Trends in population dynamics and exploitation of North Sea plaice (Pleuronectes platessa L.) since the late 1800s

Abstract

The exploitation history of North Sea plaice is reconstructed from the late 1800s to 1993 using data on catch per unit of effort, total international landings, size and age composition of landings, and growth rate. Available data indicate that fishing mortality was already high in the 1890s (F=0.4) and increased to a record high in the late 1920s and early 1930s (F=0.8). After World War II, fishing mortality was reduced, but from the 1960s onwards it increased steadily to a level of F=0.6 in the early 1990s. Indices of catch per unit of effort during the last quarter of the 19th century are concurrent with the expansion of the fisheries. Stock biomass was at a low level in the 1930s but increased between the 1960s and 1980s owing to increased recruitment and improved growth. Landings were fairly stable around 55 000 t until the mid-1950s when they increased almost threefold to a record level of 170 000 t in 1989. The causes of these changes are discussed. It is suggested that changes in both growth rate and exploitation rate in young fish have been important.