• Growth rate fluctuations of herring in the Celtic Sea: a history of life on the edge

      Lynch, D.; Wilson, J.; Clarke, M. (International Council for the Exploration of the Sea (ICES), 2011)
      The most south-western herring populations in Europe occur in the Celtic Sea, south of Ireland. Biological sampling has been conducted since the 1920s and routinely since 1958. This study collated and analysed these long term data for the first time. Overall results were examined in the context of time series of environmental data and population scale indices of population status. Size at age was low in the 1920s and 1950s, but increased to a peak in the 1970s before declining strongly until recently. Condition factor over time declined, whilst growth rates were greater in the 1960s and 1970s than in the 1980s and 1990s. Further analyses suggest that the changes are influenced by environmental factors, especially the North Atlantic Oscillation sea surface temperature, and the abundance of Calanus copepods. The implications of this work, for the rational management of this stock, are discussed.
    • Long term trends in population dynamics of NW Ireland herring revealed by data archaeology

      Clarke, M.; Egan, A.; Mariani, S.; Miller, D. (International Council for the Exploration of the Sea (ICES), 2011)
      Herring populations to the northwest of Ireland are considered to constitute a single stock. They consist of a diverse array of autumn, winter and spring spawning components. They have been subject to large catches in the past. Landings peaked at 50 000 t in 1987 and have shown a slow decline since, as 2 dominant year classes, born in the 1980s, declined. No strong year classes have appeared since, and the stock is now outside safe biological limits. The time series of data available for stock assessment only covered the period 1970- present, even though routine sampling began after the end of World War I. In order to examine the stock trajectories over time, all catch at age data for the period 1921-1970 were compiled and analysed. The study revealed large fluctuations in the size of the stock and its productivity over time. Occasional strong cohorts occurred either singly, or in close together within 5 years of each other, and were observed at roughly 20-40 year intervals. Interspersed with this were long intervals of poor recruitment. Further historical analyses of 18th and 19th century records confirm this general periodicity. Overall results were examined in the context of time series of sea surface temperature data. The study can provide a basis for development of a long term management plan for the stock, using management strategy evaluation (MSE). The implications of this work, for the rational management of this stock for the future, are discussed.