Transient responses of fished populations to marine reserve establishment

Abstract

Implementation of no-take marine reserves is typically followed by monitoring to ensure that a reserve meets its intended goal, such as increasing the abundance of fished species. The factors affecting whether abundance will increase within a reserve are well characterized; however, those results are based on long-term equilibria of population models. Here we use age-structured models of a generic fish population to analyze the short-term transient response. We show that it may take decades for a fished population to reach postreserve equilibrium. In the meantime, short-term transient dynamics dominate. During the transient phase, population abundance could either remain unchanged, decrease, or exhibit single-generation oscillations, regardless of the eventual long-term result. Such transient dynamics are longer and more oscillatory for populations with heavier fishing, older ages at maturity, lower natural mortality rates, and lower larval connectivity. We provide metrics based on demographic data to describe the important characteristics of these postreserve transient dynamics.

Marine reserves can enhance ecological resilience

“…One driver of increased biomass in reserves is larger body sizes for harvested populations (Lester et al 2009), which arise from lower mortality and therefore greater survival to larger sizes, especially if a fishery targets larger individuals (both theoretically expected and empirically verified; see Taylor & McIlwain 2010, White et al 2013. Therefore, both the age and size structure in reserves will fill in with older ages and larger sizes, eventually approaching a stable age and size distribution (White et al 2013).…”

“…Overall, increases in abundance can arise from both decreased mortality and increased reproductive output as body size increases, whereas increases in biomass can arise from both increased body size and increased abundance. The expected increase in biomass and abundance is greater for higher harvest rates outside the reserves and before reserve establishment (Guénette & Pitcher 1999;White et al 2010bWhite et al , 2013. The timescale of biomass and abundance responses inevitably depends on life history and fishing history (intensity and duration; see Gerber et al 2003, Jennings 2000; see also the sidebar, Long-Term Field Study: A Tropical Example).…”

“…Therefore, both the age and size structure in reserves will fill in with older ages and larger sizes, eventually approaching a stable age and size distribution (White et al 2013). As the age and size structure fill in, reproductive output increases because of the increased number of mature individuals and because fecundity increases with maternal age and size [e.g., modeled by White et al (2013) and verified within reserves by Diaz et al (2011) (Shears & Babcock 2003), with a benthic habitat-type shift from urchin barrens (from 27-87% to 0% cover since reserve establishment) to kelp forests (Ecklonia radiata; from 0-5% to 17-50% cover) in regions <8 m (Parsons et al 2004). Kelp forest stasis or decrease at greater depths, likely due to increased turbidity (Parsons et al 2004), illustrates weakened cascades with disturbance.…”

“…Initial increases in biomass after reserve establishment typically precede increases in abundance because increases in body size depend on the within-generation process of somatic growth, whereas increases in abundance depend on the multigenerational process of increased reproduction (Halpern & Warner 2002, Molloy et al 2009). The timescale of the age and size structure filling in response increases with increasing age at maturation and longevity (White et al 2013). Initial, short-term (transient) responses in abundances can, theoretically, include declines or oscillations at a period of the generation time, especially for species with greater age at maturity, longevity, and intensity and duration of harvest (White et al 2013).…”

Trends and variability in demographic indicators of a recovering population of green sea turtles Chelonia mydas

“…If eggs had also been harvested, recovery would likely have taken much longer, as has been observed in the Florida green turtle population (Chaloupka et al 2008, NOAA & USFWS 2015. Ultimately, examining changes in juvenile size distributions over time may be a better indicator of population-level changes than sizes of adults, as juvenile size distributions could give an early indication of changes in age structure as cohorts 'fill in' the gaps caused by exploitation (White et al 2013).…”