Discrimnation of herring populations in a northern Norwegian fjord: genetic and biological aspects
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From a trawl survey in a fjord system in northern Norway, it has been possible to separate catches of herring into two distinct groups characterized by different number of vertebrae (VS). Low- vertebral herring (VS = 52-55) belong to a local stock of spring-spawning fjord stock called the Balsfjord herring. High-vertebral herring (VS = 56-60) belong to an immigrating population of Atlanto- Scandian herring spawned off the Norwegian coast. The separation between the stocks was verified by genetic analysis of herring samples using enzyme electrophoresis. The Balsfjord -herring is characterized by a very high frequency ( 0.93) of a rare allele (designated 110) at the LDH-2 locus which is expressed in white muscle tissue. In comparison, this allele is usually not found (frequency < 0. 01) in samples of Atlanto-Scandian herring. The discrimination between the two stocks in Balsfjord was, however, evident from the biological data obtained (age distribution, VS, mean length and length distributions) as well as the genetic characters investigated. In addition, the Balsfjord herring, especially at mature stage, seem to be distributed at 60-200 m depth and was caught by using bottom trawl. The Atlanto- Scandian herring, mainly 2 or 3 years old, was found in the upper water layer (0-60 m). The genetic data indicate very limited or no gene flow between the two herring stocks in Balsfjord. This fjord system, therefore, offers unique possibility to study interaction and isolating mechanisms with regards to marine fish stocks.