Fish harvesting advice under climate change: A risk-equivalent empirical approach

2.3 Model implementation 2.3.1 Production-environment relationship. Pt/Bt was calculated from survey biomass and catch time series (fish > 40 cm) over all years which gives n-1 values for biomass surplus production. A surplus production calculation like this assumes that biomass index units are the same and of the same magnitude. This assumption would need to be evaluated for any case study in order to put catch and biomass on approximately similar scales but results always remain in relative units regardless which lessens the temptation to interpret results as absolute biomass and production levels. Recent science advice for the stock indicated that considering “stock status indicators and ecosystem conditions, a reduction in the exploit a, though express- ing the growth rate in survey units highlights the relative index nature of result annual P/B val- ues were statistically related to a time series of annual average water temperature at 150 m in the Central Gulf of St-Lawrence. This temperature index is derived annually based on various observation sources interpolated with bathymetric constraints [28]. Temperature values at depth in the Gulf of St Lawrence do not show large seasonal nor spatial variability, although a degree of mixing still occurs at 150 m in some areas. The temperature at 150 m correlates with a suite of environmental conditions in the Gulf of St Lawrence (e.g. oxygen concentration, temperature at most other, deeper depth strata occupied by Greenland halibut), all of which

may be sensitive to CC and impact Greenland halibut production. Thus we refer to tempera- ture at 150 m as the overarching summary variable of CC driven external conditions rather Fig 1. A map of the Gulf of St Lawrence showing the 150m and 300m isobaths and the main Greenland halibut fishing areas (in green). Reprinted from https://github.com/duplisea/ccca/blob/master/Fig1.png under a CC BY license, with permission from Daniel Duplisea, original copyright 2020. https://doi.org/10.1371/journal.pone.0239503.g001 PLOS ONE Climate-change conditioned risk-equivalent fisheries advice PLOS ONE | https://doi.org/10.1371/journal.pone.0239503 February 19, 2021 6 / 22