The stochastic nature of larval connectivity among nearshore marine populations

Abstract

Many nearshore fish and invertebrate populations are overexploited even when apparently coherent management structures are in place. One potential cause of mismanagement may be a poor understanding and accounting of stochasticity, particularly for stock recruitment. Many of the fishes and invertebrates that comprise nearshore fisheries are relatively sedentary as adults but have an obligate larval pelagic stage that is dispersed by ocean currents. Here, we demonstrate that larval connectivity is inherently an intermittent and heterogeneous process on annual time scales. This stochasticity arises from the advection of pelagic larvae by chaotic coastal circulations. This result departs from typical assumptions where larvae simply diffuse from one site to another or where complex connectivity patterns are created by transport within spatially complicated environments. We derive a statistical model for the expected variability in larval settlement patterns and demonstrate how larval connectivity varies as a function of different biological and physical processes. The stochastic nature of larval connectivity creates an unavoidable uncertainty in the assessment of fish recruitment and the resulting forecasts of sustainable yields.coastal oceanography ͉ fisheries ͉ marine ecology N earshore ecosystems host a wide variety of marine organisms and are among the most productive environments on Earth. Yet many species harvested from these ecosystems are overfished (1-3), a problem that is especially acute for those invertebrates and fishes with a relatively sedentary adult life stage. One potential cause of overfishing is mismanagement because of a poor understanding and accounting of stochasticity in these systems (4,5). Stochasticity caused by climate variations has long been known to influence the dynamics of ocean ecosystems and the fisheries they support (6). Climate variation affects rates of fecundity and recruitment by altering water temperature, coastal circulation patterns, or the availability of spawning grounds (7,8); such effects can be understood and, given sufficient data, may be predictable. Here, we introduce a mechanism that generates stochasticity in spatial and temporal patterns of larval transport on annual time scales. This stochasticity is inherently unpredictable because of the chaotic nature of coastal circulations and the relatively short larval time scales.Many harvested fish and invertebrates from nearshore ecosystems have a life cycle that includes a pelagic larval stage that can last up to months and a localized benthic adult stage (9, 10). These relatively sedentary adults release hundreds to millions of larvae that are advected and dispersed by ocean currents as they develop competency to settle (9-13). Spawning releases can occur continuously over months or in a few short events. Biotic factors, such as active swimming and vertical migration, also contribute to movement patterns (12,14,15). A small fraction of the larvae settle at suitable sites, and an even smaller fraction recruit to adult...

Temporal genetic homogeneity among shore crab (Carcinus maenas) larval events supplied to an estuarine system on the Portuguese northwest coast

“…Indeed, individual patches of larvae may maintain their integrity throughout larval development (Natunewicz and Epifanio, 2001), allowing aggregations of larvae from the same family group to settle in close proximity. This process is supported by oceanographic predictions along the California Upwelling System (Siegel et al, 2008) and might also occur in a similar oceanographic system, along the Portuguese west coast.…”

“…As a consequence, local populations are not constrained by the genetic makeup of the larval pool that must subsequently recruit in order to maintain large populations. Evidence shows that larval aggregations or 'parcels' (Siegel et al, 2008) may be retained in shelf waters after emission from estuaries (for example, Natunewicz and Epifanio, 2001;MartaAlmeida et al, 2006) and remain cohesive until settlement, making possible the detection of family structure within parcels if they were produced in the same spawning event. However, the longevity of such parcels may well depend locally on environmental heterogeneity, such as turbulence or small scale features of circulation that will disrupt the aggregations and mix larvae from different origins.…”

Genetic differentiation across eastern Pacific oceanographic barriers in the threatened seahorse Hippocampus ingens

“…Siegel et al (2008) demonstrate through modeling that alongshore larval connectivity is inherently a stochastic process due to chaotic coastal currents. This local process, coupled with extensive mixing due to a seasonal shift of the Inter-tropical Convergence Zone and periodic El Niño events, facilitate sporadic dispersal between regions of the eastern Pacific (Muss et al 2001;Pisias and Mix 1997).…”

Pelagic Larval Duration and Dispersal Distance Revisited

“…Our observations of the importance of behavior in determining dispersal distance have largely been ignored. Since Shanks et al (2003a) was published, a number of papers have presented models of larval dispersal that have attempted to estimate dispersal distance using PD coupled with oceanographic models with various degrees of sophistication (Siegel et al, 2003(Siegel et al, , 2008Kinlan et al, 2005;Edwards et al, 2007). Many of these models have assumed that larvae are passive; they present the null hypothesis of passive dispersal.…”