Population indicators such as spawning stock biomass (SSB) are commonly used to assess the status of marine species and set harvest levels and conservation targets. However, limitations in the data or methods used to estimate such indicators can lead to critical uncertainties that can erode the effectiveness of management strategies.
Such is the case within the Scotian Shelf and Bay of Fundy management zone (NAFO Division 4WX), where abrupt changes in the methods and data used to assess Atlantic herring have yielded levels of SSB that differ by as much as eight times over previous estimates and have led to serious concern and uncertainty regarding their conservation status.
In this study, the authors sought to reduce this uncertainty by integrating all available information from field observations, stock assessments, and published studies to understand how the heuristic status, or health, of the herring population, has changed over time. To achieve this, thirty-three individual population and higher-order indicators that describe the ecological dynamics of larval, juvenile, and adult herring were analysed individually and then integrated to produce a multivariate long-term (1965–2016) index of population health.
The results suggest that the population health of herring underwent a gradual, long-term decline since at least 1965, punctuated by a more rapid decline between 1980 and 2005, and culminating in the currently low but stable health level. Several of the individual health indices, including the average weight of herring, were found to be promising early warning indicators of future changes in SSB, and by applying this approach, we expect that SSB will reach historically low levels over the next six years.
This study suggests that integrating factors related to population health can provide deeper insight in situations where uncertainty is high and can complement existing assessment approaches. The new framework for estimating population health may be useful in assessing the status of exploited populations in general, ultimately leading to improved conservation outcomes.
