Quotas on the hunting or fishing of wildlife may be worsening the problem they seek to address. A new study has predicted cycles in wildlife population sizes over time and indicates that quota systems, introduced in response to declining numbers of wildlife, do not respond quickly or accurately enough to changes in population size and may put wildlife at greater risk.
Overexploitation is commonplace in the harvesting of fish and wildlife. There has been growing interest in 'optimal harvesting policy' that proposes control methods such as 'threshold harvesting', where an allocated number of animals are excluded from hunting or fishing. These thresholds are re-assessed regularly in order to adjust to changes in population figures.
However, in reality it is difficult to apply such control methods, especially in 'recreational harvesting', such as sports fishing and wildlife hunting. Here, the populations are more prone to random variations in size and control measures are often in the form of ad hoc quotas on the number of animals killed rather than the number that remain alive. These quotas tend to be changed from year to year and this study suggests that they are not as responsive to population size as they should be.
The research developed a model to predict fluctuations in wildlife populations where harvesting is 'open access' - this means that there are no restrictions apart from annual quotas. It assumed those in charge of quotas would adjust them according to population sizes.
The model predicted that, even in a relatively stable population, quotas take a long time to achieve their goal of maintaining the number of wildlife at a certain level. This slow adjustment would produce a pattern of variation in population size that repeats over a period of decades. This pattern is more apparent in species with slow growth rates that take longer to adapt to changes in population, such as large mammals. The model also predicted that, at high levels of harvesting, annual quotas increase the risk of population collapse (where the population declines by more than 10 per cent below the expected level).
To evaluate the model, the research compared its predictions with data collected over 20 years on three populations of hunted wildlife: one population of Canadian white-tailed deer and two populations of Norwegian moose. All population numbers fluctuated in a pattern that was similar to that predicted by the model. This pattern of changes in population numbers repeated every 8, 24 and 30 years for each of the three populations. The researchers also found that hunters did not reduce their level of hunting activity until 2 to 6 years after population levels had fallen, and that they do not respond immediately to new quotas.
The research indicated that delayed human responses to changing populations and delayed wildlife responses to quotas may lead to conservation and management benefiting from more frequent re-assessment of quotas or from using policies whose application or impacts are more stable over time. Future policies should look to maintaining a more constant effort on limiting the amount of harvesting, perhaps by using threshold policies where certain numbers of wildlife are always excluded.
Source: Fryxell, J.M., Packer, C., McCann, K. et al. (2010). Resource Management Cycles and the Sustainability of Harvested Wildlife Populations. Science. 328:903-906.