Abstract
The overarching aim of this multidisciplinary thesis was to contribute to the development of a novel shark bycatch reduction device (BRD) that both meets fishing operators’ needs and is economically feasible. Following the multidisciplinary approach evident in existing frameworks developed for addressing bycatch problems, each chapter employed a different methodology drawn from a research discipline that provided a suitable set of tools for investigating a specific problem related to the overall goal.
First, a literature review identified and evaluated global options available for mitigating elasmobranch bycatch on longline fishing gear. The aim of this review was to determine whether a shark bycatch reduction method currently exists that New Zealand longline fisheries could use to reduce their shark bycatch rates, and if not, identify promising emergent bycatch reduction methods as candidates for further development. The review found that no effective shark bycatch reduction methods or devices were commercially available to longline fishing operators, and identified electrosensory shark deterrents as an approach that showed potential for further development.
Qualitative social research methods were then used to explore New Zealand longline fishing operators’ perspectives on shark bycatch. Most interviews occurred in 2014 during a national campaign to legislate against shark finning in New Zealand waters. At this time, widespread popular opinion held that finning made sharks an economically valuable bycatch or even target species in many longline fisheries. If sharks were valued by fishermen for their fins or otherwise targeted then it was unlikely that they would consider a shark BRD useful. Interviews with ling (Genypterus blacodes) and tuna (Thunnus spp.) longline fishermen showed that they viewed spiny dogfish (Squalus acanthias) and blue shark (Prionace glauca) bycatch in their respective fisheries as a significant operational and economic challenge that they were motivated to mitigate but lacked effective tools to do so. Interviews also revealed that fishermen viewed the issue of shark bycatch in the context of maximising target species catch rates rather than minimising shark bycatch rates.
Following interviews with skippers, laboratory animal behaviour experiments tested the hypothesis that weak electric stimulus generated by a prototype BRD would deter spiny dogfish or sandbar sharks (Carcharhinus plumbeus) from eating bait. Sandbar sharks were used as a carcharhinid model for an important longline bycatch species, blue sharks. The primary function of the elasmobranch electric sense is to guide predatory strikes during the final stage of prey capture, so electrosensory stimulus could disrupt their close range feeding responses. Weak electric stimulus produced by a microcontroller attached to an array of carbon electrodes and powered by a 9 V battery was used to deter groups of sharks from eating bait. Electric stimulus significantly reduced bait consumption by each species in a laboratory setting. Bait consumption by groups of juvenile sandbar shark median declined by 85 % when bait was located 10 cm from active electrodes compared to when it was 2 m away. Bait consumption by groups of adult spiny dogfish declined by 50 % when bait was located 10 cm from active electrodes compared to when it was located 10 cm from inactive electrodes. Results from laboratory studies of electrosensory shark deterrents tend to produce larger effect sizes than similar stimuli applied in field studies. If these results translated to the field the effect size produced in the sandbar shark experiment could be adequate to meet fishermen’s expectations of a successful BRD. Conversely, the smaller effect size and wide interquartile range in spiny dogfish bait consumption means that for this species, improvements in both effectiveness and consistency would be required for the prototype electrosensory BRD to meet fishing industry needs.
Following the bait choice experiments, a spatial utilisation experiment tested the hypothesis that individual sandbar sharks avoided the location of carbon electrodes emitting weak electric stimulus. Sharks were not deterred from the location of either 4 Hz, 33 mA direct current or alternating current stimuli nor did their swimming speed change relative to a non-electric control. The absence of an avoidance response suggests that an electrosensory shark BRD based on this concept may not be applicable to non-baited fishing gears such as purse seine and trawl nets. The finding electrosensory stimuli that reduced bait consumption in the previous experiments did not also elicit spatial avoidance supports the premise that electrosensory deterrents interrupt shark feeding behaviours rather than eliciting an aversive response.
Finally, the economic impact of spiny dogfish in the inshore ling longline fishery was analysed quantitatively. Spiny dogfish and ling catch rates and export markets were assessed, then New Zealand government fisheries observer data were used to investigate a hypothesis that arose during skipper interviews, that spiny dogfish incur costs to inshore ling longline vessels by reducing ling catch rates. Overall, there was a significant weak positive relationship between spiny dogfish catch per unit effort (CPUE) and ling CPUE, which probably reflects these two species’ spatial and temporal co-occurrence. On fishing lines in the upper quartile of spiny dogfish CPUE, there was a significant moderate negative relationship between spiny dogfish and ling CPUE. Median total hook occupancy in this fishery was 21 % but spiny dogfish alone could occasionally take up to 90 % of hooks. When spiny dogfish catch rates were high, the negative impact of spiny dogfish on ling CPUE could have been the result of hook occupation by spiny dogfish reducing the number of hooks available to ling. To reduce the highest spiny dogfish catches to a level below that likely to incur an opportunity cost in this fishery, the BRD would need to have an approximately 80 % effect size in the field. Based on the ratio of spiny dogfish to ling, an opportunity cost of high spiny dogfish catches in terms of ‘lost’ ling was estimated and valued at 34 cents per hook. The novel BRD under development produced weak electrosensory stimulus which was only perceptible to sharks at close range, which meant that every hook would require an individual BRD. Therefore, our estimates suggest a price ceiling of 34 cents per unit, above which the cost of a BRD outweighs its benefit in terms of increased ling catch. Increased durability could allow for the use of a more expensive device. However, based on the views of the three interviewed skippers, willingness to pay for a BRD is very low. Skipper access to better information, including our estimates of spiny dogfish bycatch-associated costs of ling catch, may provide increased incentives for BRD adoption.
This thesis shows that an electronic BRD that produces electrosensory stimulus is a promising approach for mitigating carcharhinid shark bycatch, although it would require further development to become effective and reliable enough to be cost effective as a method of mitigating spiny dogfish bycatch in the ling longline fishery. This thesis also provides insight into an improved framework for developing bycatch reduction methods aimed at meeting fishing industry needs. Such a framework should involve initiating bycatch reduction research by conducting social research aimed at understanding fishing industry needs and perspectives on the bycatch problem. This can highlight the potential economic impacts, species of concern and key outcomes that industry would require for BRD uptake to occur. Proceeding to conduct an economic assessment of a bycatch problem’s cost in a specific fishery can contribute to BRD cost-per-unit and minimum acceptable effect size estimates, which can then guide subsequent device development.