| Summary: | M.Sc.Thesis submitted to Trinity College Dublin. The herring fishery in the Celtic Sea and Division VIIj has been commercially important for many years. The Marine Institute has been collecting biological data for this herring stock since 1959. This stock is assessed by ICES annually. However, this is the first study of long term biological trends. The biological data consists of total length, weight, sex, maturity and age of the commercial catches. This study looks at mean length and mean weight at age, growth rate, condition factor and maturity ogives from 1959 to 2007. Environmental factors that may explain the biological trends are also investigated. These data consist of sea surface temperature (SST) for the Celtic Sea and the Irish Sea from 1970 to 2004, North Atlantic Oscillation (NAO) indices from 1958 to 2001 and Calanus spp. abundance for the Celtic Sea and Division VIIj from 1958 to 2007. In addition, data from the ICES stock assessment is consulted and this consists of spawning stock biomass (SSB), fishing mortality (F) and recruitment (R) from 1958 to 2008. The results show that mean length and mean weight at age peaked in the 1970s and declined thereafter. It was found that the condition factor over time declined. The results also illustrate that the growth rates were faster in the 1960s and 1970s than in the 1980s and 1990s. It can be seen that maturation for 1 winter ring increased in the early 1970s and has remained at a high level since then. This study looks at possible explanations for the biological trends and found that it was unlikely to be due to fishing mortality or density dependence. There have been changes in the proportions of autumn and winter spawners in this area over time but it is unlikely that this influenced the trends. Evidence suggests that the changes in the biological data over time may be influenced by environmental factors. NAO shows a significant negative correlation with growth rate in length. There is evidence to suggest that increased SST in the Irish Sea and ...
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