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

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

Mating behaviour by the haddock (Melanogrammus aeglefinus)

This study examines the full repertoire of haddock reproductive behaviour, monitored in an aquarium for 3 months over each of two successive spawning seasons. Observations showed that male haddock were territorial and that visits to their territories by females triggered courtship behaviour, leading to spawning. Entry of females to the territories was induced by an acoustic and visual self-advertisement of displaying males, termed patrolling behaviour, exhibited for many hours each night for several months. The existence of sneaking behaviour by non-territorial males was observed for the first time for haddock and was supported by genetic data. Overall our observations are consistent with the behaviour expected from lekking species, as in the cod, a closely related species. Lekking is said to occur when non-resource-based aggregations of males are visited by females for the purpose of mating. Implications for the management of fish stocks are discussed