Shark Depredation and Unwanted Bycatch in Pelagic Longline

Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries
when wire is not used. Higher risk of losing target species can be
an important consequence if wire use is not permitted because of
deficiencies in maintenance of damaged monofilament line between
sets. Prior to the ban, instead of monofilament line, a short (< 1 m)
length of usually multi-strand stainless steel wire was incorporated
between the hook and a swivel, beyond which the remainder of the
branchline extended to the mainline clip.
Use of wire was recently prohibited in Australia, the intention
being that a greater proportion of sharks caught would now escape
the mono (relative to wire) which would be more likely severed.
This, to a certain extent removed the option that fishers otherwise
could exercise of killing all sharks to retain or to recover hooks. It
is true that for a fisher who only wants to discard all sharks and is
unconcerned about the extent of hook loss, mono, not wire to the
hook does provide easier discarding opportunity. But, the likelihood
that many of such discards are left with long sections of mono trailing
(as well as the hook) is high. The long-term health implications
of this are not fully understood although the anecdotal evidence
(see survey results) is that, to a shark, such potential impediment is of
little or not consequence.
The majority of fishers surveyed did not consider hook recovery from
sharks warranted and, if given the option they would not elect to
incorporate wire. But opinion varied with wire being considered
an essential tool to make fishing viable where bycatch species that
are particularly destructive to fishing gear (sharks, snake mackerel,
wahoo lancet fish) are prevalent. Without wire, fishers commented
that it was easier to discard sharks that are caught efficiently by cutting
the mono near the hook, despite loss of all hooks by doing so. This
contrasts to the practice in those fisheries where despite similar shark
retention and finning restrictions, wire usage is still permitted. Here
a common practice with wire is to use it to advantage and recover all
fishing gear, including hooks. This is made possible because, under
tension (a fish on the hook) wire, unlike mono has no elasticity. A
shark can therefore be more easily controlled with safety and in
doing so all fishing gear, including the hook is invariably recovered,
with frequent, time-consuming costly shark damage repairs greatly
minimized. (A high, elevated work deck on a vessel does limit this
capacity, however).
There is potential therefore in fisheries where the use of wire is
permitted, for improvement in shark conservation although this can
be reliant to a large extent on:
• The existence of other measures to constrain the extent to
which sharks that are caught can be retained; and
• The attitude of individual fishers to sharks and the measures
they choose to take to discard them when caught.
An important component of this can be, as it is in Australia,
the choice or not to use firearms as a routine method to
dispatch fish prior to them being landed
(see following section).
Additional advantages of wire are that a lead centre swivel is generally
used where mono attaches to wire. As this tends to be within
1 metre of the hook, line sink rate to avoid seabird interactions is
optimized. Hook storage bin tangles that reduce fishing efficiency
and exacerbate bird interaction rates are also much reduced with
branch-line configuration of the above design.
In a fishery where gear damage by fish (not only sharks) is high,
average trip length and fishing effort can also be an important
influence on the desirability of wire use. In Australia, where average
trip length is only about 4 days and daily effort around 950 hooks set
(Table A1.1), the capacity to withstand substantial gear damage is
much more feasible than in fisheries with considerably longer trips
setting many more hooks. Daily gear damage can be so great, on
voyages of extended duration that if fishing effort capacity is to be
maintained throughout the trip it is more feasible to contain damage
by using wire rather than have enough replacement gear. Regulations
that restrict the number of clips that can be carried aboard vessels
in Australia also limit this as an option. (There is insufficient time
for crew to actually repair all damaged gear except on much larger
vessels where sufficient crew keep pace with repair needs).
A1.9. Fishing Practices that Affect Shark Catches
A1.9.1. Fishing location
It would seem that shark catch rates in the AFZ particularly of
blue sharks, increase in more southern waters. However, now it is a
requirement that vessels fishing in this region must possess a minimum
quota of Southern bluefin tuna before a longline is set in a specified
period of the year. Few longline permit holders possess quota for this
species. Availability and cost of quota is now prohibitive for economic
longline efficiency, with the majority of quota now consumed on
aquaculture of this species. This scenario is likely to have greatly
reduced overall shark catch potential as it also has for seabird
mortalities. Survival rates of more southerly, cold-water caught
sharks may however be better due to reduced metabolic rate and
lower oxygen demands (Francis et al, 2001).
Questionnaire respondents considered that line setting inshore (often
by unintentional line drift) of the continental shelf break greatly
increases shark catch rates and where this is a known consequence,
this region is generally avoided. As noted previously, consideration
is currently being given to applying longline fishing restrictions in
relation to waters adjacent to or over the shelf break. But, this is not
being driven by any perceived need in relation to shark management.
Shark catch rate and species composition of the catch will change
because of this. Evaluating the extent of fishing effort in relation to
this area factor for shark interactions, based on existing logbook and
observer fishing position records would be of value.
A1.9.2. Line set, soak and haul duration, depth and timing
The average number of hooks on a longline usually has a direct
relationship to duration of line setting and hauling. The interval
between setting end and hauling, referred to as ‘the soak’, has no real
relatship to fishing effort (hooks set) in this particular fishery. But the
combined duration of these events can, not only alter catch rate but
survival prospects for species that are discarded. Francis et al (2001)
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