
& Pouvreau, S., 1999. Incidental catches of marine mammals
in pelagic trawl fisheries of the northeast Atlantic.
Fisheries Research, 41, 297-307.
Prochnow, G. & Kock, K-H., 2000. The protection of
harbour porpoise (Phocoena phocoena) in the waters off Sylt and
Amrum (German Wadden Sea): A baseline study. Archive of
Fishery and Marine Research, 48, 195-207.
SCOS, 2003. Scientific advice on matters related to the
management of seal populations. Natural Environment Research
Council.
Silvani, L., Gazo, M. & Aguilar, A., 1999. Spanish
driftnet fishing and incidental catches in the western
Mediterranean. Biological Conservation, 90, 79-85.
Spencer, N., Santos Vázquez, M.B. & Pierce, G.J., 2001.
Evaluation of the state of knowledge concerning by-catches of
cetaceans. Final Report Tender No XIV/1999/01 Lot 7(31/12/99 -
31/10/00). 212pp
Tregenza, N.J.C., Berrow, S.D., Hammond, P.S. & Leaper,
R., 1997. Harbour porpoise (Phocoena phocoena L.) by-catch in
set gillnets in the Celtic Sea. ICES Journal of Marine
Science, 54, 896-904.
Vinther, M., 1999. Bycatches of harbour porpoises
(Phocoena phocoena L.) in Danish set-net fisheries. Journal of
Cetacean Research and Management, 1, 123-135.
Wade, P.R. 1998. Calculating limits to the allowable
human-caused mortality of cetaceans and pinnipeds. Marine
Mammal Science, 14, 1-37.