Theoretical study of the between-haul variation of haddock selectivity in a diamond mesh cod-end

We use PRESEMO, an individual-based structural model of cod-end selection, to investigate some of the mechanisms that may give rise to what has been termed ‘uncontrolled’ between-haul variation. This is the variation in selection that can occur from haul-to-haul even though there may have been no changes to the cod-end.We demonstrate how haul-to-haul variation of the population density, the population structure and the spatial distribution of both the target and by-catch species may lead to between-haul variation of cod-end selection.

We also show that for reasonable levels of variation of these parameters PRESEMO can simulate selection estimates that are consistent with and have comparable levels of between-haul variation as those obtained experimentally.