A document by Ken H Andersen. Click on the document to view its contents.

Robust projections of future trends in global fish biomass, production and catches under different fishing scenarios are needed to inform fisheries policy in a changing climate. Trust in future projections, however, relies on establishing that the models used can accurately simulate past relationships between exploitation rates, catches and ecosystem states. Here we use fisheries catch and catch-only assessment models in combination with effort data to estimate regional fishing exploitation levels (defined as the fishing mortality relative to fishing mortality at maximum sustainable yield, F/FMSY). These estimates are given for large pelagic, forage and demersal fish types across all large marine ecosystems and the high seas between 1961 and 2004; and with a ‘ramp-up’ between 1841-1960. We find that global exploitation rates for both large pelagic and demersal fish are consistently higher than for forage fish and reached their peaks in the late 1980s. We use the exploitation rates to globally simulate historical fishing patterns in a mechanistic fish community model. We find a good match between model and reconstructed fisheries catch, both for total catch as well as catch distribution by functional type. Simulations show a clear deviation from an unfished model state, with a 25% reduction in fish biomass in large pelagic and demersal fish in shelf regions in the most recent years and a 50% increase in forage fish, primarily due to the release of predation pressure. These results can set a baseline from which the effect of climate change relative to fishing could be estimated.

Fish community biomass is generally thought to decline with increasing temperature due to higher metabolic losses resulting in less efficient energy transfer in warm-water food webs. However, whether these metabolic predictions explain observed macroecological patterns in fish community biomass is virtually unknown. Here we test these predictions by examining the variation in demersal fish biomass across 21 productive shelf regions using high-resolution monitoring data from the North Atlantic and Northeast Pacific. We find that biomass per km2 varies 40-fold across regions and is highest in cold waters and areas with low fishing exploitation. We find no evidence that temperature change has impacted biomass within marine regions over time. Yet, the cross-regional patterns suggest that long-term impacts of warming will be negative on biomass. These results provide an empirical basis for predicting future changes in fish community biomass and its associated services for human wellbeing i.e., food provisioning, under global warming.