Reconciling Species Assessment

Marine fish species and populations are evaluated in a number of different ways, with two main methods including traditional stock assessment and conservation based metrics, such as the IUCN's Red List.  To make the Red List of Threatened Species, a species must have declined ≥50% within either a three-generation period or 10 years (whichever is longer).  The growing number of species included on the Red List has headlined many reports and articles; however, many fisheries scientists pay little attention to these analyses.  On the other hand, stock assessments use often complex mathematical models to describe the population biomass, both the overall levels as well as its comparison to benchmark levels of biomass, known as reference points.


The main disconnect between these two evaluation methods comes from the fact that for an exploited species to reach its maximum level of production, it needs to be fished down.  Although the levels to which a stock is fished down varies by population and can be hotly debated, it often requires removing ≥50% of the original population, in order for the population growth rate to be maximized.  Yet, despite the well-document and management-driven fishing down phase of a fishery, it is often assumed that this decrease in biomass may trigger a negative conservation status.  For this reason, some conservationists see managed species at risk of extinction, while fishery managers—even for overfished species—rarely think about extinction.

Counts of Red List species by category. See Davies and Baum for details.

Measuring the actual disconnect between these two evaluation methods is what Davies and Baum set out to examine in a Scientific Report in Nature last week.

The basic setup for their analysis was to collect Red List and stock assessment data for a number of marine species, and then classify the species or stock into one of four categories: positive hit, negative hit, false alarm, or miss.  (Check out the figure below for more information.)  The Red list criteria was simply threatened or not, and the stock assessment criteria was whether the stock was above or below its reference point, for example BMSY.

What they found was higher agreement than initially thought.  The total of hits (evaluation agreements) was 76%, although this number increased to 81% when a lower, riskier reference point was used.  From there, much of their discussion focused on the disagreements.  False alarms—Red List threatened, but not overfished according to stock assessment—occurred, but were not biasing the overall disagreement as some might think.  Misses—Red List not threatened, but overfished according to the stock assessment—were characterized by species with short generation times or those that have had low biomass levels for a long time, and contributed greatly to the overall disagreement.

The authors are wise to note several considerations, such as the varying ecological levels of the two assessments (species vs. populations), the difference in how the two metrics are calculated (rate-based vs. biomass-based), and the fact that many Red List species do not have a stock assessment (not to mention the error involved in even the best stock assessments and the range of reference points available).

Western Pacific striped marlin are an example of a "miss"—below its biological reference point, but not Red Listed. (Photo)

Davies and Baum do acknowledge that some species flagged by the Red List are likely just below their reference points and not in danger of extinction, but the real upshot here is the relatively high agreement between the two evaluation methods.  Ultimately, they suggest that Red List status could serve to inform fisheries that do not have stock assessments and serve as warnings of overfishing.

This paper is open and can be downloaded freely here.

Steve Midway

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