Growth, sexual maturation and spawning in central North Sea plaice (Pleuronectes platessa L.), and the generation of maturity ogives from commercial catch data

This paper investigates sexual maturation in North Sea plaice using data from the commercial catch of plaice landed on Lowestoft fish market from 1983–1996, along with data from the English summer groundfish surveys from 1977–1993. Two offshore spawning areas were identified in the central North Sea – central-northern and central-southern. A general linear model (GLM) showed plaice sexual maturation was influenced by body weight, age and geographical location and was subject to annual and seasonal variation. Few three year old female place appeared to spawn in the central North Sea, similar to the situation early in the 20th century. The condition factor of plaice peaked at the start of spawning and declined sharply during spawning, with older females investing the most heavily in reproduction, both in absolute and relative terms. The loss of older females through fishing is, therefore, likely to compromise the overall reproductive fitness of the remaining population, comprising of young females. The reproductive fitness of the males was also compromised, since young males tended to be in running condition for about a month less than older males. Reproductive investment in females was about twice the level found in males. There was evidence that plaice in the northern North Sea matured at a slower rate than fish in the south. From the mid-1980s until the mid-1990s there was an increase in the proportion of young plaice reaching sexual maturity, following an earlier decline from the late 1970s. Problems with using maturity ogives derived from market sampling data in order to assess the proportion of spawning plaice in the North Sea are discussed.