Contrasting latitudinal patterns in spawning phenology of three gadoids in the northeast Atlantic

Abstract The timing of phytoplankton spring blooms varies geographically, typically with later blooms occurring at higher latitudes. This is expected to lead to a latitudinal gradient in timing of spawning of species that depend on synchronizing spawning with the spring bloom to optimize larval feeding opportunities. Here, we contrast and compare spatial variation in spawning phenology of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), and saithe (Pollachius virens) in three areas spanning from the North Sea in the south to the Barents Sea in the north. Data on macroscopically determined gonadal development stages were obtained from fish collected by research surveys and by fishing vessels. We found evidence of active spawning over extensive areas, including areas outside the currently recognized spawning grounds. Among the three species, haddock spawned the latest in all areas, and saithe earliest, except in the North Sea, where cod spawned first. Size-related differences in peak spawning were only observed in the south for cod and saithe, and in the north for haddock, with smaller fish spawning later. However, the results on size were associated with high uncertainty. Consistent with the expectation, the peak of the spawning season for cod and haddock was later at higher latitudes. In contrast, the latitudinal variation in spawning time in saithe was limited. Our results illustrate the need for both species and region-specific investigations of spawning phenology, as it may affect recruitment dynamics, especially under climate change, and have implications for survey design, stock assessment, and ultimately, population persistence.