Small‐scale genetic structure in a marine population in relation to water circulation and egg characteristics

Abstract

Abstract. Until the last decade it was assumed that most marine species have pronounced gene flow over vast areas, largely because of their potential for dispersal during early life stages. However, recent genetic, modeling, and field studies have shown that marine populations may be structured at scales that are inconsistent with extensive dispersal of eggs and larvae. Such findings have stimulated the birth of new studies explaining the mechanisms that promote population structure and isolation in the oceans, in the face of high potential for dispersal. Here we study the vertical and horizontal distribution of cod (Gadus morhua) eggs in relation to small-scale circulation and water column hydrography in a coastal location of southern Norway. Previous studies conducted in this region have shown that cod populations inhabiting fjord locations, which are on average 30 km apart, are genetically differentiated, a remarkable outcome considering that Atlantic cod have pelagic egg stages and long pelagic larval duration. We document that cod eggs are found in greater abundance in shallow water layers, which on average are flowing up the fjord (away from the open ocean), and in the inner portion of the fjord, which is subject to lower current speeds compared to the outer or mouth of the fjord. Eggs were found to be neutrally buoyant at shallow depths, a trait that also favors local retention, given the local circulation. The same patterns held during two environmentally contrasting years. These results strongly suggest that population structure of Atlantic cod is favored and maintained by a balance between water circulation and egg characteristics.

Habitat effects on population connectivity in a coastal seascape

“…In contrast, when comparing among sheltered areas, correlations in recruitment decreased rapidly with distance, remaining above background levels essentially only for stations sampled within a single fjord. An analysis of egg buoyancy and circulation patterns within fjords found that eggs are concentrated in inward-flowing currents, acting as a retention mechanism for eggs spawned locally within fjords (Ciannelli et al 2010). Tagging studies on recently settled age-0 cod also indicate that this early life stage is very stationary (Grant & Brown 1998, Olsen et al 2004).…”

“…Marine reserves are increasingly being implemented to aid in the conservation and recovery of exploited populations; however, the effectiveness of marine reserves depends critically on the spatial scale of processes such as larval and adult dispersal (Botsford et al 2003, Kritzer & Sale 2004, Grüss et al 2011. In 2012, Norway's first no-take marine reserve was established in the Tvedestrand fjord between our Arendal and Risør study sites, spanning 1.5 km 2 of the inner fjord habitat and including a major cod spawning and nursery area (Ciannelli et al 2010). Marine reserves are generally not considered for highly migratory species such as Atlantic cod, but what we now know about movement and dispersal of Skagerrak coastal cod suggests that even a relatively small reserve could be an effective conservation tool in this region, protecting sedentary adults from harvest (see also Moland et al 2013).…”

“…2). Previous work has suggested that cod eggs are largely retained within fjords by inward-flowing currents and shallow sills, promoting the maintenance of local population structure (Ciannelli et al 2010). The fjords near Lillesand and Risør have such sills, whereas Arendal does not and is more appropriately described as a bay, partially protected from the Skagerrak by coastal islands.…”

“…However, genetic and demographic structure is increasingly being documented at relatively fine spatial scales (Conover et al 2006, Jorde et al 2007, Hauser & Carvalho 2008, Knutsen et al 2011. The mechanisms used to explain such population structure in marine environments include the retention of eggs and larvae in currents or eddies (Sponaugle et al 2002, Morgan et al 2009, Ciannelli et al 2010, high mortality of dispersing individuals (Koehn et al 1980), and the homing of larval and mature fish to natal habitats (Jones et al 1999, Thorrold et al 2001. While the scale of larval dispersal has often been the focus of studies on marine population connectivity, adult behavior can also be a crucial factor (Frisk et al 2014).…”

Climate and population density drive changes in cod body size throughout a century on the Norwegian coast

“…Since the early days of this survey, the spatial structuring of populations within and among fjords has been evident (21), and recently supported by genetics (22,23) and life-history studies (17,24). This structure is maintained through retention of early life-history stages (eggs and larvae) (25) and limited juvenile and adult dispersal (26,27). Spatial population structure can provide stabilizing benefits to fisheries and ecosystems (28,29) but is not always considered in management of fish stocks or incorporated into ecological studies.…”