Estimating end effects in trawl catches

Abstract

The end effect in trawl catches is defined as the proportion of the fish catch taken during shooting and hauling of the net, a period excluded from that nominally referred to as haul duration. If important, this effect will lead to biased abundance estimates, because the swept area will be underestimated. An experimental survey was carried out to compare catch numbers obtained in standard research 30-min hauls with those from 0-min hauls, the latter referring to the trawl being hauled as soon as the trawl geometry stabilized on the seabed. Average catch ratios (0-min/30-min hauls) ranged from 0.05 (s.d. 0.06) for sole to 0.34 (s.d. 0.64) for hake, indicating that the end effect might be more important and more variable for highly mobile species. As a consequence, the bias in abundance indices derived from swept area estimates that ignore end effects will be species-dependent.

Fish assemblages inside and outside marine protected areas off northern Iceland: protection effects or environmental confounds?

“…Apart from environmental factors, tow duration, if varying widely from tow to tow, may also be considered an important factor (Godø et al, 1990;Somerton et al, 2002;Battaglia et al, 2006). Although detected in some cases in the present study, this effect proved difficult to describe qualitatively.…”

Can we project changes in fish abundance and distribution in response to climate?

“…This indicates that there is something systematically wrong with the way that the SS‐DBEM handles those species, even though the general allocation of biomass to pelagics is reasonable. Note, however, that the bottom trawl surveys are not designed to sample pelagic species (ICES, 2016) and relatively short tows with limited time in the water column can provide a misleading picture of abundance and distribution for predominantly shoaling species (Battaglia, Trenkel, & Rochet, 2006). At the ‘Species by cells’ (1) level the species for which the models perform best are pout, haddock, plaice and lesser‐spotted dogfish, all bottom‐dwelling species that are effectively sampled by the survey gear (Fraser et al, 2007; Walker, Maxwell, Quesne, & Jennings, 2017).…”

“…This indicates that there is something systematically wrong with the way that the SS-DBEM handles those species, even though the general allocation of biomass to pelagics is reasonable. Note, however, that the bottom trawl surveys are not designed to sample pelagic species (ICES, 2016) and relatively short tows with limited time in the water column can provide a misleading picture of abundance and distribution for predominantly shoaling species (Battaglia, Trenkel, & Rochet, 2006).…”

We can reduce the impact of scientific trawling on marine ecosystems

“…Reducing haul duration entails the risk of reducing species richness as well as modifying sampled length and density estimates (Moriarty et al 2018). The importance of the so called "end effect" (Battaglia et al 2006) consisting of individuals caught during shooting and hauling will also be larger in shorter hauls, while the potential for trawl clogging will be reduced. Implementing handling methods that increase survival, such as keeping protected or fragile species in water while on board, should become standard practice on all surveys.…”