Decades of Fishing and Climate Change Eroded Nature's Insurance Policy for Pacific Herring

Our analysis of 65 years of Pacific herring data from Haida Gwaii reveals how a natural insurance system has eroded over time. We assembled records from 1950 to 2015 showing how Pacific herring populations across 11 locations around the British Columbia archipelago have fundamentally changed. What was once a portfolio of populations that boomed and busted at different times—providing reliable resources to predators and fishermen even when individual groups crashed—has become dangerously synchronized.

We wanted to understand what was driving this transformation. We built a Bayesian state-space model using spawn surveys and catch records to tease apart the relative effects of fishing pressure, environmental changes, and population growth on both local herring groups and the archipelago as a whole.

Our results painted a clear picture of decline. We documented a severe decline in herring population growth over the 65-year period, along with erosion of the herring portfolio itself. Commercial harvest had historically played a key role in herring dynamics, with typical annual exploitation rates hovering around 15% across the archipelago. But local harvest rates reached as high as 65% when fishing occurred in specific areas. The Pacific Decadal Oscillation and population growth had equally strong effects on both local and regional population dynamics.

What struck us most was how dramatically the portfolio effect—nature's way of spreading risk across space—had eroded. When populations become synchronized, they lose their ability to buffer against regional collapse. If one crashes, they all crash together.

This matters because herring are a cultural keystone species for indigenous peoples and a central node in Northeast Pacific food webs supporting top predators. Our findings suggest that developing herring management strategies at a finer spatial scale may help recover previous levels of spatial population asynchrony and ensure greater regional resource reliability.

Citation

Stier, Adrian C.; Olaf Shelton, Andrew; Samhouri, Jameal F.; Feist, Blake E.; Levin, Phillip S. (2020). Fishing, environment, and the erosion of a population portfolio. Ecosphere.

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