A meta-analysis of vertical stratification in demersal trawl gears

A meta-analysis is presented of fishing trials that use trawl gear with horizontal separator panels to direct fish into an upper or lower codend. The analysis is applied to eight North Atlantic species: the gadoids cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), and whiting (Merlangius merlangus), the flatfish lemon sole (Microstomus kitt) and plaice (Pleuronectes platessa), and monkfish (Lophius piscatorius) and Nephrops (Nephops norvegicus). The proportion of fish that rise above the separator panel decreases as the height of the leading edge of the panel increases for six of the eight species. Only monkfish and Nephrops have no significant dependency on panel height. Cod is the only species for which separation depends on the horizontal distance of the leading edge of the panel from the ground gear, with the proportion of cod going above the panel increasing the farther the panel is from the ground gear. The time of day only affects the separation of plaice, with a greater proportion going above the panel at night than during the day.