Factors affecting elasmobranch escape from turtle excluder devices (TEDs) in a tropical penaeid-trawl fishery.

The study quantified the impact of turtle excluder devices (TEDs) on the catches of various elasmobranchs caught off northern Australia using data collected during a previous study (Brewer, et al., 2006). The effect of fish size and various aspects of TED design such as grid orientation, grid angle, and bar space were quantified to determine their effect on the escape of elasmobranchs from shrimp trawls. The relatively low number of elasmobranchs encountered during sampling resulted in a lack of power to isolate the effects of the various factors tested. Generally, the study showed that TEDs facilitated the escape of large elasmobranchs, including several species of conservation interest (the scalloped hammerhead, Sphyrna lewini and the zebra shark, Stegostoma fasciatum). Bar space and orientation were important TED design factors affecting the escape of elasmobranchs. Top-shooter TEDs enabled more Carcharhiniformes (“ground sharks”, including requiem sharks and hammerhead sharks) to escape penaeid trawls, while bottom-shooter TEDs increased the escape of Myliobatiformes (rays, including stingrays and cownose rays). However, the TED bar space that facilitates maximum escape of elasmobranchs while maintaining catch of the target species is more difficult to quantify given the low catch rate of elasmobranchs in the trawls.