Fishing in a stirred ocean: Sustainable harvest can increase spatial variation in fish populations.
Berkley, Heather*,1, Kendall, Bruce1, Siegel, David1, Costello, Christopher1, 1 University of California, Santa Barbara, Santa Barbara, CA
ABSTRACT- Most marine fisheries models that examine the effects of harvest on population dynamics focus on changes in mean density or temporal variability. In contrast, a spatially explicit model can be used to examine the spatial effects of harvest on population size and recruitment. Due to turbulent motion in the ocean, the release of larvae from several locations may be grouped together so their dispersal trajectories are correlated. Also, the dispersal kernel we used is determined by oceanographic conditions, distance traveled, and planktonic larval duration. Combined, this produces the possibility of dispersing many larvae to one location and none to another. Therefore, our model generates stochastic and "spiky" recruitment, which produces heterogeneity in population size in the absence of a spatially heterogeneous environment. We asked the question: what is the impact of harvest on the spatial variability of adult density and recruitment? This was tested using 3 different harvest policies: a constant fraction harvested, a total allowable catch for the region, and a constant escapement level for each location. Intuition suggests that removing adults would decrease the spatial variation because at high adult densities the harvest pressure is greatest, which might be expected to drive the population down to a fairly constant level across the spatial domain. Counter intuitively, we find that increasing harvest from zero to the highest sustainable level for all harvest policies increases the spatial variability in both recruitment and adult abundance. This relates to density dependent post-settlement recruitment as well as the "spiky" recruitment. When adult populations are reduced below the natural carrying capacity, recruitment is less impacted by density dependence so that each larval recruitment group is larger with more intense levels of harvest. Therefore, harvesting fish at sustainable levels can increase the spatial variability in adult populations.
Key words: variation, density dependence, recruitment, harvest
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