Population diversity in Puget Sound, WA, Pacific herring: Is there a “portfolio effect?"

Background/Question/Methods

Pacific herring play a key role in the Puget Sound estuary food web, and their populations have declined over the last several decades. The underlying cause of these declines is unknown, however the success of any management strategy aimed at recovery relies on understanding the degree to which single actions are likely to impact the entire herring stock. This is particularly important in Puget Sound, where herring spawn at 19 discrete nearshore sites. The aim of this work was to determine the extent of demographic diversity among these 19 populations, using time series data of adult spawner biomass. We asked the following questions: Is there coherence among Puget Sound herring populations, indicating they respond similarly to environmental drivers; and does coherence vary spatially? Is there population diversity among Puget Sound herring that is likely to buffer the entire stock against environmental or human perturbations? To answer these questions we conducted time series and statistical analyses using annual estimates of adult herring spawner biomass from 1973-2011, calculated based on egg deposition and acoustic surveys.

Results/Conclusions

Variability of individual stock biomass often far exceeded that of Puget Sound-wide biomass. A large proportion (0.32) of correlations among individual herring population were negative, demonstrating asynchrony among populations. Synchrony did not increase substantially within regions. In some populations, the maximum proportional contribution to the total Puget Sound biomass exceeded the mean contribution by nearly 5x. Different populations contributed the bulk of biomass in different years, suggesting the importance of maintaining all populations in the system.

The lack of coherence among individual populations, high variability in abundance of individual populations relative to the entire stock, and changing proportional contributions by individual populations to the Sound-wide biomass through time, all suggest that the stock’s resilience depends on maintaining all populations in the ecosystem. Genetic data from earlier research suggest that Puget Sound herring belong to 3 separate populations, but our results point to additional diversity among populations not reflected in the genetic data. The regional incoherence among populations, and the negative correlations between some, suggest that drivers of variation in spawning biomass occur on neither a regional scale nor a Sound-wide scale, but are local in nature. This further emphasizes the importance of conserving individual herring populations in Puget Sound.