Otolith shape: a population marker for Atlantic herring Clupea harengus.

Otolith shape variation of seven Atlantic herring Clupea harengus populations from Canada, the Faroe Islands, Iceland, Ireland, Norway and Scotland, U.K., covering a large area of the species' distribution, was studied in order to see if otolith shape can be used to discriminate between populations. The otolith shape was obtained using quantitative shape analysis, transformed with Wavelet and analysed with multivariate methods. Significant differences were detected among the seven populations, which could be traced to three morphological structures in the otoliths. The differentiation in otolith shape between populations was not only correlated with their spawning time, indicating a strong environmental effect, but could also be due to differing life-history strategies. A model based on the shape differences discriminates with 94% accuracy between Icelandic summer spawners and Norwegian spring spawners, which are known to mix at feeding grounds. This study shows that otolith shape could become an accurate marker for C. harengus population discrimination.

Thermal stratification influences maturation and timing of spawning in a local Clupea harengus population.

Maturation and timing of spawning in relation to temperature were studied in a local Atlantic herring Clupea harengus population inhabiting a small semi-enclosed ecosystem (7 km(2)) separated from the larger outer fjord system by narrow sills on the west coast of Norway. Ambient temperatures varied annually up to 4° C during both the pre-spawning and spawning periods from February to April, but without affecting the spawning time. Instead, the timing of spawning was found to be related to thermal stratification in response to spring warming, which occurred about the same time every year regardless of initial temperatures.