Abstract:
Spiny dogfish, Squalus acanthias, are a major bycatch species in commercial bottom longline fisheries in New Zealand and internationally. This study investigated the use of weak electric stimuli to deter spiny dogfish from taking bait by stimulating the elasmobranch electrosensory system. Rig, Mustelus lenticulatus, were used as a model for spiny dogfish in behavioural experiments. A 4 mA d.c. electric field associated with bait significantly reduced a range of rig feeding behaviours relative to a non-electric control. These behaviours included bait removal, feeding latency and foraging effort. A 4 mA d.c. electric field with a 4 Hz pulse significantly reduced rig foraging effort in comparison to a constant 4 mA d.c. field. The voltage gradient of experimental stimulus was estimated to be greater than 50 μV cm-1 at sites less than 15 cm from the electrodes. It is unlikely that this voltage gradient was intense enough to affect non-electrosensitive tissue, so it is reasonable to assume that the response of rig to experimental stimulus was mediated by the electrosensory system. A morphological comparison of the external electrosensory systems of spiny dogfish and rig was used to assess the suitability of rig as a model for spiny dogfish electrosensory-mediated behaviour. When adjusted for body size, the number and density of pores in some pore fields varied significantly between the two species, and spiny dogfish had more proportionally more ventral pores than rig. However, the total number of electrosensory pores of spiny dogfish and rig was not significantly different. These species attain similar adult body lengths, so it is reasonable to conclude that size-matched individuals of each species have the same total electrosensory pore density. These findings show that a weak electric field can be used to reduce bait removal by rig, and suggest that rig were a reasonable model for spiny dogfish electrosensory-mediated behaviour, based on morphological similarities between the electrosensory systems of these species. This indicates that an electric bycatch reduction device may be a feasible solution to spiny dogfish bycatch in commercial bottom longline fisheries and support further investigation in the form of a field study.