Growth of cod (Gadus morhua) in the western Baltic Sea: estimating improved growth parameters from tag–recapture data

Abstract

Tag–recapture data contain valuable information about individual fish growth, which can enhance the estimation of growth parameters. Tag–recapture data are especially useful when age-determination uncertainties throw age-based growth estimates and stock assessments into question, as is the present situation with several important gadoid stocks. Length-based (GROTAG method) and age-based (LEP method) growth functions were fit to a large data set of tagged and recaptured cod (Gadus morhua) from an artificial reef in the western Baltic Sea to calculate improved growth parameter estimates. The LEP method allowed integration of different data formats, including tag–recapture, length frequency, and length-at-age, to estimate a more robust and comprehensive growth function (von Bertalanffy growth parameters: L∞ = 154.56 cm, k = 0.11, t0 = –0.13). Two heavily exploited cod stocks inhabit the Baltic Sea, subsisting at the upper thermal and lower salinity limits of the species. Otolith shape analyses indicated that, unexpectedly, individuals from both populations were resident at the reef. Compared with cod populations elsewhere, cod in the western Baltic Sea grow relatively slowly and with weak seasonal fluctuations in growth rates, potentially due to adverse conditions for growth.