Improving fishing effort descriptors: modelling engine power and gear-size relations of five european trawl fleets

Based on information from an international inventory of gears currently deployed by trawlers in five European countries, the relationship between vessel engine power and trawl size is quantified for different trawl types, trawling techniques and target species. Using multiplicative modelling it is estimated that the fishing circle (or circumference) of trawls targeting shoaling species such as mackerel (Scomber scombrus) and herring (Clupea harengus) increases approximately 44.1m with each 100 hp increase, whereas the increase for trawls targeting demersal species such as Nephrops (Nephrops norvegicus) and monkfish (Lophius piscatorius) is only approximately 9.4mper 100 hp. Trawling technique also affects the relationship between vessel horsepower and fishing circle in that trawls used for pair trawling have a significantly (P < 0.001) lower rate of fishing circle increase with hp of a factor 0.56 of that of both twin and single trawls. Underlying these results is the definition of four geometrically different trawl typologies and corresponding target species, driven by the assumption that fishing mortality for a trawl gear is governed by its geometry and proportional to its size, as understood by existing knowledge of the interactions between trawl gear and target species. The modelling results have implications for the reliability of kilowatt days as descriptor of effective effort and point to the need of including metrics relating to the size and geometry of gear deployed in routine monitoring of fishing effort.