Offshore pelagic shark and teleost surveys
Objectives:
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To assess the biodiversity and composition of the pelagic shark and teleost assemblage
in shelf slope and offshore regions of the GAB.
To determine the spatial and temporal distributions of pelagic sharks and teleosts in
shelf slope and offshore regions of the GAB.
To identify key habitats of pelagic sharks and teleosts in the shelf slope and offshore
regions of the GAB.
To collect dietary information for the pelagic sharks and teleosts in the shelf slope and
offshore regions of the GAB to contribute to aligned trophodynamic models for the
region.
Methodology:
Pelagic shark and teleost capture for tagging
Pelagic sharks and teleosts will be captured using tuna circle hooks (size 14/0 and 16/0) baited
with blue mackerel attached to a short (1 m) stainless steel wire leader (2 mm). Tuna circle
hooks will be used to minimise the chance of deep ingestion of hooks and harm to the animals.
This hook pattern is designed to hook the jaw hinge which minimises injury and optimises the
post-release survival probability. The leader will be attached to a 9 mm floating rope and
suspended from a 70 mm diameter rubber buoy, or multiple leaders (200-400) will be attached
to a set long-line that will be constantly monitored from a nearby vessel.
Deep-bait setting techniques will be deployed during all sets to minimise seabird interactions.
This involves use of a device that takes baited circle hooks from the deck of the vessel to
several metres below the water surface in a weighted, enclosed capsule during each set. This is
designed to: stop seabirds from gaining physical access to long-line baits at the surface during
the set; minimise visual contact between seabirds and baits being prepared on the deck, and
minimise seabird bycatch. In addition to use of deep bait setting, other standard mitigation
techniques, including use of tori lines, weighted main lines, and zero discard policies (any loose
food and fish scraps will be jettisoned from the vessel at night contained in heavily weighted
biodegradable bags) will be used routinely during the survey.
Restraint of captured sharks and teleosts for tagging
Once captured, sharks will be removed from the water using an aluminium chute or a soft
rubber sling and restrained using a dense foam mattress. Circle hooks will be removed
immediately using bolt cutters or pliers. The gills of each shark will be irrigated with saltwater
using a deck-hose covered with PVC pipe. The eyes will be covered with a soft, black damp
cloth and the mattress to minimise any potential capture related stress or eye damage. Sharks
will be sexed, and length will be measured to the nearest 10 mm by total length (TL).
Conventional tags
Conventional tags do not have an energy source, and are solely used to identify individuals if
they are re-captured at a later date by commercial or recreational fishers. Sharks will be tagged
using Hallprint stainless steel dart tags (SSD). Dart tags are inserted in the dorsal musculature
near the first dorsal fin depending on the species. There is no automated mechanism to release
conventional tags, but shedding can occur. Sharks with conventional tags are regularly
recaptured after several years at liberty.
Dorsal fin mounted satellite tags
Dorsal fin mounted satellite tags, including the Wildlife Computers SPOT and Mk10A models,
(position only), will be deployed on selected pelagic shark species (shortfin mako and white
shark). These tags report to the Argos tracking satellite network. The tags are attached directly
to the first dorsal fin of sharks by drilling two 3. 5 mm holes through the fin cartilage and affixing
the tag using two stainless steel bolts, nylex locknuts and small lower grade stainless steel
washers. These tags will not be retrieved from sharks, but the small lower grade stainless steel
washers are expected to grow out of the fin over time. The small washers have been shown to
grow out of the fin over time. The bolts are joined to the tags using a water-proof epoxy that is
expected to break away from the tag as the fin grows in width, therefore allowing the sharks to
shed the tag over time. Once each tag is fastened to the dorsal fin, the lock nuts will be
loosened by two turns to allow for growth of the fin during the deployment period. The battery
life of satellite tags (SPOT tags) is dependent on how the duty cycle of the tag is programmed
(i.e. how many times the tag transmits data each day). SPOT tags deployed previously on
shortfin makos have been shown to transmit data for up to 680 days. SPOT tags record and
transmit Argos location estimates and temperature data. Shortfin makos have been recaptured
following long-term (1 + yrs) satellite tag deployments in the NE Pacific Ocean and recently off
Southern Australia have shown good healing responses at the tagging site. While this
information has not been published, the PI has observed the dorsal fin of two shortfin makos
that were satellite tagged. The drill holes used to fasten the satellite tags to the dorsal fin are
small in diameter (4.5 mm). The tag dry weights of dorsal mounted tags is 0.1 % of the body
weight of an average sized shortfin mako (2 m), and therefore it is expected that this would have
little to no effect on individual motility or behaviour. When the tag is fastened to the dorsal fin
each of the lock nuts are loosened by one to two turns to allow for growth of the fin in width
during the tag deployment period. This is a standard technique that is also used by other
leading fisheries research agencies. Stainless steel dart tags are also known for being shed.
Dorsal fin mounted satellite tags and stainless steel dart tags have a low chance of being
recovered. For example, a recent review by Kohler and Turner (2001) indicated that 55% of 191
reported recapture rates for 72 shark species examined during 52 tagging studies reported
return rates of <5%. Tags will only be recovered if the animal is captured by a fishery and the
capture is subsequently reported. Each tag has the relevant recovery information (phone and
email) affixed on a label on the facia. PSAT tags have a slightly higher probability of recovery as
they may also be found by beachcombers and fishers at sea following the programmed release
of the tag after 6 months.
External tagging
Sharks will be attracted to the vessel using fish-based berley and a baited line. Vemco V16
coded acoustic and pop-up satellite archival tags will be inserted into the dorsal musculature
using a tagging pole and modified spear-gun and stainless steel applicator. Tags are attached
to a plastic dart head via 260 mm of 2 mm diameter, multi-strand, high grade (316) stainless
steel wire trace.
Euthanasia
If an animal requires euthanasia due to unforeseen circumstances, the procedure will be
performed by Dr Paul Rogers by severing of the spine on the dorsal side adjacent to the first gill
arch. This is the most suitable and humane method for euthanasing pelagic sharks.