Larval Mortality and Subsequent Year-Class Strength in the Plaice (Pleuronectes platessa L.)

Abstract

Most marine fish of commercial importance have extremely high fecundities and the correspondingly high mortalities known to occur during the early part of their life cycles, particularly during the pelagic egg and larval phase, mast be part of the natural process of maintaining the populations in balance. Hjort (1914) was the first to suggest that good and bad year-classes may be determined during this pelagic phase. These investigations use data collected from several sources to illusrate the life history of the plaice in numerical terms. The pelgaic phase has been described from surveys of known spawning grounds using high-speed plankton samplers. Nursery grounds where 0 and l group fish live were sampled with push nets and beam trawls; while information on adult stocks was obtained mainly from the market sampling of landings by commercial trawlers. Larval abundance and mortality estimates in the southern North Sea spawnings have been correlated with subsequent year-class strengths in the English plaice landings and give credence to Hjort's original hypothesis. In particular the high survival of the 1963 yearclass has been shown to be of utmost importance; for catch, effort and price statistics and age composition data show very clearly that the recruitment of 1963 year-class to the fisheries has been a major factor in the recent exceptional landings of North Sea plaice by Denmark, the Netherlands and the UK.