Bycatch mitigation assessment for sharks caught in coastal anchored gillnets

Published source details Thorpe T. & Frierson D. (2009) Bycatch mitigation assessment for sharks caught in coastal anchored gillnets. Fisheries Research, 98, 102-112.
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This study is summarised as evidence for the following.

	Action	Category
	Modify gillnet or entangling (trammel/tangle) net configuration Action Link

Modify gillnet or entangling (trammel/tangle) net configuration

A replicated, paired, controlled study in 2000 of two coastal fished areas in the Atlantic Ocean, off the coast of North Carolina, USA (Thorpe & Frierson 2009) found that modifying the configuration of a gillnet by increasing the tension did not typically reduce the catch rates of four unwanted shark species in a commercial gillnet fishery, compared to unmodified nets. Shark catch rates were reduced in gillnets with increased tension only in nets of the larger mesh size (10.2 cm) and only for two of four species: Atlantic sharpnose Rhizoprionodon terraenovae, (modified: 0.35, unmodified: 0.58 kg/gillnet/hr) and blacknose Carcharhinus acronotus (modified: 0.04, unmodified: 0.13 kg/gillnet/hr); but not blacktip Carcharhinus limbatus (0.09 vs 0.13 kg/gillnet/hr) or bonnethead sharks Sphyrna tiburo (0.23 vs 0.31 kg/gillnet/hr). Catch rates of all four species were not significantly different between nets of 7.6 cm mesh size (modified: 0.05–2.11; unmodified: 0.08–2.46 kg/gillnet/hr). In addition, there was no difference in catch rates of the target fishery species Spanish mackerel Scomberomorus maculatus between modified and unmodified gillnets of the same mesh size (see paper for data). Data was collected from deployments of four gillnets of two mesh sizes (7.6 and 10 cm) by a commercial fishing vessel in May-September 2000. For each mesh size, one gillnet had increased tension (using larger floats on the top-rope and heavier weights on the bottom rope) and was set end to end (15 m apart) with the other, unmodified, net. Between 24–34 sets were made with each mesh size.

(Summarised by: Natasha Taylor)