GILL NETS

However, research in the US suggests that pingers are less effective as
fisheries progress. In the Atlantic swordfish fishery bycatch levels at the
end of the fishery, when pingers were used, were as high as they had
been when pingers were not used.
It is thought that porpoises become habituated to the sound, and therefore
the pingers fail to prevent entanglement.

In 1998, when only 238 hauls of the English bottom set gill nets fishery in the Celtic Sea were
observed, there was a bycatch of 16 harbour porpoises, 7 of which were caught in nets with pingers.

Some scientists have suggested that it is not the porpoises that have an aversion to the noise from the pingers, but that it is their prey, the fish, that have the aversion, and that the porpoises are just following their prey.

Evidence and anecdotal reports suggest that pingers are not successful in reducing dolphin bycatch.

Many are of the opinion that pingers should only be as an emergency measure, as multitudes of pingers in certain areas could inhibit the normal foraging behaviour of cetaceans and effect their migratory routes.(See also lost gill nets)

However, very few European fishermen have been willing to test pingers on their nets.

It would appear that the majority of entanglements occur at night.
Therefore, if the soak times were reduced so that nets were not in
the water at night, the bycatch rates would be reduced significantly.
There is a negative correlation between soak times and bycatch rates.
There has also been a considerable amount of research concerning
the relationship between mesh size, twine size, floatlines, tie downs
and soak times, with some promising results.

However, European fishermen seem unwilling to make such radical
changes to their fishing practices.

It has been demonstrated that common dolphins become entangled in gill nets.
It would seem that, unlike harbour porpoises, they are attracted by the loud clatter of the gear used to shoot and haul the nets. It has been suggested that a “skirt” around the gear to reduce the noise would reduce the levels of bycatch.

However, it would seem that this suggestion has not been investigated by European fishermen.

As mentioned earlier, cetaceans are often aware of the net when they
are almost upon it. Possibly signals from the float line alert them to
the presence of the net, and in an attempt to swim under the floatline,
they swim into the net. It has been suggested that if a significantly
large gap existed between the floatline and the mesh of the net,
a cetacean could pass safely through it, when attempting to avoid the
floatline. However, we are not aware of any research involving nets of this type.

Acoustically Reflective gill nets: Perhaps the most promising development to date has been the invention and development of the acoustically reflective gill nets. This gill nets was invented by a scientist in the US. It is heavier than a normal gill nets as it has barium sulphate filler. Fishermen are not required to attach anything. The unit is complete.

Trials in the Bay of Fundy (1998 & 2000) were very promising.
In his Report to ASCOBANS (27 -12 -2000), Dr. Andrew Read stated
"experimental trials indicated that acoustically reflective nets are effective in reducing the
bycatch of harbour porpoises in a statistically significant fashion.
In 1998 and 2000, 7 of 242 control strings, had at least one porpoise bycatch, whereas no
porpoises were taken in 124 strings of acoustically reflective gill nets.
In the Bay of Fundy tests, no significant difference was observed in the catch rate of target
species between control and acoustically reflective nets (Trippel 2000)."

Dr. Read continued, to state that the Scientific Committee of the IWC endorsed further
experimentation with these nets and that the
Gulf of Maine TRT, at its meeting in December 2000,
recommended that a large scale trial be conducted to
determine whether the acoustically reflective gill nets
would reduce the bycatch rates of harbour porpoises in
the groundfish sink gill nets fishery.
He also stated "If they prove to be effective in reducing
the bycatch of small cetaceans in gill nets fisheries, and do
not reduce the catch of target fish species, acoustically
reflective gill nets hold great promise as a mitigation measure".

In an article written by Gretel H. Schueller, (19 - 02 - 2001), Dr. E. Trippel -
Canadian Department of Fisheries and Oceans, is reported to have said that the
results involving the acoustically reflective nets "are almost to good to be true". In the same article, Dr. D. Potter, NMFS, is reported to have said, "If the net works as well as tests indicate, the net has the potential to be useful......it might turn out to be a global solution".

There was also a significant reduction in seabird bycatch, and anecdotal reports suggest that seals were not attracted to the nets. Pingers act as dinner bells to seals with obvious consequences.
Anecdotal reports also suggest that these nets could assist in
reducing dolphin bycatch, and trials will take place in the near
future in US waters.

Trials of acoustically reflective nets were carried out in Danish waters.
However, the filler used was iron oxide, as opposed to the barium sulphate, and the nets were not hung by an expert team.
Additional floatation should have been added to the nets, as they were 12% heavier than normal nets. This was not done and consequently, although the nets did not collapse towards the bottom, they did sag.

Even though the tests were not carried out as per instructions, the results regarding harbour porpoise bycatch were as promising as those in the Bay of Fundy, but there was a reduction in fish catch.

It is imperative that the barium sulphate filled acoustically reflective nets are tested in European waters.