This study represents one of the few attempts at assessing the importance of ground gears on the engineering and catching performances of penaeid trawls and, in doing so, contributes towards a growing body of research focusing on minimising the environmental impacts of this important component of mobile gear (Brewer et al., 1996; Rose, 1999; Ball et al., 2003; Shepard et al., 2009). In particular, by eliminating the potential for confounding effects from varying trawl geometries, we have obtained fine-scale estimates of ground-gear contributions towards total system drag and species selectivity; outcomes that can be discussed according to the associated geometric and species-specific responses, and then used to prioritise future research.
In terms of trawling-system drag, the ground gear contribution can be delineated as the sum of components caused through(i) physical benthic contact (i.e. ‘scraping’) and (ii) hydrodynamic forces (viscous friction and pressure). For the three chainlinkground gears tested here, the above combined to provide a resis-tance that was some 15–22% of the total system. Given the relatively low profiles of the chain-links, much of their drag can be attributedto (i) above; manifesting as a clear positive relationship with chain-weight, and presumably caused by the heavier chains penetrating more sediment and creating more scraping resistance.