Evidence of Segregated Spawning in a Single Marine Fish Stock: Sympatric Divergence of Ecotypes in Icelandic Cod?

Abstract

There is increasing recognition of intraspecific diversity and population structure within marine fish species, yet there is little direct evidence of the isolating mechanisms that maintain it or documentation of its ecological extent. We analyzed depth and temperature histories collected by electronic data storage tags retrieved from 104 Atlantic cod at liberty ≥1 year to evaluate a possible isolating mechanisms maintaining population structure within the Icelandic cod stock. This stock consists of two distinct behavioral types, resident coastal cod and migratory frontal cod, each occurring within two geographically distinct populations. Despite being captured together on the same spawning grounds, we show the behavioral types seem reproductively isolated by fine-scale differences in spawning habitat selection, primarily depth. Additionally, the different groups occupied distinct seasonal thermal and bathymetric niches that generally demonstrated low levels of overlap throughout the year. Our results indicate that isolating mechanisms, such as differential habitat selection during spawning, might contribute to maintaining diversity and fine-scale population structure in broadcast-spawning marine fishes.

Settling-depth vs. genotype and size vs. genotype correlations at the Pan I locus in 0-group Atlantic cod Gadus morhua

“…Various mechanisms have been suggested to hamper this intermingling and potential for hybridization between the 2 populations, like differences in peak time of spawning, possible differences in temperature and depth preferences for spawning, and differences in spawning behavior (Nordeide 1998, Nordeide & Folstad 2000, Sarvas & Fevolden 2005a; cf. also Grabowski et al 2011 for similar features for spawning Icelandic cod). Eggs and larvae spawned offshore will drift northeast-ward with the prevailing Norwegian coastal current.…”

Consistency in the behaviour types of the Atlantic cod: repeatability, timing of migration and geo-location

“…This also indicates that the coastal cod may travel some distance along the coast during the feeding migrations, a result corroborated by tidal locations of this behaviour type during feeding migrations. On the other hand, the frontal cod migrated out of their spawning grounds at variable temperature and depth, and displayed an extensive, vertical and horizontal Those fish nevertheless displayed a temperature history of a coastal cod (Grabowski et al 2011). The timing of a migration from spawning to feeding ground and vice versa, which is crucial for individual survival and reproductive success, has been shown to be related either to food availability and temperature (Bell 2011, Schaefer et al 2011 or to genetic inheritance (Quinn et al 2000, Pulido et al 2001.…”

“…The classification of behaviour types as coastal versus frontal was performed according to previous studies (Pálsson & Thorsteinsson 2003, Pampoulie et al 2008a, Grabowski et al 2011. In general, coastal and frontal behaviours were defined according to the annual temperature and depth history of the tagged individuals.…”

“…a coastal, relatively stationary type in shallow waters, and a migratory frontal (offshore) type, breeding in shallow waters but migrating to deeper waters to commonly feed near thermal fronts (Nordeide 1998, Godo & Michalsen 2000, Pálsson & Thorsteinsson 2003, Neat et al 2006. Both behaviour types have been studied in Norwegian (Nordeide 1998, Godo & Michalsen 2000 and Icelandic waters (Pálsson & Thorsteinsson 2003, Pampoulie et al 2008a, and have been shown to segregate by depth both during spawning and feeding time (Nordeide 1998, Grabowski et al 2011). Recent studies have also shown genotype differences at the pantophysin locus (Pan I, see Fevolden & Pogson 1997, Sarvas & Fevolden 2005, Skarstein et al 2007, Pampoulie et al 2008a.…”

Evidence from data storage tags for the presence of lunar and semi-lunar behavioral cycles in spawning Atlantic cod

“…Two distinctive behavioral ecotypes occur within each geographic component. Coastal cod tend to remain in the relatively shallow water of the continental shelf, while frontal cod move to deeper waters along the shelf break and exhibit large daily vertical migrations (Pálsson and Thorsteinsson 2003;Pampoulie et al 2008;Grabowski et al 2011). These stock components exhibit differences in their reproductive phenologies (Grabowski et al 2011) and thus it is possible that their reproductive behavior has responded differently to environmental cycles.…”