Stochastic larval recruitment reduces the likely utility of rotating marine reserve schemes

Background/Question/Methods

No-take marine reserves that rotate over space have been proposed as an approach for preserving fish populations and improving fishery yield.  Earlier theoretical models have suggested that rotation schemes can allow fish biomass to repeatedly build up inside reserves and then be fished down, affording larger catches of bigger fish.  However, those models omit two key factors known to affect the time scale of post-reserve recovery: 1) the age structure of the fished population, which must ‘fill-in’ the empty older age classes; and 2) high stochastic variability during the larval stage, which can slow down the filling-in process.  Recent work by our group has shown the effect of the first factor: including realistic age structure in the model effectively eliminates the advantage of rotational schemes over fixed reserves.  Here we explore the role of stochastic larval recruitment on rotation success by analyzing a spatially explicit age-structured model of a generic fish species in a rotating reserve system, with varying amounts of interannual variation in larval recruitment.

Results/Conclusions

As expected, increasing levels of stochastic variation in larval recruitment produced increasingly variable transient dynamics.  This high variability greatly lengthened the optimal rotation period for reserves.  In a deterministic system, the optimal rotation period is approximately 1/M, the natural mortality rate of the species.  With stochasticity, the optimal period increases beyond that value and approaches infinity (i.e., the optimal solution is static reserves).  This result arises because only very old reserves does sufficient biomass build up in very old age classes to buffer the population against year-to-year variability in larval recruitment.  When a recently fished area is protected, the trajectory is highly sensitive to large recruitment pulses or recruitment droughts.  These results highlight the potential disadvantages of rotational schemes in variable environments as well as the complications that recruitment variability introduces into the adaptive management of reserves.