Fish stock development in the Central Baltic Sea (1974-1999) in relation to variability in the environment

Cod, sprat, and herring are the commercially most important fish species in the Central Baltic Sea. In the present study, dynamics of stock abundance and biomass of these species were reconstructed using Multispecies Virtual Population Analysis. The cod stock declined from an historic high during the early 1980s to its lowest level on record at the beginning of the 1990s, showing no sign of recovery afterwards. The sprat stock size increased to a historic high level concurrently, while herring abundance was slightly reduced. However, a substantial reduction in herring weight at age resulted in a continuous decline of the total biomass from the early 1980s. A review of recruitment processes influenced by the variable physical environment was performed for cod and sprat, i.e. the species most intensively studied and showing the largest variability in stock sizes. The most important processes are (i) egg production in dependence of ambient hydrographic conditions and food availability, (ii) egg developmental success in relation to oxygen concentration and temperature at depths of incubation, (iii) egg predation by clupeids dependent on predator-prey overlap, (iv) larval development in relation to hydrographic processes and food availability, and (v) predation on juveniles. All the above processes negatively affected the cod population, while the sprat stock benefited from them, despite a developing industrial fishery, resulting in a regime shift from a cod to a sprat dominated system in the Central Baltic.

Article from Marine Science Symposia Vol. 219 - "Hydrobiological variability in the ICES Area, 1990-1999", symposium held in Edinburgh, 8-10 August 2001. To access  the remaining articles please click on the keyword "MSS Vol. 219".