Dependency of larval fish survival on retention/dispersion in food limited environments: the Baltic Sea as a case study
A three-dimensional hydrodynamic model has been used to analyse temporally and spatially resolved circulation patterns in the Baltic Sea with special emphasis on drifting particles representing larval fish. The main purpose of this study was (i) to investigate potential drift patterns of larval fish, (ii) to identify its intra- and inter-annual variability for time periods based on the timing of spawning and (iii) to analyse its seasonal and spatial variability in dependence of the atmospheric forcing conditions. For the time period 1979–1998 temporally and spatially resolved simulated flow fields were used to describe the potential drift from the centre of main reproductive effort of Baltic cod (Gadus morhua). The results of the model runs demonstrate a general change in circulation pattern from retention during a first decade (1979–1988) to dispersion in the following decade (1989–1998). This increase in dispersion was related to an increase in the variability of the local wind forcing conditions over the Baltic. The more frequent occurrence of dispersion in spring of the recent decade was accompanied by a strong decay in biomass of one of the main larval fish feeding component in the central basin, the calanoid copepod Pseudocalanus elongatus. Larger dispersion of this prey organism may have affected the spatial overlap and thus the contact rates between predator and prey. Hence, this may have resulted in a food limitation for early life stages of Baltic cod and potentially contributed to the pronounced shift in cod peak spawning time from spring to late summer. Early life stages of cod originating from late spawning fish, benefited from a stronger dispersion in late summer and autumn, into shallow coastal areas with higher calanoid abundance.