The Net Effect? - Whale and Dolphin Conservation Society

44
The Net Effect?
A WDCS report for Greenpeace
In Denmark, following various trials in the
late 1990s, the Danish action plan to reduce
bycatch of porpoises in the North Sea
(Ministry of Environment and Energy 1998)
was adopted. As part of this, a regulation
was introduced in 2000 requiring pingers
to be used in all Danish bottom-set gillnet
fishing using nets up to 300m long in the
North Sea from August to October (Larsen
et al. 2002a). In effect, this requirement
applies only to wreck-net fishing for cod,
and in the period of highest observed
porpoise bycatches. Observations during
2000 and 2001 recorded no porpoise
bycatches in 129 wreck-net sets with pingers,
whereas two porpoises were caught in 11
wreck-net sets without pingers during the
same period in 2000 (Larsen et al. 2002a).
The researchers conclude that the use of
pingers in the Danish North Sea wreck-net
fishery has eliminated bycatch of harbour
porpoises, although they acknowledge that
even with functional pingers on the nets they
should expect to see some porpoise catches
in future. They also note that it is important
to continue monitoring the wreck-net fishery
to assess the efficacy of pinger use and any
signs of porpoise habituation.
Recently there has been some research
into interactive pingers, where the deterrent
sounds are triggered by the sonar clicks of
the approaching porpoises (Amundin et al.
2002). This approach aims to address several
concerns as it reduces noise pollution by only
transmitting sounds when they are needed,
and thus also delays habituation. To date,
results of this work have only been reported
from trials carried out using captive harbour
porpoises in Denmark (CEC 2002a),
although sea trials were scheduled for
summer 2002.
4.2.3 Acoustic deterrent devices
(pingers) in pelagic trawls
The potential for pingers to reduce bycatch
of cetaceans in pelagic trawl nets has been
investigated recently. In response to the high
dolphin catch rate recorded in the UK sea
bass pelagic pair-trawl fishery, pingers were
deployed during 2001, the first year of mitigation
trials. Dukane pingers were simply
placed around the mouth of the trawl of one
pair with the intention of deterring animals
from entering (Northridge 2003b). However,
the rate of dolphin bycatch was found to be
higher in the 15 tows conducted with pingers
than it was in the 37 tows without pingers.
This trial was therefore abandoned. During
further trials in the UK sea bass fishery in
2003, Aquamark pingers were deployed,
this time placed well back in the trawl in
order to deter animals from swimming into
the rear part of the net. The pingers were
intended to be removed and replaced on a
daily basis on two of the observed pairs.
However, rigorous trials were not completed
because in both of the pairs, high catches of
dolphins occurred in the first tow of the trial
when no pingers were in place (20 animals
in two tows with no pingers deployed),
and both the skippers asked to have pingers
installed and kept in place for the rest of the
trial (catching eight dolphins in 32 tows with
pingers deployed) (Northridge 2003a).
Although the bycatch rate appears to be
much lower in the pingered nets than in the
unpingered nets, dolphins were caught in
both cases and, because the trial was unbalanced,
no conclusions could be drawn on
the effect of the pingers on bycatch rates.
Pingers are also being investigated in the
Irish pelagic pair-trawl fishery for albacore
tuna (BIM 2003). The alarm system being
developed looks at deterring dolphins from
the vicinity of the trawl when the risk of
cetacean bycatch is believed to be high, i.e.
when there is a change in net geometry, for
instance resulting from the vessel changing
course or hauling the net. A prototype
system was tested during the tuna season
from July to September 2002, consisting of
a control unit in the wheelhouse of the boat
which communicates with an underwater
pinger, via a through-water acoustic link.