UW Aquatic & Fishery Sciences Quantitative Seminar
UW School of Aquatic & Fishery Science
Global changes in surplus production of fish stocks: a preliminary analysis based on stock assessment data
Fishing represents one of the largest and best documented perturbations to fish populations in the world’s oceans. While the economic benefits and ecological costs of fisheries receive most of the attention, fisheries also provide a unique opportunity to learn about the dynamics of populations, communities, and ecosystems. A global stock assessment database has recently been built in conjunction with an NCEAS working group that brought together marine ecologists and fisheries scientists from around the world. Using catch and biomass timeseries from this database, we can calculate a simple metric of productivity for exploited fish populations: surplus production or the net change in biomass after accounting for fishery removals. Trends and correlations in productivity allow us to assess the interaction of productivity across fish stocks and ecosystems and their response to external forces such as climate change and fishing. Synchronous patterns among stocks in any of the components of surplus production (recruitment, growth, or natural mortality) result in larger interannual variability in overall fishery productivity of an ecosystem. Conversely, negative correlations in surplus production among stocks can result in relatively stable productivity for an ecosystem as a whole – essentially dampening variability in the productivity of individual stocks. This second pattern has been referred to as the “portfolio effect,” and has been used as a practical argument for conservation of biodiversity and protection of “weak” or less productive stocks. We use surplus production calculated from stock assessments to examine productivity patterns from several large marine ecosystems around the world including the Eastern Bering Sea, California Current, Northeast U.S. Continental Shelf, North Sea, and the Baltic Sea.