Relative selectivity in trawl, longline and gillnet fisheries for cod and haddock

Abstract

A full-scale fishing experiment on north-east Arctic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), involving a trawler, a longliner and a gillnetter was conducted in order to determine how the length distributions and life history parameters of the catches were affected by gear type. Relative selectivity was analysed for catches taken when the boats fished simultaneously in the same area, and when the skippers were allowed to fish under conditions as close as possible to commercial operation. Trawl and longline-caught cod had similar length distributions, but cod caught by trawl were on average 2.3 cm smaller than longline-caught cod. Cod caught by longline had a lower condition factor and length at age. The longliner caught smaller haddock than the trawler when the boats fished in the same area. When the skippers were allowed to change fishing ground, the mean length of cod increased in the trawl catches and became larger than the mean length in the longline catches. The mean length of haddock in the longline catches increased by 2.8 cm and became larger than that of the trawl catches. Gillnet catches consisted almost solely of large cod. Mean length of gillnet caught fish in 186 mm nets decreased from 86.2 cm to 82.3 cm even if the boat did not change area. Separate selectivity experiments for trawl and gillnet were conducted, and the parameters for the mesh selection are presented. To find the functional form of the longline selectivity we used relative catch proportion in each 5 cm length-group for longline compared to trawl and gillnet, whose selectivities were assumed to be known. When compared to that of the gillnet, the plots indicate that the longline selectivity curve takes the form of a unimodal distribution. When longline catches was compared to trawl catches none of the suggested models seemed to fit, and to conclude anything about the form of the longline selection curve is not possible. International Council for the Exploration of the Sea

Quantifying the roles of competing static gears: comparative selectivity of longlines and monofilament gill nets in a multi-species fishery of the Algarve (southern Portugal)

“…However, in most cases the studies have focused on one or two species, with few examples of a multi-species nature and, with the exception of a few studies such as those of Huse et al (2000), Halliday (2002) and Stergiou and Erzini (2002), most did not avoid the above mentioned confounding effects. In the current study bias due to spatial variability was minimised by ensuring that fishing with the two gears took place on the same fishing grounds and at the same time.…”

“…The problem of gear competition is particularly important from the socioeconomic point of view, especially so for artisanal fishers (Durand et al, 1991;Anon., 1995). While a number of studies have compared two or more gears in terms of catch composition, catch rates and size selectivity (Rollefsen, 1953;Russell et al, 1988;Elliott and Beamesderfer, 1990;Engas et al, 1993;Nedreaas et al, 1993;Hareide, 1995;Jorgensen, 1995;Huse et al, 1999), very few studies have been based on the comparison of gear fished commercially and simultaneously on the same fishing grounds (Huse et al, 2000;Stergiou and Erzini, 2002).…”

Length-based stock assessment of Oreochromis mossambicus and Oreochromis niloticus (Actinopterygii: Perciformes: Cichlidae) in multi-mesh gillnet fisheries in reservoirs of Sri Lanka

“…This gillnet selectivity analysis was based on the assumptions that fishes of size classes are equally available to different mesh sizes, and that catchability in gillnets is dependent on the geometric shape of the fish and the mesh size (Baranov 1948). These assumptions are a useful simplification and the basis of the majority of gillnet selection studies (Hamley 1975, Kirkwood and Walker 1986, Huse et al 2000, Baremore et al 2012.…”

Hook selectivity of the pacu Piaractus mesopotamicus (Holmberg, 1887) in the Pantanal, the state of Mato Grosso do Sul, Brazil

“…This author emphasizes that there is no validation of these models and argues that they are very simple. By comparing selection criteria in many fishing gears, Huse et al (2000) mentioned that fish format in trawling and emeshing nets is a more rigorous selection criterion than those based on competition and behavior, which are important mechanisms for species and size selection in hook fishing. Moreover, comparative captures may not offer conclusive evidence of a selection curve because data adjustment may result in an infinity of selection curve models (Millar, 1995).…”