Using otolith microstructure to analyse growth of juvenile Baltic cod Gadus morhua

Pelagic and demersal juvenile Baltic cod Gadus morhua L. were collected on the slope and the top of Ronne bank in the Baltic Sea during 2 cruises in November and December 1998. The growth, age at settling and vertical migration pattern were studied by otolith microstructure analysis. The relationship between fish and otolith size were found to change at settling, with an increase of fish size in relation to otolith size after settling. This change was more pronounced on the slope compared to the top of the bank. The timing of first settling at the 2 localities did not differ with respect to fish age. At both localities, fish that hatched early in the season spent a shorter time in the pelagic stage than late-hatched fish. However, significant differences in growth rate during the pelagic stage were observed, where fish captured on the slope grew faster. On the bank, individuals with fast otolith growth rates before settling continued to grow fast after settling. On the slope, no relationship between growth before and after settling was found. These results indicate that the different settling habitats occupied by juvenile Baltic cod have different potential for settling and nursery areas.