Fish lost at sea: the effect of soak time on pelagic longline catches

Our analyses of observer records reveal that abundance estimates are strongly influenced by the timing of longline operations in relation to dawn and dusk and soak time—the amount of time that baited hooks are available in the water. Catch data will underestimate the total mortalityof several species because hooked animals are “lost at sea.” They fall off, are removed, or escape from the hookbefore the longline is retrieved. For example, longline segments with soak times of 20 hours were retrieved withfewer skipjack tuna and seabirds than segments with soak times of 5 hours.The mortality of some seabird species is up to 45% higher than previously estimated. The effects of soak time and timing vary considerably between species. Soak time and exposure to dusk periods have strong positive effects on the catch rates of many species. In particular, the catch rates of most shark and billfish species increase with soak time. At the end of longline retrieval, for example,expected catch rates for broadbill swordfish are four times those at the beginning of retrieval.Survival of the animal while it is hooked on the longline appears to be an important factor determining whether it is eventually brought on board the vessel. Catch rates of species that survive being hooked (e.g. blue shark) increase with soak time. In contrast, skipjack tuna and seabirds are usually dead at the time of retrieval. Theircatch rates decline with time, perhaps because scavengers can easily remove hooked animals that are dead.The results of our study have important implications for fishery management and assessments that rely on longline catch data. A reduction in soak time since longlining commenced in the 1950s has introduced a systematic biasin estimates of mortality levels and abundance. The abundance of species like seabirds has been over-estimatedin recent years. Simple modifications to procedures for data collection, such as recording the number of hooksretrieved without baits, would greatly improve mortality estimates.

Bibliographic Details

Main Authors:	Ward, Peter, Myers, Ransom A., Blanchard, Wade
Format:	article biblioteca
Language:	English
Published:	2004
Subjects:	Ecology, Fisheries, Management,
Online Access:	http://hdl.handle.net/1834/30894
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