Including Historical Catches in Stock Assessments Reveals Comparable Depletion Rates in Northeast Atlantic Stocks

Whether fish stocks are sustainably exploited is judged against reference points, such as Maximum Sustainable Yield (MSY), estimated from a stock’s exploitation history. In the Northeast Atlantic, commercial fishing began in the Middle Ages and trawling developed around 1900, yet most stocks are assessed by ICES (International Council for the Exploration of the Sea) using time series that start after 1950. We collated historical catches not currently used by ICES into a new database for 83 stocks, and fitted Bayesian state-space surplus production models to 62 stocks with and without them, comparing terminal depletion and the shape and scale of the production function. Contrary to expectations, including historical catches did not result in more depleted terminal biomass for most stocks. Across the 62 stocks, B/BMSY, F/FMSY and catch relative to MSY were similar with and without historical catches, and BMSY/K was only marginally more often higher when historical catches were included. The effect was stock-specific, because the representativeness and bias of the historical series vary among stocks and periods. On average, 82% and 83% of six diagnostic tests were passed by the models fitted without and with historical catches, respectively. Historical catches nonetheless matter for sustainable management. They provide a plausibility check on initial depletion, can anchor stock status when they capture contrast between early low and later peak catches, and help constrain productivity and carrying capacity, and hence the scale of MSY-based reference points. We therefore, recommend that historical catches be evaluated routinely when setting sustainability baselines, with inclusion decided stock by stock.