Optimal management of a deteriorating fishery
As a result of climate change, populations may find themselves in a continually degrading habitat as temperatures (or pH, etc.) become increasingly unfavorable. For commercially important species, harvesting is another stressor. For marine fish, harvest is a major source of mortality. Understanding how harvesters will modify their harvesting strategies in the face of climate change is crucial for understanding whether exploited populations will persist in the future and how they might be protected.
To better understand the relationship between degrading habitat and optimal harvesting, we constructed a simple Beverton-Holt model in which per-capita growth rate declines over time and imagine the population is harvested by a profit-maximizing harvester. We analyzed and compared three scenarios under which the harvester either manages this population with (1) perfect knowledge of the decline, (2) imperfect knowledge of the decline, or (3) no knowledge of the decline. The outcomes—in terms of population size over time as well as profits—were derived for different biological and economic parameters.
When the growth rate declines, a harvester with perfect knowledge of the changing conditions will often initially decrease harvest (increase escapement) as conditions worsen. Once conditions degrade sufficiently, the harvester will increase harvest. When the population growth rate decreases to the point that the population would eventually become extinct even in the absence of harvesting, the harvester will fish the population to the lowest level that is economically profitable.
When the harvester instead has imperfect knowledge, we use passive adaptive management to allow the harvester to learn about the unknown impact of the degrading environment on the population. Profits may be approximately 6% lower, relative to perfect knowledge, while population sizes may be approximately 2% lower. Worse outcomes (in terms of profit and population size) are obtained when the harvester does not take the environmental decline into account at all, as he harvests 'too hard' relative to the optimal and fails to fish down the population at the appropriate time.
These results suggest that there can be significant economic and conservation value in improving our understanding of the effects of climate change on growth rate. They also suggest that ignoring such effects can have dramatic negative consequences.