The distribution and population dynamics of O-group plaice ( Pleuronectes platessa L.) on nursery grounds in the Firth of Forth

Abstract

Midsummer (1 August) population estimates of about 2 million O-group plaice ( Pleuronectes platessa L.) were derived for sandy bays around the Firth of Forth in 1979–1980. This is an order of magnitude less than similar estimates made for the Clyde Sea Area in 1973–1974. Autumn population estimates of 0·4–1·0 million fish were comparable to estimates by the Ministry of Agriculture, Fisheries and Food for the area between the Scottish border and Flamborough Head (2·3 million for 1970 and 1973) which represented 4·8% (1973) to 5·3% (1970) of the total number of O-group fish on the English east coast. Largo Bay was the most important nursery area holding 25% of the total population. It is particularly well situated to receive newly metamorphosed plaice carried in water currents along the north side of the Forth from the spawning ground off Fife Ness. Plaice in the Forth are mainly distributed on fine to medium sandy beaches (186–480 μm), the mean number per haul in midsummer ( D) being correlated with the median diameter (m.d. in μm) of the low water sediments by the equation: D=−45·7666+0·2327 m.d. (n=11,r=0·68,P<0·02 but>0·01). The shallow inshore water in sandy bays in the outer Firth was well mixed and more marine than estuarine (27·7–35·0‰). The correlation coefficient between fish density and water temperature was low, while that with salinity ( S‰) was: D=6·1618+0·2238S (n=23,r=0·62,P<0·005). Regression analysis demonstrated that the relationship between the instantaneous mortality rate ( Z) and the initial population density ( D p) was: Z×100=0·7480+0·0546d p (n=12,r=0·87,P<0·001). The mean mortality rate for the O-group plaice in the Forth nursery areas was 53% month −1.