© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Variability in maturity and growth in a heavily exploited stock: cod (Gadus morhua L.) in the Irish Sea
a Department of Agriculture and Rural Development, Agricultural and Environmental Sciences Division Newforge Lane, Belfast BT9 5PX, UK
b The Queen's University of Belfast, Department of Agricultural and Environmental Sciences Newforge Lane, Belfast BT9 5PX, UK
c Department of Agriculture and Rural Development, Biometrics Division Newforge Lane, Belfast BT9 5PX, UK
*Correspondence to H. D. Gerritsen: Marine Institute, Galway Technology Park, Parkmore, Galway, Ireland; tel: 353 91780368; fax: 353 91730470. e-mail: hans.gerritsen{at}marine.ie.
Maturity, sex ratio (proportion female by number) and length-at-age of cod in the Irish Sea were examined using data collected during groundfish surveys in spring 1992–2002. Skewed sex ratios with a predominance of males were observed in the survey catches of 2- and 3-year-old cod taken on the spawning grounds. In contrast, commercial midwater trawl catches of cod in the same area had consistently higher sex ratios, suggesting that mature males and females have different vertical migration behaviour. Estimates of proportion mature were not affected by method of capture. In a GLM analysis, the factor age explained most of the variation in maturity in each sex, whilst length, although a significant factor, explained less variation than year and region (spawning and non-spawning grounds). Maturity was observed in a variable proportion of 2-year-olds, whilst virtually no 1-year-olds and all 3-year-olds and older were mature irrespective of area or year of capture. Population estimates of proportion mature in 2-year-olds, from all years' data combined, were 0.87 in males and 0.45 in females. Estimates were higher in spawning areas than in non-spawning areas, and showed a general increase over time throughout the survey area. However, in any year, the proportion mature at age 2 varied little with length and was reduced only in the smallest fish (<40 cm) at this age in some years. The increase over time in proportion mature coincided with rising sea surface temperature (SST) and a decline in recruitment and stock biomass at high rates of fishing mortality. Mean length-at-age differed consistently between year classes, the differences being largely established in the first year of life. The fastest growth rates were apparent in very weak year classes produced in the late 1990s when SST was relatively high. The interrelationships between growth, maturity, SST and abundance are examined, and changes in growth and maturity since earlier studies in the 1970s are investigated.
Keywords: cod, Gadus morhua, growth, Irish Sea, maturity, sex ratio
Received 17 July 2001; accepted 6 October 2003.
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