Sampling estuarine fish and invertebrates using demersal otter trawls: Effects of net height, tow duration and diel period

Trawl surveys are done in many parts of the world but few studies have used pilot experiments to examine the effects of gear design and sampling practices on retained samples. We did two separate experiments in two south-eastern Australian estuaries to test the hypotheses that samples of fish and invertebrates taken using a demersal otter trawl would be affected by: (i) vertical net height (standard 0.8 m vs. high-rise 1.2 m) and tow duration (5 vs. 10 vs. 20 min); and (ii) diel period (day vs. night) and tow duration. Mean catch-per-unit-effort (standardised to numbers of individuals caught 5 min−1) was significantly larger in the high-rise net for some variables, but in most cases, CPUE was correlated proportionately between the two heights of net. There were no differences in the structure and composition of assemblages between net heights or among tow durations. The size-frequencies of fish differed between net heights and among tow durations, but there were no clear patterns in the proportions of fish caught across different size classes. Several species were caught mostly, or in larger numbers, at night and contributed to differences in assemblages of fish and invertebrates between diel periods. We stress the importance of using pilot studies to develop more reliable and cost-effective surveys and conclude that future sampling with our trawl should be done at night using the standard net and short (5 min) tows.