Mortality through the early life-history of fish: What can we learn from European plaice (Pleuronectes platessa L.)?

It is difficult to estimate natural mortality for many marine fish populations, especially during the transition period from larvae to juveniles, because the appropriate data are scarce. Plaice (Pleuronectes platessa) is an exception since it has been studied extensively. The study of mortality rates in juveniles is made easier because the nursery grounds are inshore and generally less than 5 m deep. This contribution considers the factors affecting mortality rates for eggs and larvae, and for settlement and nursery ground phases. There are problems associated with estimating mortality rates, especially for juveniles because immigration into nursery areas at the beginning of the season and emigration of larger individuals off nursery grounds in the latter part of the season confound losses due to mortality. The shifts in mortality schedules and the causes through the early life history are investigated in relation to concepts such as ‘nursery ground carrying capacity’ and ‘self-thinning’. Predation in the pelagic phase is probably density independent and a source of inter-annual variability in survival of early life-history stages. Predation mortality during the nursery ground phase is most likely density dependent. However, there is a need for further in-depth study, especially during the period of settlement.