Towards the inclusion of broadband acoustic methods in the assessment of benthopelagic fish stocks

Reliable estimates of fish stock size are a prerequisite for sustainable exploitation. Active acoustic methods are cost-effective and non-intrusive means of quantifying pelagic organisms, delivering data with high spatial and temporal resolution. Recent advances in broadband echosounders have substantially improved range resolution, enabling effective detection of individual fish and opening opportunities for echo-counting - an approach not possible with traditional continuous wave acoustics. These advances now make it feasible to study benthopelagic fish stocks, i.e. fish stocks that are found both in the water column and in close proximity to the seabed, using echo-counting. However, the lack of standardized procedures has so far hindered the inclusion of echo-counting surveys in stock assessments. Here, we present a comprehensive method for estimating the total stock size of Atlantic cod (Gadus morhua), a benthopelagic species, using echo-counting and modern acoustic instruments. We compare results from echo-counting with traditional echo-integration and with the ICES assessment of Atlantic cod in the Nuuk fjord system – a favourable test scenario for benthopelagic fisheries acoustics. All approaches yielded comparable results. Based on these findings, we recommend echo-counting for benthopelagic fish stocks, where single echoes can be reliably resolved (Sawada index is