Investigating Sound and Gear Modification as a Means of Reducing Shark bycatch

Sharks are unintentionally captured during commercial fishing operations and discarded, known as bycatch. Sharks captured in this manner have high mortality rates resulting in hundreds of millions of sharks killed annually. Longline operations have the highest instances of bycatch. In this study, two methods for reducing shark bycatch were investigated. A simple gear modification on longline leaders was tested by substituting monofilament (mono) leaders with ultra-high-molecular-weight polyethylene (spectra) leaders. A recording of thunder was tested during longline fishing for a deterrent effect. The experiments were conducted simultaneously on a 915 m longline with 50 leaders with each leader alternating between mono and spectra leaders. An underwater speaker was suspended over the center of the longline. Longline sets alternated receiving sound and no sound treatments. Additionally, rod-and-reel sampling was conducted to test the sound treatment with four rods, two lines set at 10 m and two lines set at 5 m. A set was fished for two hours with an hour of sound treatment and then an hour of no sound. One hundred and thirteen sharks were captured, 76 on control leaders and 37 on experimental leaders, resulting in a 50% reduction in overall shark bycatch. When only sharks of larger size were considered an 84% reduction in bycatch was observed. Unexpectedly, more sharks were caught during longline sound treatment than during control sets. This was also observed during rod-and-reel sampling. Based on the most conservative estimate for the bycatch reduction rate observed in this study, the change in leader material would result in millions of sharks being released from commercial longlines annually. Thunder appeared to have no deterrent effect on sharks, rather it attracted sharks. Based on the results of this study, it is recommended that spectra be substituted for monofilament on all commercial longline operations