The Net Effect? - Whale and Dolphin Conservation Society

Table 3.
Comparison of catches between pairs with standard tows and
one fitted with a selection grid and associated experimental equipment
fully through the grid. In the event, no
dolphins were seen during the grid’s four-day
trial in 2002, although a shark was recorded
escaping through the hatch apparently
without problem (Northridge 2003b).
In 2003, after some modification of the
device, 42 days of sea trials were conducted
and monitored with two cameras, one inside
and one outside the trawl (although the latter
was removed for part of the trial) (Northridge
2003a). An electronic grid sensor was also
used to monitor the angle of the grid and the
speed of water passing through the grid.
During the trial, observers monitored bycatch
in the Scottish pair fishing with the grid, and
the others without it, all of which were fishing
in the same general location.
The results from 2003 show that bycatch
rates for the three pairs not using the grid
were high, with 37 dolphins caught in 62
tows, a mean of 0.6 animals per tow. The
highest bycatch rate was recorded in
February (1 animal per tow). The grid pair
was only operational during March and
April, therefore the total bycatch rates are
not comparable. However, in the 82 gridtows
observed only two dolphins were killed
(Northridge 2003a; see Table 3).
The two dolphin fatalities occurred during a
period when the camera was not functioning.
However, it was evident that one of the
dolphins had passed up to the escape hatch
and then become caught by its beak in the
mesh of the cover net (which is a flap
designed to reduce fish losses). The second
animal had become trapped behind the first.
It was concluded that the two deaths were
the result of using a mesh size for the cover
flap that was too large and that the problem
could be rectified by using a smaller mesh
(Northridge 2003a).
The very much reduced bycatch rate in the
pair using the grid was clearly not achieved
by the intended mechanism as no animals
were observed to swim through the escape
hatch. Instead, it seems that the experimental
set-up had the effect of deterring animals
from venturing too far down the trawl net
(Northridge 2003a). Two possible reasons
for this effect have been put forward. First,
the grid sensor, which emits a loud noise at
typical dolphin echolocation frequencies,
may have caused the animals to turn back
before they reached the final narrow section
of the net. Second, the grid itself, which is
made of solid stainless steel bars, would be
easily detectable to dolphins either visually or
acoustically and this alone may have deterred
animals from entering the narrow tunnel. If
the latter explanation is correct, it is suggested
that the simple placement of a large metal
grate further forward in the net, at the start
The Net Effect?
A WDCS report for Greenpeace
Non-grid No. of tows No. of No. of tows Dolphin tows as Dolphins caught
dolphins caught with dolphin catches % of total tows per tow
February 16 16 3 0.19 1.00
March 29 21 6 0.21 0.72
April 17 0 0 0.00 0.00
Total 37 9 0.15 0.60
Grid pair
March 40 2 1 0.03 0.05
April 42 0 0 0.00 0.00
Total 82 2 1 0.01 0.02
From Northridge 2003a
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