Investigating density-dependent catchability in bottom-trawl surveys

The relationship between density of groundfish in the area swept by a trawl and the catchability was explored. We studied Norwegian and Canadian underwater video observations of cod (Gadus morhua), haddock (Melanogrammus aeglefinis), and American plaice (Hippoglossoides platessoides) in the mouth of bottom-survey trawls and found that there were qualitative differences in escapement and capture behaviour at various densities. Based on these observations, available data from trawl efficiency experiments carried out in both regions were analysed to look for density effects on catchability. For both areas and all species examined, video and trawl efficiency experiments support the hypothesis that the density of fish ahead of the bottom-trawl affects catchability.