Vikebø, F. B., Husebø, Å., Slotte, A., Stenevik, E. K., and Lien, V. S. 2010. Effect of hatching date, vertical distribution, and interannual variation in physical forcing on northward displacement and temperature conditions of Norwegian spring-spawning herring larvae. – ICES Journal of Marine Science, 67: 1948–1956. Early hatching has been shown to be associated with increased survival of Norwegian spring-spawning herring (Clupea harengus) larvae. We investigated whether the process behind this association is related to larval drift and ambient temperature. A three-dimensional hydrodynamic model was used to simulate the effect of hatching date on northward displacement and temperature conditions of larvae from spawning grounds off western Norway for 1989–2008. The simulations revealed that the displacement during a period of 60 d was greatest if the larvae hatched early in the season and if they were located near the surface. The relationship between drift speed and ambient temperature was significantly negative, but less so later in the season, because the coastal current became progressively warmer. Results from the simulated interannual variations in larval drift compared with the observed survival suggest that a rapid northward displacement to the main nursery area in the Barents Sea is more important for larval survival than ambient temperature. The significant effect of northward displacement on survival could be explained by reduced overlap with predators and/or higher prey densities, but the causal processes involved remain to be investigated.
Husebø, Å., Stenevik, E. K., Slotte, A., Fossum, P., Salthaug, A., Vikebø, F., Aanes, S., and Folkvord, A. 2009. Effects of hatching time on year-class strength in Norwegian spring-spawning herring (Clupea harengus). – ICES Journal of Marine Science, 66: 1710–1717. Effects of mean hatching date, post-hatching temperature, wintering temperature of adults, spawning stock size, and percentage of recruit spawners on larval survival in Norwegian spring-spawning herring (Clupea harengus) during the period 1987–2004 were analysed. In the final model, only hatching date proved to be significant. However, hatching date was itself negatively correlated with wintering temperature and positively correlated with the percentage of recruit spawners. This suggests indirect effects on larval survival, whereby low percentages of recruit spawners and high temperatures during gonad development lead to early spawning. Early hatching could be favourable for survival by allowing the larvae to drift away from areas where potential predators concentrate in spring, before predation pressure increases. Indirect support for this hypothesis was found in the activity of the purse-seine fishery for immature saithe (Pollachius virens) along the Norwegian coast. This fishery starts as soon as the saithe aggregate into large schools, which is presumed to reflect their feeding activity. The commercial catch data indicated that the saithe became active 2 months earlier in the area south of 67°N, than in areas to the north. Both field data and larval drift models confirmed that the majority of the early hatched larvae had passed across this border by that time of year.
Husebø, Å., Slotte, A., and Stenevik, E. K. 2007. Growth of juvenile Norwegian spring-spawning herring in relation to latitudinal and interannual differences in temperature and fish density in their coastal and fjord nursery areas. – ICES Journal of Marine Science, 64: 1161–1172. Norwegian spring-spawning herring (Clupea harengus) spawn in February and March along the Norwegian coast from 58°N to 69°N. The larvae are transported north with the coastal current, and in autumn, the main part of the 0-group is found in the Barents Sea, and a smaller and variable fraction ends up in coastal and fjord nursery areas that experience a wide range of environmental conditions and fish densities. Based on data from herring 0–2 years old collected from 1970 to 2004, there is a positive relationship between temperature and the growth of this coastal component, in terms of length, weight, condition factor, and annual otolith increment width, and a negative relationship between acoustic abundance and the same growth indices. In general, juvenile growth decreased northwards along the coast concurrently with decreasing summer and autumn temperatures and increasing acoustic abundance. It seems, therefore, that there may be interference in the relationship between juvenile herring growth and temperature, attributable to variable recruitment, currents, larval drift, and advection into the fjords, causing latitudinal and interannual differences in fish density, and hence variable competition for food.
Vikebø, F. B., Korosov, A., Stenevik, E. K., Husebø, Å., and Slotte, A. 2012. Spatio-temporal overlap of hatching in Norwegian spring-spawning herring and the spring phytoplankton bloom at available spawning substrata. – ICES Journal of Marine Science, 69: . Analyses of bottom substrata demonstrated that a potential spawning area of 13 490 nautical miles2 is available for Norwegian spring-spawning (NSS) herring on the Norwegian shelf between 59 and 71°N. However, the availability of suitable substrata differed significantly on a latitudinal scale; 60% of the available spawning area was north of 67°N. However, the spatial distribution of larvae indicated that the bulk of the adult population wintering off northern Norway in the period 1998–2007 migrated farther south and upstream to spawn. This suggests benefits attributable to extra costs accompanying the spawning migration. To test whether southern spawning is beneficial with regard to overlap between herring larvae and their prey, remotely sensed chlorophyll a data from SeaWiFS were used as a proxy for the onset of phytoplankton bloom (OSB) in discrete bins along the Norwegian coast and linked to interannual variation in the observed average timing of larval hatching during the years 1998–2007. There was a delay in OSB with increasing latitude of 37 d within the spawning habitat of NSS herring. Overlap between larval hatching and OSB was clearly highest in the south, suggesting that match with prey availability may be an important factor in the selection of spawning grounds by NSS herring.
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