Climate impacts on fished populations. Part 1: Simulating bottom-up, physiological, and fishery-induced changes in production potential

Climate influences fish stocks via direct physiological effects from changes in temperature, dissolved oxygen, and acidity. Nevertheless, concomitant impacts from indirect bottom-up and top-down effects of changing environmental factors, such as food resources and predation, may play an equally important role in determining productivity changes in marine environments. Together, direct and indirect influences can lead to changes in productivity of fish stocks, especially if stocks are unable to move in space to offset environmental shifts. Changes in productivity, in turn, interact with fisheries, which also affect the productivity of stocks through plastic, density-dependent effects such as increased growth of fish at reduced densities.

To assess the influence of climate on fish stocks, the present study developed and applied a model of individual eco-physiological response to environmental factors to derive population level outcomes. It then investigated how fished stocks respond to climate variation at various levels of fishing intensity, and how these changes interact with fishing-induced changes in productivity.

The model outcomes showed that fishing led to rapid and expected density-dependent per-capita declines in natural mortality (M) and growth (periodically faster growth and larger body size) over the period of initial depletion; there was higher average M due to age- and size-structure truncation from fishing. Climate responses were relatively small, but changed in the opposite direction to fishing for M (increasing per-capita M) and body size (smaller individuals). Increased growth rates at higher temperatures in climate scenarios further enhanced density-dependent changes from fishing. Nevertheless, the present approach also suggests that even moderate changes in environmental suitability are sufficient to offset changes determined by direct temperature impacts on focal species.

Our results suggest that climate responses of productivity parameters need to be considered in combination with density-dependent responses. For many stocks, it may not be possible to separate these processes, owing to the limitations of available data.