This paper examines the relationships between maturity, length and age of whiting sampled on a length-stratified basis from groundfish surveys of the Irish Sea during spawning in spring 1992 – 2001. Maturity, defined by the triggering of vitellogenesis or milt production, was a function of both length and age. Proportions of mature individuals in 1-year-old males increased successively from almost zero in length classes below 15 cm to around 0.9 at 25 cm, whilst almost all 2-year-old males were mature from their smallest length of around 19 cm. Maturity in females was more strongly linked to age than to length. Most 1- year-old females were immature, the proportion of mature individuals not exceeding 0.3 in any length class. Most 2-year-old females were mature and immature fish were found in the smallest length classes only (20–25 cm). Almost all 3-year-olds of both sexes were mature in all length classes. Proportions of mature individuals in 1-year-olds increased substantially after 1997, particularly in males. Significant positive cross-correlation between proportion mature and mean length was found for 1-year- olds of both sexes. Length at 50% maturity (L50) averaged around 19 cm in males and 22 cm in females. Variability in L50 was negatively cross-correlated with average sea surface temperature in the preceding year. There is no evidence for substantial changes in maturity of whiting since the 1950s, despite an order-of-magnitude reduction in biomass caused by high fishing mortality. Concomitant decreases in mean length-at-age and weight-at-age in recent decades indicate that conditions may have been unfavourable for compensatory changes in maturation.
Knowledge of the proportion mature at size or age is necessary for estimation of spawning stock biomass (SSB), which is central to the management of many fisheries (Trippel and Harvey, 1991). Length-related maturity patterns are also an important factor in the design of selective fishing gear. In a number of stocks of gadoid fish, which comprise a high proportion of commercial catches in North Atlantic coastal seas, reduced size and/or age at maturity have been recorded during periods of stock decline (Beacham, 1983; Ponamorenko and Yaragina, 1995; Trippel et al., 1997; Cardinale and Modin, 1999). Stocks of cod (Gadus morhua) and whiting (Merlangius merlangus) in the Irish Sea declined substantially in abundance during the 1990s due to high fishing mortality (ICES, 2001a). Whilst there is some evidence for a reduction in mean age at maturity in Irish Sea cod since the 1970s (Armstrong et al., submitted), trends in whiting in this area have not previously been investigated.