Jan Horbowy
Sea Fisheries Institute, Kołłątaja 1, 81-332 Gdynia, Poland

Key words: cod, Baltic, production model, difference model.

Abstract.
In light of large inconsistencies in age reading of the eastern Baltic cod stock, production and difference models were tested and compared with results provided by age structured analysis (XSA). In one of the difference models cod cannibalism was simulated. The models were fitted under non-equilibrium conditions by minimizing the sum of squares of modeled and observed quantities. The survey results were used to estimate fishing effort (used in all models) and recruitment data (used explicitly in difference models). The bootstrap procedure was applied to estimate coefficients of variation of model parameters. It is shown that both production and difference models produce cod dynamics estimates which exhibit similar trends of development as XSA. The CVs of some model parameters are high; however, they are especially so for the carrying capacity and maximum sustainable yield. All the models produced rather similar estimates of stock size in 2002, which ranged from 140 to 190 thousand tons (age 3 and older). Retrospective analysis indicated that there is a relatively consistent pattern of trends in biomass estimates.