Electropositive metals and shark bycatch

During this year’s survey we tested a strategy to reduce incidental shark bycatch in fisheries using metals from the highly electropositive lanthanide series. Since sharks and other elasmobranchs are endowed with a sensitive electro-sensory system, which detect very small changes in electric fields, it has been hypothesized that the strong electric field emitted by these metals would repel sharks away from baited hooks. To test this hypothesis we fitted each hook with either a piece of the electropositive metal or a control lead weight and compared catch rates on the metals and controls. The results from this experiment are still being compiled and may vary between species.

Common thresher sharks or the ‘unicorn of the sea’

On this years’ survey only three common thresher sharks were recorded from over 50 longline-sets. Most of the survey sets occurred offshore in warm water areas frequented by mako and blue sharks, and this may partly explain why so few common threshers were recorded. The more scientists learn about common thresher sharks the more we start to think that they are patchily distributed through coastal areas where small forage fish tend to aggregate. Thresher sharks have a very long caudal fin that is uniquely adapted for feeding by striking small forage fish and then turning around and eating them. These forage fish are often small pelagic species, such as sardines and anchovies, which form dense schools. Scientists also know that thresher sharks are gregarious and they are often found in groups. It is therefore possible that the formation of these groups is a social tactic adopted to round up the forage fish in a similar way that some dolphin species do? Threshers still have a certain mystique about them and because of their relative rarity some fishers in Australia call them the ‘unicorn of the sea’.