Investigating the behavioural response of grey nurse sharks to recreational lures and baited lines
Investigating the behavioural response of grey nurse sharks to recreational lures and baited lines.
Drs Will Robbins & Vic Peddemors, Cronulla Fisheries Research Centre of Excellence
Grey nurse shark taking the bait during this study
Grey nurse shark seen with fishing gear trailing out from gill
Grey nurse sharks, Carcharias taurus, are typically found inhabiting the bottom of rocky gutters and caves near inshore reefs. Unfortunately these habitats are often targeted by line fishers seeking species such as snapper and kingfish, placing grey nurse sharks at risk of incidental hooking. Historical catch data, combined with observations of hooks and wounds on many grey nurse sharks off NSW have provided evidence of ongoing interactions with commercial and recreational line fishing gears. However, the underlying mechanisms associated with these interactions were unclear. Specifically, it was unknown whether grey nurse sharks are more vulnerable to particular types of fishing gears or techniques, and at what rate interactions occur.
This study assessed the interaction rates of grey nurse sharks to bottom-set baits, towed lures and artificial jigs deployed. To prevent injuries, all hooks were disabled on the gear, with underwater video cameras used to record shark and fish interactions. We found that grey nurse sharks clearly interact with bottom-set baits deployed close to their aggregations. All bait types were taken at all times of day, and grey nurse sharks were the only bait takers after dusk. Even the least-taken bait types resulted in frequent (10%) shark interactions, demonstrating that bottom-set baits pose a high interaction risk when deployed around grey nurse shark aggregation sites.
None of the trolled attractants (lures and bait) elicited any reaction from grey nurse sharks. Although there may be indirect effects of sharks targeting fish which have been caught by lures, the lack of any interactions with trolled lures and the low number of sharks recorded carrying lures indicates that trolling represents minimal direct risk to grey nurse sharks.
Knife jigs and soft plastic jigs pose some risks to grey nurse sharks while close to the substratum. Most interactions occur due to the jigs hitting the shark, potentially resulting in foul hooking. This can occur relatively frequently when fishing in close proximity to grey nurse shark aggregations. Benthic-oriented jigs had higher interaction rates than vertically-moving jigs because they are deployed close to the substratum for longer periods of time.
To investigate the behavioural responses of grey nurse sharks to various recreational line-fishing gears. This involved examining interaction rates with bottom-set baited lines, towed lures and vertical jigs when deployed close to grey nurse shark aggregations. Research was focused at Fish Rock, South West Rocks, a NSW Critical habitat where year-round aggregations of grey nurse sharks occur.
Dr Robbins holding an underwater video camera and a bethnic jig (minus hooks) about to be tested
Gear types used in this research were commonly used by recreational fishers at the study site. Sampling was undertaken from a runabout, with all areas of Fish Rock sampled. Hooks were disabled on all gear types, with fish and shark interactions monitored using underwater video cameras which fed a signal directly back to the boat. Previous studies have found these cameras have no effects on the behaviour of sharks.
Sampling was conducted as three discrete experiments looking at bottom-set baits, trolled lures and jigs. Experiments consisted of
- 800 bait trials of four different baits
- 625 lure tows (537 km) of 16 different lures
- 861 jig drops of eight jig types.
Grey nurse shark taking a research bait at Fish Rock
Baits were fished using a modified paternoster rig, where the baits were positioned ~70 cm above the substratum. Sampling consisted of 5 minute replicates typically conducted in sandy gutters within ~40 m of grey nurse shark aggregations. Sampling was conducted at dawn, morning, afternoon, dusk and night. Night sampling was undertaken within 50 m of the position where sharks had aggregated that day and used red-filtered lights to illuminate the area around the baits. Four bait types were tested - whole and slabbed blue mackerel, whole pilchard and whole squid. A strike was recorded when a shark or fish took the bait, or completely engulfed the bait in its mouth.
The 800 bait trials were distributed equally across the five times of day (i.e. 40 replicates per bait per time of day). Baits were taken by sharks, by fishes targeted by local anglers, and by smaller, non-target fishes. Key findings from the bait trials were:
- Grey nurse sharks took 22% of all baits deployed.
- Larger baits of slabbed and whole blue mackerel were taken most by grey nurse sharks, with 34-35% of baits taken.
- Smaller baits of pilchard and squid were also taken by grey nurse sharks, at a lower, but persistent rate of 10%.
- Grey nurse sharks took all bait types at all times of day, with no consistent time-of-day preference.
- The vast majority of bait takes by grey nurse sharks occurred by the shark approaching the bait, rather than the bait drifting onto the shark.
- Grey nurse sharks were the only takers of baits at night. Fish took baits only between dawn and dusk.
- Pilchard and squid baits were taken more often by fish species targeted by anglers.
Towed attractant trials
Examples of some of the lures used
Trolling was conducted at dawn, morning and afternoon, at both shallow (1-2 m) and deep (7-10 m) depths. Fifteen different large and small artificial lures and one fish bait type were trolled 24 m behind the boat at 5 -7 kts, as per manufacturer recommendations. Trolling was conducted as circuits around the Fish Rock complex to ensure the maximum number of habitat types and area were exposed to the lures.
Gears tested were:
- Hard plastic lures (blue/white, red/white, yellow/green, yellow/red)
- Hard plastic rattling lures (blue, blue/silver, green/blue, silver)
- Metal lures (silver)
- Soft plastics (white, green/orange)
- Plastic poppers (red/white, blue/silver)
- Feather jigs (red/white, purple/black)
- Whole fish (blue mackerel)
The 625 lure tows around the Fish Rock complex allowed us to draw the following conclusions:
- Fish strikes were most prevalent during dawn (42%) and afternoon (33%), with fewer interactions (25%) during morning sampling.
- Over 18,000 sightings of fish were recorded.
- Over 3,600 fish reactions to lures were recorded.
- Over 100 fish strikes were recorded on the lures.
- Only one grey nurse shark was seen during the lure tows.
- No grey nurse interactions were recorded in 537 km of lure towing.
Researcher with jig and inline camera
Footage from the inline camera showing a jig about to land on a shark
Vertical jigs were deployed using rod and reel, with results captured by an underwater video camera attached inline ~ 1 m above the jig. The camera allowed the lure to maintain a constant position in the camera’s field of view as it was jigged upwards. Two metal knife jigs (pink and green), and four types of soft plastics (blue, brown/white, green/orange and white) were tested in a series of five minute replicates. Two benthic jigs were also tested using the live-feed video cameras.
Vertical jigs were dropped and retrieved over 800 times in the presence of sharks. Over 92% of these drops landed in the gutter where sharks were located (proximate drops), with the jig passing within a body-length of a grey nurse shark in over 50% of these occasions. Key findings from the jig experiments are as follows:
- One in 20 proximate jig drops (5%) hit a grey nurse shark. 62% of hits occurred on the way down, 38% on retrieval. These may have snagged grey nurse sharks had the gear possessed working hooks.
- A further 6% of proximate drops were “line hits” or “near misses” passing within millimetres of sharks.
Grey nurse sharks attempted to bite the jig on only 6 occasions.
- All interactions occurred when the jig was close to the substratum, with grey nurse sharks not seen to actively chase any jigs.
- Grey nurse sharks interacted with benthic-oriented jigs in 55% of replicates. This usually involved sharks rubbing up alongside the jig, or the jig drifting onto the shark.
Conclusions of this study
Grey nurse sharks clearly interact with static baits deployed close to their aggregations. All bait types were taken at all times of day, and grey nurse sharks were the only bait-takers after dusk. Even the least taken bait types resulted in frequent (10%) shark interactions, demonstrating that bottom-set baits pose a high interaction risk when deployed around grey nurse shark aggregations.
None of the trolled attractants (lures and bait) elicited any reaction from grey nurse sharks. Although there may be indirect effects of sharks targeting fish that have been caught by lures, the lack of any interactions with trolled lures, and the low number of sharks recorded carrying lures suggests that trolling represents minimal direct risk to grey nurse sharks.
Knife jigs and soft plastic jigs pose risks to grey nurse sharks while fished close to the substratum. Most interactions occur through jigs hitting the shark, resulting in foul hooking. This can occur relatively frequently when fishing in close proximity to grey nurse shark aggregations. Benthic-oriented jigs had higher interaction rates than vertical jigs as they are deployed close to the substratum for longer periods of time.