Taking account of catchability in groundfish survey trawls: implications for estimating demersal fish biomass

Abstract

Fraser, H. M., Greenstreet, S. P. R., and Piet, G. J. 2007. Taking account of catchability in groundfish survey trawls: implications for estimating demersal fish biomass. – ICES Journal of Marine Science, 64. Groundfish surveys are a key component of current scientific data monitoring and data-collection activities undertaken in support of fisheries management. Recent requirements to develop and implement an ecosystem approach to management are placing increasing and varied demands on such datasets. Successfully incorporating ecosystem and environmental objectives within fisheries management will, for example, require greater understanding of foodweb trophodynamics, which in turn requires detailed information on the abundance and distribution of fish predators and prey species on spatial scales hitherto rarely considered. However, no trawl gear catches all the fish in its path, so density estimates provided by such trawl samples do not reflect true densities of fish. Catchability in a trawl gear is affected by many factors and varies both between species and between different sized conspecifics, and therefore has the capacity to confound our understanding of predator–prey interactions and of the relative abundance of different species and size classes of fish at any point in time or space. To overcome such problems, estimates of the catchability of each size class of each species sampled in a given survey are required for trawl sample densities to be raised to the actual densities of fish present at each location sampled. Here, we present a method for estimating catchability coefficients for 1-cm size classes of fish species sampled by the Grande Overture Vertical trawl during the third quarter ICES International Bottom Trawl Survey. The catchability coefficients obtained are applied to survey data collected between 1998 and 2004 to examine annual variation in actual abundance of the demersal fish assemblage in the North Sea.

Size-selective fishing drives species composition in the Celtic Sea

“…Notably, the s.d. of biomass distributions for eight species (Figure 4a) were quite robust to "correction" (Table 1) using three arbitrarily chosen catchability curves (Fraser et al, 2007). There was also evidence that variations in mean logarithmic length over time were of similar magnitude.…”

“…Although this Figure 4a) and survey data raised using each of three catchability curves (curve 1, cod; curve 2, haddock; curve 3, whiting) from Fraser et al (2007). Figure 6.…”

“…Comparative studies find clear signals of length-dependent catchability in bottom-trawl surveys similar to that considered here (Korsbrekke and Nakken, 1999;Benoît and Swain, 2003), but absolute catchabilities are difficult to measure. Fraser et al (2007) estimated catchability curves for several demersal species by comparing North Sea bottom-trawl survey data with VPA assessments. Those specific curves are unlikely to apply directly to the Celtic Sea WCGFS, but provide an opportunity to evaluate the effect on community length distributions of applying arbitrary but realistic catchability curves.…”

“…The s.d. values of biomass-weighted logarithmic length (Table 1) were calculated for the eight species anglerfish (Lophius piscatorius), monkfish (Lophius budegassa), blue whiting, cod, haddock, hake, megrim, and whiting in the WCGFS in 1998, using raw survey data ( Figure 4a) and survey data raised across all eight species using each of three catchability curves (cod, haddock, and whiting) from Fraser et al (2007). The s.d.…”

Sea floor litter monitoring : International Bottom Trawl Survey 2016

“…The majority of the items is small (Figure 3-3), even smaller than most fish for which a catchability of less than 5% is assumed, e.g. being caught randomly rather that representative (ICES 2003, Fraser et al 2007, Piet et al 2009). Therefore the probability of catching these small litter items is assumed to be minute and random.…”