Capture time, size and hooking mortality of bottom longline-caught sharks

Published source details Morgan A. & Carlson J.K. (2010) Capture time, size and hooking mortality of bottom longline-caught sharks. Fisheries Research, 101, 32-37.
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This study is summarised as evidence for the following.

	Action	Category
	Deploy fishing gear at selected depths to avoid unwanted species Action Link
	Reduce duration of fishing gear deployments Action Link

Deploy fishing gear at selected depths to avoid unwanted species

A replicated study in 2005–2007 of fishing grounds in the North West Atlantic Ocean off Florida, USA (Morgan & Carlson 2010) found that different species of sharks were hooked on bottom-set longline gear at different water depths. Sandbar sharks Carcharhinus plumbeus were only caught at depths >20 m (21–40 m: 43, 41–60 m: 50, >60 m: 12 sharks/10,000 hook hours). Blacktip sharks Carcharhinus limbatus were caught less frequently at depths <60 m (<20 m: 41, 21–40 m: 18, 41–60 m: 15, >60 m: 91 sharks/10,000 hook hours). Three other shark species were most frequently caught between 41 and 60 m depths: tiger shark Galeocerdo cuvier (<20 m: 3, 21–40 m: 8, 41–60 m: 33, >60 m: 28 sharks/10,000 hook hours), Atlantic sharpnose Rhizoprionodon terraenovae (<20 m: 49, 21–40 m: 13, 41–60 m: 43, >60 m: 13 sharks/10,000 hook hours) and blacknose Carcharhinus acronotus (<20 m: 15, 21–40 m: 10, 41–60 m: 34, >60 m: 6 sharks/10,000 hook hours). Fifty-five gear deployments were undertaken, each with 8–10 km of longline with 250 branch lines set with 18/0 circle hooks with a 10° offset, deployed either overnight for 6–10 h or for 4–6 h during the day.

(Summarised by: Leo Clarke)
Reduce duration of fishing gear deployments

A replicated study in 2005–2007 of a fished area of seabed in the Atlantic Ocean off Florida, USA (Morgan & Carlson 2010) found that catch rates of unwanted sharks (Chondrichthyes) on bottom-set longlines were lower at shorter times the gear had been in the water, and varied between species with depth, at fishing durations of up 10 hours. For the four main species, the overall probability of capture (hook being bitten) increased most from 5 hours after the start of gear deployment compared to the first 5 hours of the sets, and for individual species the average amount of time hooks were in the water prior to being bitten was 4 hours for sandbar Carcharhinus plumbeus and blacknose sharks Carcharhinus acronotus, 5 hours for blacktip sharks Carcharhinus limbatus, and 9 hours for bull sharks Carcharhinus leucas, respectively (data reported as statistical model results). Sandbar sharks were only caught at depths >20 m (21–40 m: 43, 41–60 m: 50, >60 m: 12 sharks/10,000 hook hours). Blacktip sharks were caught less frequently at depths <60 m (<20 m: 41, 21–40 m: 18, 41–60 m: 15, >60 m: 91 sharks/10,000 hook hours). Blacknose sharks and two other shark species were most frequently caught between 41 and 60 m depths: blacknose (<20 m: 15, 21–40 m: 10, 41–60 m: 34, >60 m: 6 sharks/10,000 hook hours), tiger shark Galeocerdo cuvier (<20 m: 3, 21–40 m: 8, 41–60 m: 33, >60 m: 28 sharks/10,000 hook hours), Atlantic sharpnose Rhizoprionodon terraenovae (<20 m: 49, 21–40 m: 13, 41–60 m: 43, >60 m: 13 sharks/10,000 hook hours). Fifty-five longline deployments were undertaken (8–10 km of longline, 18/0 circle hooks with a 10° offset). Longlines were deployed either overnight for 6–10 h or for 4–6 h during the day. Hook timers on each hook recorded shark capture times.

(Summarised by: Leo Clarke)