Effects of headline height on catches in southeastern Australian fish trawls

Benthic fish trawls are used off southeastern Australia to target multiple species that have similar sizes and morphologies to non-target species. These characteristics mean there are limited options for improving selectivity via changes to meshes and/or bycatch reduction devices in the codend. Consequently, simple modifications to the anterior trawl body that potentially exploit behavioural differences to separate species before entry have been prioritised for testing. Optimising trawl headline height is recognized as important for refining selectivity, but causal effects can be difficult to isolate owing to variability among confounding operational or technical factors (such as variable spread ratios, ground gears, and/or towing speeds). Here we attempted to isolate the effects of headline height by alternately fishing a conventional trawl (mean headline height ± SE of 3.76 ± 0.07 m) with a design that was identical in key aspects, but had a ∼22 % reduction in fishing circumference at the mouth and therefore a similar lower average headline height (2.75 ± 0.07 m) across homogenous operational variables. There were no significant differences in the numbers and weights of retained and discarded species between the two trawls, although mean catches of the primary target, tiger flathead, Platycephaus richardsoni were ∼25 % lower in the low trawl, implying this species orientates across the range of conventionally fished heights. Regardless of the trawl, there was a significant, negative main effect of headline height on the weights of total discards and two abundant batoids (smooth stingray, Bathytoshia brevicaudata and greenback stingaree, Urolophus viridis) attributed to lower ground-gear contact pressure and/or an increased footrope height of both trawls, facilitating the escape of some individuals under the trawl. The only other species affected by headline height was velvet leatherjacket Meuschenia scaber, with catches increasing as headline increased; possibly owing to changes in netting panel angles at the trawl mouth that precluded escape. It should be possible to marginally reduce the circumference at the mouth of regional trawls and so reduce headline height and at least some drag while maintaining target catches. More broadly, it appears that regardless of the trawl-body circumference, headline height can indirectly affect species selectivity via concomitant effects on other gear components.