Predator-prey relationships can have profound effects on fish populations, but they are generally not considered explicitly in fisheries management. Several methods are available today to incorporate predators, a practical step toward ecosystem-based management.
Managing fisheries is a complex undertaking. This is partly because it is difficult or impossible to directly observe fish populations and the many factors affecting them. So managers and scientists use mathematical models to estimate fish abundance and evaluate possible management actions. At the most basic level, models use estimates of deaths and births, among other things, to calculate how many fish can be caught without reducing the longterm health or productivity of the fishery.
Most of the models in use today focus only on one species. Known as single-species models, they often assume that predators eat a low and constant share of fish. In reality, consumption by predators can vary widely and account for a significant share of the prey population. For example, Overholtz et al. (2008) found that predators take 20 to 40 percent of the Atlantic herring per year in the Gulf of Maine- Georges Bank area. If managers ignore this factor, they risk setting catch limits that are too high.
Animation: Fisheries and Predators
Methods to incorporate predators
Fortunately, several methods are available to incorporate predator-prey interactions into fisheries management. They include:
- “Second fleet.” Managers can use a single-species stock assessment model and treat predators as a second fishing fleet. The “catch” for this fleet can be derived from data on predator population and diet. Overholtz et al. (2008) found that incorporating data on predators led to a larger estimate of the mortality that the herring population can sustain. However, if one assumes predators will take their usual large share of this mortality, that leaves a smaller yield available to fishermen. Figure 1 presents details of this research, much of which was considered or implemented in the 2012 Atlantic herring stock assessment.
- Multi-species models. If more data are available, it may be possible to use a multispecies model. In South Africa, Punt and Butterworth (1995) used one such model to investigate interactions between fur seals and two species of hake—shallow-water and deep-water. The seals eat both hake species, and shallow-water hake occasionally eat deep-water hake, the smaller of the two species. One proposal was to kill fur seals in order to increase the hake catch, but the model showed that this would allow shallow-water hake to increase in number and eat more deep-water hake. As a result, killing seals would have little impact on the overall catch and could even reduce total hake biomass (see figure 2).
- Precautionary buffers. Even when fisheries managers cannot model predators, they can still explicitly account for them. For example, the Lenfest Forage Fish Task Force (Pikitch et al. 2012) recommended that when a predator-prey relationship is unclear, fishing on the prey species should be reduced, as a precaution to help protect both predators and prey. The size of the reduction would depend on the amount of scientific information available. (For more detail, click for the full report, summary, or table showing the tiered approach to forage fish management). Using a different approach, Smith et al. (2011) found a similar result: Halving exploitation rates of forage fish would protect predators while still achieving 80 percent of maximum sustainable yield (MSY). A widely discussed goal for fisheries is ecosystem-based management, which would consider a broad range of factors beyond the dynamics of a single species. Some have questioned whether this is feasible, but it is clear from the methods described here that managers can already take one step toward that goal, by incorporating predator-prey interactions into their decisions.
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Fact Sheet File: Methods to Consider Predators in Fishery Management (PDF)
This piece was originally published on May 7 on the Pew Charitable Trusts Environmental Initiatives blog.