ALBERT

Description: In the Bornholm Basin cod eggs occur exclusively in the intermediate and deep water layers, because their buoyancy is insufficient for allowing them to float in the low salinity surface layer. In the lower range of their vertical distribution they can be exposed to low oxygen contents, which might affect their mortality thus influencing recruitment. At three cruises carried out during the main spawning season of cod in the Bornholm region, hydrographic measurements were made and ichthyoplankton samples were taken by means of a Bongo net and a multiple opening/closing zooplankton sampler. In the middle and at the end of May large numbers of cod eggs were found especially in the north-eastern part of the Bornholm Basin, whereas in the middle of June considerable frequencies were observed only in the southeast. The eggs were concentrated at depths of 60 to 75 m, but they occurred also below that depth range down to the bottom. In this lower part of the water column an intermediate minimum of the oxygen concentration was observed with values of less than 1 ml/l. Due to the increase of specific gravity during embryonic development of cod the older egg stages were relatively more abundant in the deeper water, which caused them to be more exposed to low oxygen levels than the younger ones. Instantaneous daily mortality rates (Z} were estimated by comparing the daily production of a given stage during the first survey with that of its corresponding stage during the second survey. For two cohorts the Z-values amounted to 0.314 and 0.322, respectively. An extrapolation for the entire incubation period leads to an overall egg mortality of 99.9 %.