The Net Effect? - Whale and Dolphin Conservation Society

12
The Net Effect?
A WDCS report for Greenpeace
7,366 in 1994, reducing to a low of 3,887
porpoises in 2001. The figures provided for
2000 and 2001 are assumed to be overestimates
(by 570 and 405 animals respectively)
as they do not take into account effects of the
mandatory use of pingers in the cod wrecknet
fishery that was introduced in 2000
(Vinther & Larsen 2002). The highest annual
catch figure of 7,366 represents 4.3% of the
porpoise population in the relevant part of
the North Sea (170,000 animals) as estimated
by SCANS (Hammond et al. 1995).
Harbour porpoises are also caught in the
North Sea by UK bottom-set gillnet and
tangle-net fisheries targeting cod, sole, skate
and turbot. Observer studies of these fisheries
have allowed bycatch levels to be assessed for
the period 1995-99, with annual catches estimated
to total from 818 porpoises in 1995 to
436 animals in 1999 (CEC 2002a). While
porpoise bycatch rates were found to be
highest in the skate fishery, the estimated total
porpoise mortality was greatest in the inshore
cod fishery due to the very large fishing effort
(Northridge & Hammond 1999). The
decrease in bycatch levels over the study
period is attributed to the decline in fishing
effort (measured in days at sea).
The Celtic Sea bottom-set gillnet fishery for
hake presents one of the most acute bycatch
problems for harbour porpoises recorded in
the north-east Atlantic. The UK and Irish setnet
fisheries in the Celtic Sea were investigated
using onboard observers from 1992 to
1994. This study recorded a catch of 43
porpoises, all but one of them caught in
hake gillnets and one caught in a tangle net.
Extrapolation to the total UK and Irish fleets
produced an estimated total mortality of
2,200 harbour porpoises per year, which
represents 6.2% of the estimated population
in the Celtic Sea (Tregenza et al. 1997a).
Latest strandings figures from the UK
demonstrate that harbour porpoise strandings
have been steadily increasing in the UK
since the beginning of the 1990s, with more
marked increases in 2001 and 2002 (Sabin et
al. 2003). In particular, there has been a
general increase over this period in the
number of bycaught harbour porpoises
that stranded annually in south-west
England. There has also been an increase
in the number of porpoises reported stranded
around the south-west in the winter months
and into early spring. Bycaught harbour porpoises
that strand in the UK typically show
external signs consistent with wide-meshed
monofilament-type gear (gillnets), in contrast
to common dolphins which tend to have
injuries consistent with smaller-meshed gear
such as trawl netting. However, it is notable
that many of the bycaught harbour porpoises
examined in the first quarter of 2002 lacked
the usual monofilament-type net marks. It is
suggested that these porpoises may have been
caught in smaller-mesh mobile gear (trawl
nets) since they often stranded in the same
areas and in a similar state of decomposition
to large numbers of common dolphins that
were diagnosed as having died due to
bycatch (Sabin et al. 2003).
The Baltic Sea, where the population is
already severely depleted, is an area of considerable
concern for the harbour porpoise.
Bycatch is considered to have played an
important role not only in reducing the abundance
of porpoises, but also in preventing
their recovery (e.g. Berggren et al. 2002).
Bycatches of harbour porpoises are known to
have occurred in many parts of the Baltic in
salmon driftnets and bottom-set gillnets (for
cod and other demersal species) (ASCOBANS
2002). As the density of porpoises in the
Baltic is now extremely low, the animals are
only rarely seen or caught, which makes
assessing bycatch rates extremely difficult.
However, porpoise bycatches have been
reported in recent years in Swedish driftnets,
various Polish gillnet fisheries and Finnish
fisheries (CEC 2002a). It has been estimated
that the current minimum bycatch is seven
porpoises per annum (Berggren et al. 2002).