Shark Depredation and Unwanted Bycatch in Pelagic Longline

Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries
If operators were intent on landing and processing all sharks caught,
the revenue from this was considered by half the respondents
insignificant when offset against the time lost and crew safety
compromise involved. Time lost has the following important
consequences: (i) The longer it takes to haul a longline the more likely
it is that target catch will die or be damaged, which reduces its value;
and (ii) Increased time taken to haul a line can have a repercussion
on subsequent fishing effort potential and/or the appropriate timing
of this to maximise target species catches.
Here it is worthwhile to note that only one respondent operates in
a fishing ground with a 500 hook per set limit imposition (a game
fish catch and release alive, management strategy). As the average
number of hooks set (which equates to time and this is thought to
determine a fish’s likelihood of survival if discarded) in the fishery is
nearly 50% more than this, it would be interesting to compare shark
catch data here to quantify the relationship between the number
of hooks set on a line and the survival or otherwise of sharks (and
other species). And, whilst there are obvious problems in comparing
species catch rates across different fishing grounds, there can be (on
average) more than twice this fishing effort per line (and therefore
time) elsewhere. The question here is whether species survival and
discard rates alter as a result of increased hook numbers set (and
the increase in total set time). Obvious economic and operational
efficiency issues arise in relation to the impact of a change of this
nature on target species catches.
While all respondents agreed that more sharks can be caught if a
wire trace is used, avoiding setting hooks in certain areas such as
on the east coat continental shelf, setting hooks at a greater depth
or avoiding especially oily (pilchards for example) or larger baits
(especially squid) were all factors believed to help reduce sharks
catches. Of all respondents, two had experience in using artificial
baits and both considered them ineffective for catching sharks
compared to conventional baits. But, the suitability of such bait for
target species catches is the other factor of importance.
Respondents who considered that the use of light sticks (also
referred to as “shark sticks”), caused higher rates of shark capture
were strongly of this opinion, but had different views about which
color correlated most to high shark capture. Light sticks in blue,
yellow and green were all suggested to be colors, which increased
shark catch, rates. But surprisingly most did not consider the act of
burleying (to deliberately discharge offal and or spent bait) had any
bearing upon shark catches. In relation to this point it would be of
value to ascertain whether shark catch rates increase as fishing effort
persists in the same location, potentially as a consequence of shark
aggregation in response to persistent offal discharge. This may be
of relevance in those fisheries where strategically discharging offal
is actually a requirement, one entirely at odds with its objective of
assisting to minimise seabird interactions. Likewise, there is a
relationship between hook box tangle decrease (hook box tangles
cause more birds to be killed) and branchlines configured with
wire to a 45 g or 60g swivel at less than 1 metre away from the hook
(a faster bait sink rate to avoid birds). The logistics of retaining fishing
gear whilst successfully discarding live sharks is much improved
too. Put simply, wire and an appropriate size swivel near each hook
can be used to achieve several objectives, less birds killed for two
reasons, more sharks released alive with more fishing gear retained
and perhaps even fewer sea turtles killed because gear sets and stays
set at a consistently greater depth (this too will keep shark catches
down). One respondent also pointed out that heavy swivels are too
dangerous to use unless wire can be incorporated, an additional issue
of specific relevance to management decision-making with multiple
not single species bycatch reduction objectives in mind.
Even without a reduction in the incentive for operators to not catch
and retain sharks as a consequence of regulations to limit this, all
operators believed that shark catches could be increased by 100% or
more if they chose to do so. This ability was thought to be relevant
in the debate over the actual abundance of shark species as indicated
perhaps by catch rate on hooks set to target other species.
Opinions on how to minimize shark captures in the first instance,
included not re-setting in a location found to contain a lot of sharks
at that time, confining the set and haul to daylight, and intervessel
alerts to problem waters, (these areas being unpredictably
changeable). In a locality where target species occur in addition to
sharks, as is often the case, several respondents considered sharks
could be avoided and target species still caught by setting the line at
greater depth. The most prevalent opinions amongst respondents as
to how survival of sharks that are caught could be improved, included
keeping line sets shorter, to not use wire (otherwise more will die on
the hook before hauling) and to not have guns aboard.
Hook type, considered by most to be either an unknown or irrelevant
factor for its impact on shark catches was by several respondents
implicated in altering the way in which sharks are actually hooked
(and therefore ease of release or for hook recovery). ‘J’ hooks catch
and hold more sharks whereas circle hooks tend to mostly mouthhook
sharks, making these easier to release, hooks more likely to be
recovered, and the survival prospects of the shark improved.
Most respondents consider sharks to be responsible for their greatest
gear damage and economic cost, giving average estimates of this
to be around AU$100 per set but also estimate that target species
economic loss could be up to 20% of catch. For those who considered
sharks to be less problematic than other causes of gear or economic
loss, pilot whales, lancet fish, snake mackerel, marlin, sunfish as well
as crew were listed. Pilot whales were described as being considerably
more destructive, but not so consistently as sharks. They were seen
to be a greater potential problem locally and where this is so, no
problem is greater!
Most respondents considered the act of gaffing sharks alongside
either to retain a shark, or recover fishing gear in the process of
discarding the shark to be dangerous and unacceptably timeconsuming.
Similarly, it was considered inefficient to deal with sharks
by trying to use de-hookers or line-cutters, with these increasing
safety risks by trying to do so. Several respondents however do
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