Bycatch mitigation assessment for sharks caught in coastal anchored gillnets

Fishing with modified gillnets was conducted to elucidate their potential for reducing shark bycatch. Experimental fishing focused on two commercial fisheries, Spanish mackerel (Scomberomorus maculatus) and spot (Leiostomus xanthurus). The modification took the form of increasing the gillnet tension using larger floats on the head-rope and increasing the lead-core lead-line weight. Gillnet mesh sizes were 7.2cm (spot fishery) and 7.6 and 10.2cm (Spanish mackerel fishery). Gillnet selectivity of the four most commonly encountered shark species were fitted to fork-length distribution data using the SELECT method. There was no significant difference in the catch rate of the target species between control and modified gillnets for all mesh sizes used. Catch rates of some shark species were significantly reduced in modified gillnets. Model deviance values indicated good fits to the data for blacknose (Carcharhinus acronotus), blacktip (Carcharhinus limbatus) and bonnethead (Sphyrna tiburo) sharks with lowest deviance values (i.e., best fit) generally associated with the normal scale (spread proportional to mesh size) form. The selectivity results demonstrated that all life stages of Atlantic sharpnose (Rhizoprionodon terraenovae) and blacknose sharks were available to the gillnets. The mesh selectivity of bonnethead sharks was largely uniform due to their exaggerated cepholaphoil that resulted in the majority being hammer-wrapped. Further, capture data indicated a dominance of large juveniles and adult bonnethead sharks resulting in poor model fits. The capture mortality rate for all shark species was high (78.6%) with higher mortality rates associated with heightened locomotor performance and wrapping as an entanglement mode. These results demonstrated that modified gillnets have the potential to reduce shark bycatch, particularly for those species for which wrapping was the primary entanglement mode.