Description: Recruitment of central/eastern Baltic cod critically depends on favourable oceanographic conditions in the deeper basins of the Baltic Sea creating a suitable habitat for the development of early life stages. The decline in the size of the spawning stock since the mid-1980s initiated a series of investigations on recruitment, which were continued through a partial recovery of the stock in the mid-1990s. The principal factors influencing recruitment and recognized at present are: (i) the volume of water with temperature, oxygen and salinity conditions which meet the minimum requirements for successful egg development ('reproductive volume'); (ii) the age-structure of the spawning stock; (iii) the timing of spawning; and (iv) predation mortality on eggs due to sprat (Sprattus sprattus) and herring (Clupea harengus), as well as cod cannibalism. We relate recruitment at age 2 to parent stock size using updated time series of these variables, comprising the period 1966 to 1994. Spawning stock biomass and egg production are compared as measures of parent stock size. The influence of wind energy and zooplankton abundance on cod recruitment are discussed. A modified Ricker model is outlined explicity accounting for environmentally-induced oscillations around the two observed levels of cod stock size.

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 %.