The existence and population connectivity of Icelandic spring-spawning herring over a 50-year collapse period

Abstract

Abstract In the late 1960s, two local herring stocks and one seasonal migratory herring stock in Icelandic waters collapsed primarily due to overfishing. While the stocks of Icelandic summer-spawning herring (ISSH) and Norwegian spring-spawning herring (NSSH) have recovered, Icelandic spring-spawning herring (ISPH) has not. The main objective of this work is to explore the existence of ISPH and identify potential signs of recovery. Analyses were based on samples from the autumn fishery and survey data from 1962 to 2016. In the autumn, local ISSH and ISPH cohabitated and mixed in the main fishery, where sampled fish were categorized into stocks on basis of maturity stage. The results indicate that the proportion of ISPH was in the range of 13–33% in the 1960s while below 5% and on average 1.4% during 1970–2016. Several year classes of ISPH exhibit notable differences from the general low levels and caused a slight increase in ISPH spawning-stock biomass for corresponding years. An increase in proportion of generally old ISPH around 2004 and 2013 occurred simultaneously with the reappearances of NSSH in the feeding areas east and north of Iceland, respectively. Altogether, it supports earlier suggestions that ISPH might require immigration from NSSH to recover, as these were considered closely related stocks. This is consistent with the source-sink theoretical model of stock size regulation.