Paradigm shifts in marine fisheries genetics: ugly hypotheses slain by beautiful facts

Abstract

Introduction 334Population structure in marine fishes 335 Paradigm shift 335Genetic differentiation in space and time 336Population connectivity in marine fishes on a fine scale 338Spatial and temporal scale of adaptation 339 Paradigm shifts 339Spatial scale of adaptive differentiation 340Temporal scale of adaptive differentiation 342 AbstractBy providing new approaches to the investigation of demographic and evolutionary dynamics of wild populations, molecular genetics has led to fundamental changes in our understanding of marine ecology. In particular, genetic approaches have revolutionized our understanding in three areas: (i) most importantly, they have contributed to the discovery of extensive genetic population structure in many marine species, overturning the notion of large, essentially homogenous marine populations limiting local adaptation and speciation. (ii) Concomitant differences in ecologically important traits now indicate extensive adaptive differentiation and biocomplexity, potentially increasing the resilience to exploitation and disturbance. Evidence for rapid adaptive change in many populations underlies recent concerns about fisheries-induced evolution affecting life-history traits. (iii) A compilation of recent published research shows estimated effective population sizes that are 2-6 orders of magnitude smaller than census sizes, suggesting more complex recruitment dynamics in marine species than previously assumed. Studies on Atlantic cod are used to illustrate these paradigm shifts. In our synthesis, we emphasize the implications of these discoveries for marine ecology and evolution as well as the management and conservation of exploited marine fish populations. An important implication of genetic structuring and the potential for adaptive divergence is that locally adapted populations are unlikely to be replaced through immigration, with potentially detrimental consequences for the resilience to environmental changea key consideration for sustainable fisheries management.

Population assignment and local adaptation along an isolation‐by‐distance gradient in Pacific cod (Gadus macrocephalus)

“…However, the confirmation of the slope by a completely different marker system further supports the genetic pattern itself and the underlying basis of limited dispersal distances, despite a lengthy larval period and highly migratory adults. It also supports the increasingly acknowledged pattern of subtle population structure in species with no obvious barriers to dispersal, such as marine fishes (Hauser & Carvalho, 2008). …”

“…Moreover, populations at the southern edge of the species’ distribution may be old and harbor genetic variation important for the survival and evolution of a species (i.e., rear edge effect; Hampe & Petit, 2005). These stabilizing characteristics may become increasingly important as climate change progresses (Hauser & Carvalho, 2008). …”

Fine-scale spatial and temporal genetic structure of Atlantic cod off the Atlantic coast of the USA

“…Adaptive divergence can occur over much shorter time scales than can neutral genetic differentiation (Endler 1986, Cano et al 2008, resulting in genetic differentiation of a greater magnitude for selected versus neutral loci (Beaumont & Balding 2004) : 177-195, 2010 therefore inappropriate for making inferences about population processes, such as migration rates and effective population size, they can serve an important function in genetic stock identification (Nielsen et al 2006, ICES 2009). The adaptive differences underlying variation at selected loci may have important management implications (Crandall et al 2000, Conover et al 2006, Hauser & Carvalho 2008, ICES 2009.…”

“…This classical view of marine fish species (Hauser & Carvalho 2008) was supported by tagging studies that demonstrated long distance migrations (Templeman 1974) and by early genetic studies that revealed high levels of gene flow (Ward et al 1994). This perspective influenced management regimes, such that many marine fisheries are managed as broadly distributed, panmictic populations .…”

“…The implications of such fine-scale population structure are important for effective management (Palumbi 2003, Bradbury et al 2008, Reiss et al 2009). One marine fish species known to exhibit fine-scale population structure in parts of its distribution is Atlantic cod Gadus morhua L. (see reviews in Waldman 2005 andCarvalho 2008). Atlantic cod is one of the most commercially important marine fishes in the world and comprises a principal component of the northeastern US groundfish assemblage.…”