Estimating and accounting for fish losses under the footrope of a survey trawl: The case of northern shelf anglerfish

Abstract

Anglerfish, or, monkfish, (Lophius piscatorius and L. budegassa) are two of the most valuable commercial fish species in northern Europe. The stock which occupies the northern European shelf is monitored by an annual bottom trawl survey which aims to estimate absolute abundance. This estimation includes corrections for herding by the trawl gear but also requires an estimate of the capture efficiency of the net. To determine the latter, losses of fish under the footrope were quantified by 1 cm length class using trawls fitted with sub-footrope collection bags. The results demonstrate clear length dependency with the smallest fish being the most likely to escape under the gear. Overall, approximately 27 % of the anglerfish were lost under the footrope, with approximately 77 % of those below 30 cm being found in the ground gear collection bags. A length-based retention model for the gear was fitted here with a day-night effect, following appropriate model selection. This estimated higher proportions of fish escapes at night than during the day. The model was then used to estimate total stock numbers at length using data from the 2006 monkfish survey data to examine their impact on the stock estimates. As expected, this demonstrated a significant increase in the abundance of smaller recruiting fish when escapement under the footrope is accounted for. The estimates at length and age will provide better inputs for future developments of an age- or length-based analytical stock assessment which in turn will contribute to better stock management.