Spillover from marine reserves related to mechanisms of population regulation

Abstract

Spillover of fish from marine reserves to adjacent harvested waters may be mediated by density-independent movement, density-dependent movement, or both. If dispersal is by random movement, populations within the reserve must be regulated by density-dependent population growth (DDG). Density-dependent movement (DDM) can also regulate the population if accelerated emigration from a reserve to the surrounding fishing grounds leads to substantially increased mortality. Using spatially explicit models, we show that stock per unit area is bounded for DDG and increases with size for DDM. With DDG, spillover rate per unit area of reserve is maximized with reserves around 50% larger in linear dimension than the minimum size for population persistence. With DDM, spillover per unit area of reserve increases with reserve size. The results highlight the need for the mechanism of population regulation to be incorporated into theoretical and empirical investigations of marine reserve ecology.

The Ecological and Evolutionary Consequences of Marine Reserves

“…Increased movement out of reserves with increased density can, theoretically, lead to increased spillover with increased reserve size (Kellner et al 2008), as opposed to the typical expectation of decreased spillover with increased reserve size because of greater retention. Conversely, increased movement into reserves can lead to greater within-reserve abundance and less spillover with greater movement rates , Langebrake et al 2012, as opposed to smaller movement rates as typically expected.…”

“…However, declines can occur if the fishing effort displaced by reserves leads to increased mortality that outweighs the amount of spillover (Karnauskas et al 2011, Rassweiler et al 2012. Overall, the typical expected (Kellner et al 2008, Pelc et al 2010, White et al 2011 and observed (e.g., Hackradt et al 2014, Willis et al 2003) spatial distribution of abundance peaks within reserves and declines toward the reserve boundaries, depending on the spatial pattern of fishing in harvested areas (e.g., concentration of fishing effort near reserve boundaries; see Kellner et al 2007). The current standard for empirical inference of reserve-driven population responses is a before-after control-impact design-in other words, measurement of populations both inside and outside a reserve and before and after reserve establishment, which can control for the effect of environmental variability on population changes in time and the effect of habitat quality on population differences in space (Guidetti 2002, Jennings 2000.…”

“…This change in diversity and a 9% decrease in prey abundance and 3% increase in habitat complexity (coral cover, rugosity, and steepness) adjacent to the reserve (Russ & Alcala 2011) exemplify the potential for coupled community-level responses inside and outside reserves (Section 3.3). (Kellner et al 2008) Less movement out of reserves Increased residency to avoid negative interspecific interactions with increased crowding E: (Parsons et al 2010) Greater movement into reserves …”

Spillover from marine reserves and the replenishment of fished stocks

“…Kellner et al (2007Kellner et al ( , 2008 and Perez-Rusafa et al (2008) have previously shown that this form of a reaction-diffusion model produces the sigmoid shaped spillover curves exhibited across many marine reserve boundaries. Accordingly, population density, n, is described at each point in space as follows:…”

“…Growing evidence from theoretical models and empirical studies suggests that higher abundances inside reserves can lead to spillover of adults to nearby fished areas (Rowley 1994;Roberts et al 2001;Goni et al 2008;Kellner et al 2008;Perez-Ruzafa et al 2008). Spillover is typically observed as declining patterns of abundance or catch across reserve boundaries, although there may be no detectable pattern of decline in abundance if fishing effort is intense surrounding the reserve.…”