Shark Depredation and Unwanted Bycatch in Pelagic Longline

Australia Longline Tuna and Billfish Fishery
A1.3.11. Trip length
Trip length is generally around 4 days, but frequently shorter.
The longest trip (based on information from 178 trips during which
at least one set occurred) was 30 days.
A1.3.12. Daily fishing pattern
The typical fishing pattern is a set of 950 hooks commenced in the
late afternoon at around 3 pm. Setting takes about 3.5 hours, after
which the vessel drifts until line hauling commences in the early
morning around 5 am. Hauling takes about 8 hours.
A1.3.13. Market
The majority of the catch is exported to the Japanese market.
A1.4. Shark Species Encountered by
Pelagic Longline Vessels in Australian Waters
Of the 166 species of sharks that occur in Australian waters, almost
half of which are endemic to Australia (Last and Stevens 1994),
fewer than 12 are commonly caught by pelagic longliners (Stevens
and Wayte 1998). Blue sharks are caught in greatest numbers, with
oceanic white tips, porbeagle, short fin mako, thresher spp., silky and
crocodile sharks also being frequently caught. Less frequently caught
are hammerhead, tiger, dogfish (family squalidae) and longfin mako
sharks. There is uncertainty in the species composition of shark
catch due to observer identification inaccuracies that can arise due
to similarities in appearance between species.
In addition to the ‘true’ sharks, a further 130 chondrichthyans,
117 rays and 13 chimaeras occur here. At least several species of
pelagic rays are caught regularly on pelagic longlines. For instance,
approximately 1500 rays are estimated by Dambacher (2005) to be
caught annually. In total, about 80 fish species have been recorded in
catches of Australian pelagic longlining (Hobday et al 2004).
A1.5. Shark Catch Rates in Australian
Pelagic Longline Fisheries
Extrapolation of observed blue shark catch rates from total fishery
effort by foreign longline vessels fishing in Australian waters up until
their exclusion in 1997 indicates that around 86,000 or 1,100 tons
(approx 0.1% of the estimated world catch of this species (Bonfil 1994))
were being caught annually. This species comprised 84.7% of the
total shark catch. Catch rates and total catches by species are largely a
consequence of changes with latitude in species abundance (Stevens
1992) and over 80% of the fishing effort in this instance was South of
30° South latitude. Catch rates of the same species in New Zealand
and Australia from similar latitudes were not the same. For example,
blue shark CPUE in New Zealand was typically < 15 blue sharks per
1000 hooks, but could occasionally be as high as 114 (Francis et al
2000) while in Australia, the average blue shark catch rate was 5.5
per 1000 hooks, occasionally exceeding 100 (Steven 1992, Stevens
and Wayte 1999).
For other less frequently caught species, catch rates were less than 0.5
individuals per 1000 hooks (Stevens and Wayte 1998). For shortfin
makos, the observed catch rate was 0.2 per 1000 hooks or some 3,100
each season (3.3% of all sharks species caught). The porbeagle catch
rate was 0.5, with about 4,800 caught (South of 39 o South latitude)
representing 5.5% of all species caught. At a time when, although
finning had been banned, there were no restrictions on the quantity
of sharks being retained or perhaps killed in order to recover hooks,
at least 75% of sharks caught were released alive: Between 1991 and
1998, from observation of 9% of fishing effort, 25,000 of 32,314 blue
whalers, 1,100 of 1,894 porbeagles and 38 of 44 crocodile sharks were
released alive (the status of a further 11% was unknown) (Hobday et
al 2004). But, the extent to which the presence of an observer may
have influenced the fate of sharks caught must be considered.
Catch rates on Australia’s east coast varied from around 1.3 sharks
per 1000 hooks between 10 and 30 degrees south latitude, to around
7.7 between 40 to 50 degrees south. The best estimate of the catch rate
average of all shark species in the fishery was around 5.5 per 1000
hooks. This is a rough estimate based on Australian Commonwealth
Scientific and Research Organization unpublished data from a
subset of the fleet and time period, and is possibly not representative
of the entire fleet. Most recently, data collected from 4% (402 sets) of
observed fishing effort by Dambacher (2005) indicate that in 2004/05
discarded shark catch rate was around 1.3/1000 hooks, (this figure
derived from patchy data with respect to area and season). Of the 504
individuals of 17 species (sharks and rays) observed discarded, 84%
were alive. The three most commonly caught were blue whaler (47%)
pelagic ray (17%) and Mako (10%), with the next most abundant at
around 3% being Hammerhead, Manta ray and Oceanic white-tip.
If these data could be reliably used to calculate total annual catch, it
would be around 12,000 sharks (1.2/1000 hooks) representing 4.6%,
by number of all fish caught in the fishery. However from these data
it is not possible to ascertain what actual proportion of sharks caught
were either retained or discarded. Although there is some degree
of uncertainty about the accuracy of shark catch rate overall, even
if this was 5.5 per 1000 hooks as has been suggested, vessels would
seldom reach their maximum trip limit (even if they kept every
shark). It is possible therefore that aside from shark retention rates
perhaps declining simply because of the influence of legislation on
attitude, the restrictions of the legislation alone may in fact have had
little bearing on shark conservation. But it is necessary to remember
that without regulations shark catch can be dramatically increased
deliberately and can vary substantially from day to day.
The biological implications of catches are potentially complicated by
the fact that, at least for blue sharks, whilst known to have an extreme
but variable distribution throughout tropical and temperate waters,
they display very apparent sex and size segregation. Furthermore the
impact of this fishery on pelagic sharks cannot be calculated due to
the restricted time series of catches and effort data (Stevens and Wayte
1998). There was no consistent trend apparent even for the species
caught in greatest numbers, the blue shark. This species of shark is
however considered likely to be the most resilient to fishing pressure.
Although Stevens and Wayte (1998) state that at the time the majority
of pelagic sharks are finned and the carcasses discarded, no data
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