Managing fisheries for maximum nutrient yield

Abstract

Wild-caught fish are a bioavailable source of nutritious food that, if managed strategically, could enhance diet quality for billions of people. However, optimising nutrient production from the sea has not been a priority, hindering development of nutrition-sensitive policies. With fisheries management increasingly effective at rebuilding stocks and regulating sustainable fishing, we can now begin to integrate nutritional outcomes within existing management frameworks. Here, we develop a conceptual foundation for managing fisheries for multispecies Maximum Nutrient Yield (mMNY). We empirically test our approach using size-based models of North Sea and Baltic Sea fisheries and show that mMNY is predicted by the relative contribution of nutritious species to total catch and their vulnerability to fishing, leading to trade-offs between catch and specific nutrients. Simulated nutrient yield curves suggest that vitamin D, which is deficient in Northern European diets, was underfished at fishing levels that returned maximum catch weights. Analysis of global catch data shows there is scope for nutrient yields from most of the world's marine fisheries to be enhanced through nutrient-sensitive fisheries management. With nutrient composition data now widely available, we expect our mMNY framework to motivate development of nutrient-based reference points in specific contexts, such as data-limited fisheries. Managing for mMNY alongside policies that promote access to fish could help close nutrient gaps for coastal populations, maximising the contribution of wild-caught fish to global food and nutrition security.

Keywords: fisheries management; food security; nutrition; overfishing; seafood; sustainable fisheries.

FIGURE 1

Theorised maximum nutrient yield curves for multispecies fisheries. (a) shows the effect of exploitation rate on total catch (blue), fishable biomass (green), mean size (yellow) and number of collapsed stocks (orange). Nutrient yield curves may be maximised at fishing levels (b) below mMSY (nutrient overfishing), (c) above mMSY (nutrient underfishing) or (d) similar to mMSY. Catch curves were generated using a generic size‐based fisheries model of 15 interacting species with varying nutrient concentrations

FIGURE 2

Nutrient yield curves in North Sea and Baltic Sea fisheries. (a) In the North Sea, maximum vitamin D yield occurred at fishing mortality above F_mMSY, owing to (b) high contribution of sandeel (purple), herring (orange) and sprat (green) to vitamin D yields. The remaining nine species (grey) contributed <5% of the maximum vitamin D yield owing to their low productivity and/or low vitamin D concentration. (c) In the Baltic Sea, maximum vitamin A yield occurred at fishing mortality below F_mMSY, owing to relatively high contribution of (d) sprat and herring to selenium yields. Fishing mortality is total catch/total biomass

FIGURE 3

Predicting Maximum Nutrient Yield from catch evenness and species’ vulnerability to fishing. (a) In fisheries with high nutrient‐catch evenness, nutrients are supplied by several species, such that reaching mMNY will require fishing effort to be optimised over multiple species. In fisheries with low nutrient‐catch evenness, few or one species contribute to nutrient yields, such that single‐species management might be used to achieve MNY. In both multispecies and single‐species contexts, nutrient catches that are dominated by species resilient to fishing will have F_mMNY > F_mMSY, such that nutrients are underfished at mMSY. Nutrient catches that are dominated by species vulnerable to fishing will have F_mMNY < F_mMSY, indicating nutrients are overfished at mMSY. (b) North Sea vitamin D yield was more uneven and less vulnerable to fishing than total catch, indicating nutrient underfishing at F_mMSY where few species contributed to nutrient yields. (c) Baltic Sea vitamin A yield was more vulnerable to fishing than total catch at mMSY, indicating nutrient overfishing at F_mMSY where few species contributed to nutrient yields. Points are the catch evenness and mean F_MSY for each nutrient and total catch at mMSY, coloured by F_MSY (orange = vulnerable, green = resilient). F_MSY scales (b, c) are reversed to correspond with fishing vulnerability in (a) (i.e. high F_MSY = low fishing vulnerability)

FIGURE 4

Nutrient‐catch evenness and vulnerability of commercial marine catches from EEZs of 185 countries. (a) Points are the mean evenness and vulnerability to fishing of nutrient catches across six nutrients (calcium, iron, selenium, zinc, omega‐3 fatty acids and vitamin A) (±2 SEM), coloured according to their vulnerability to fishing from resilient (green) to vulnerable (orange). Labelled points indicate countries with even catches that were particularly resilient (<30) or vulnerable (>50), as well as the 20 most uneven countries (shaded area). Marginal histograms show data distributions along each axis. (b) Density plots show the vulnerability to fishing of nutrient catch relative to total catch, for each nutrient among all 185 countries. Distribution shading and annotated percentages indicate the proportion of countries where species that provided nutrient catch are less (negative values) or more (positive values) vulnerable to fishing than species that provided total catch, indicating potential nutrient under‐ or overfishing respectively