Estimating gear efficiency in a combined acoustic and trawl survey, with reference to the spatial distribution of demersal fish

Abstract

Doray, M., Mahévas, S., and Trenkel, V. M. 2010. Estimating gear efficiency in a combined acoustic and trawl survey, with reference to the spatial distribution of demersal fish. – ICES Journal of Marine Science, 67: 668–676. Few analyses have been performed to estimate the efficiency of trawls targeting demersal fish using the ratio of catches and acoustic densities. In summer 2006, acoustic and fishing data were collected simultaneously over 3 d by three fishing vessels equipped with identical pelagic trawls in the Bay of Biscay. Variography identified moderate spatial autocorrelation in the acoustic backscatter at a mean scale of 3 km, a scale slightly smaller than the mean haul length (3.5 km), indicating that fish horizontal availability did not influence trawl efficiency. Acoustic backscattering densities expressed as nautical area scattering coefficients (NASCs) recorded in the trawled layer were compared with equivalent NASC (ENASC) values calculated from the species composition in the trawl, fish-length structure, and available relationships between target strength and fish length. Estimates of trawl efficiency for hake-dominated trawls were computed as the slopes of the relationships ENASC = 0.008 NASC and ENASC = 0.18 NASC0.31 for trawls made by day and night, respectively. For the whole demersal community, the relationships were ENASC = 0.022 NASC and ENASC = 0.17 NASC0.33 for trawls made by day and night, respectively.

Surveying while fishing in the slope areas off Brazil: direct assessment of fish stock abundance from data recorded during commercial trawl fishing operations

“…Whereas rarely a realistic choice, this assumption may be sometimes favored as a conservative measure that will result in more precautionary management choices. On the other hand, adopting any estimated value below 1 will result in higher (and potentially more realistic) biomass estimates of the targeted stocks (Harden-Jones et al, 1977;Dickson, 1993aDickson, , 1993bGunderson, 1993;Hoffman et al, 2009;Doray et al, 2010). REVIZEE trawl surveys have been an example of the former case where biomass tended to be 'underestimated' by using the trawl full efficiency assumption, as a conservative option that could compensate for other sources of error.…”

Catch of pelagic hauls in Mediterranean acoustic surveys: Is it the same between day and night?

“…In order to obtain an indication regarding the possible bias of fishing during daytime and night-time, the trawl efficiency was estimated according to the approach described by Doray et al (2010). For this purpose we used the available trawl data from monospecific hauls (i.e.…”

“…The values of fish backscattering echo were then summed over each ESU at the depth range sampled by the pelagic trawl plus 5 m above and below this depth. This was done considering that the effective fishing height of the trawl is expected to be wider than the actual trawl opening (Doray et al 2010). Total NASC values (NASC t ) recorded at trawl station t were calculated as the total NASC values of the ESUs along the haul tracks.…”

“…To transform catch data to equivalent acoustic data, equivalent NASC (Simmonds and MacLennan 2005), ENASCs(t), were computed for each of the main species (s) caught at station t (Doray et al 2010) by…”

Resolution of biodiversity and assemblage structure in demersal fisheries surveys: the role of tow duration

“…The catchability of different species depends on many factors, including fish behaviour in relation to the gear type (otter trawl or beam trawl), herding efficiency, and the probability of escape at the entrance to the net (Wardle, 1993;Engås, 1994). For some species, catch rates may vary because their behaviour changes throughout the day (Trenkel et al, 2008;Doray et al, 2010); while for other species catch rates may also vary over the duration of the tow due to spatial heterogeneity (Kingsley et al, 2002) .…”