Large-scale climatic conditions prevailing over the central Baltic Sea resulted in declining salinity and oxygen concentrations in spawning areas of the eastern Baltic cod stock. These changes in hydrography reduced the reproductive success and, combined with high fishing pressure, caused a decline of the stock to the lowest level on record in the early 1990s. The present study aims at disentangling the interactions between reproductive effort and hydrographic forcing leading to variable recruitment. Based on identified key processes, stock dynamics is explained using updated environmental and life stage-specific abundance and production time-series. Declining salinities and oxygen concentrations caused high egg mortalities and indirectly increased egg predation by clupeid fish. Low recruitment, despite enhanced hydrographic conditions for egg survival in the mid-1990s, was due to food limitation for larvae, caused by the decline in the abundance of the copepod Pseudocalanus sp. The case of the eastern Baltic cod stock exemplifies the multitude effects climatic variability may have on a fish stock and underscores the importance of knowledge of these processes for understanding stock dynamics.

Climate change could lead to mismatches between the reproductive cycles of marine organisms and their planktonic food. We tested this hypothesis by comparing shrimp (Pandalus borealis) egg hatching times and satellite-derived phytoplankton bloom dynamics throughout the North Atlantic. At large spatial and long temporal (10 years or longer) scales, hatching was correlated with the timing of the spring phytoplankton bloom. Annual egg development and hatching times were determined locally by bottom water temperature. We conclude that different populations of P. borealis have adapted to local temperatures and bloom timing, matching egg hatching to food availability under average conditions. This strategy is vulnerable to interannual oceanographic variability and long-term climatic changes.

Interannual variations in spawning time, defined as the peak in egg abundance, of cod (Gadus morhua) in the Bornholm Basin, Baltic Sea, were analysed. Effects of water temperature, size and age structure of the spawning stock, abundance of food, and timing of spawning in preceding years were studied as possible determinants of annual spawning time. During the 1970s and late 1980s, peak spawning took place between the end of April and mid-June. A remarkable shift in the timing of spawning to the end of July was observed in the 1990s. The key factors governing the timing of spawning are water temperature during the period of gonadal maturation, density-dependent processes related to the size of the spawning stock, and food availability. The age structure of the spawning stock is suggested to have an additional effect. A high proportion of first-time spawners and decreasing water temperature have caused progressively delayed spawning since the early 1990s. Late spawning involves several processes that are detrimental to the survival of the early life stages. Recruitment in the mid-1990s was below what could be expected from spawning stock biomass and favourable hydrographic conditions. It is therefore suggested that the rebuilding of the Baltic cod stock could be improved by reduced fishing pressure in spring on early spawners. 2000 International Council for the Exploration of the Sea

Spatial and temporal variability in environmental factors can exert major influences on survival and growth of livlng organisms. However, in many key areas of fisheries sclence (e.g. growth, survival and recruitment determination), environmental heterogeneity is usually ignored because of insufficient environmental or fisheries data or lack of evidence that such heterogene~ty impacts response variables. For the eastern Baltic Sea (ICES Subdivisions 25 to 321, we evaluated spatial and temporal differences in conditions affecting the sunrival of cod Gadus morhua L. eggs at survival on four distinct spawning sites within the assessment area. We intercalibrated ways of quantifying the volume of water ('reproductive volume') at each site where salinity, oxygen and temperature conditions permitted successful egg development. We have developed and compared a time series (1952 to 1996) of reproductive volumes among the areas to identlfy spatial differences. The results of 2 independent volume-estimation methods are comparable, indicating that highly significant differences exist among the sites, and that the westernmost spawning ground, Bornholm Basin, has on average the highest reproductive volume and the lowest variability among the 4 sites. These findings may be useful in evaluating how spatial and temporal variability in environmental conditions affect egg hatchng success and possibly recruitment in the Baltic stock.

Cod eggs in the Baltic Sea are neutrally buoyant at depths exceeding 55 m. When these eggs hatch the larvae must enter the upper photic portion of the water column to locate and capture sufficient prey to feed and grow. In this study we investigated the time during ontogenetic development at which this vertical migration occurs. The vertical distribution of cod larvae, microzooplankton, light intensity and the physical characteristics of the water column in the Bomholm Basin were investigated during 3 cruises in May, June and July 1994. Larvae designated as pre-feeding were usually located at the depths where they had hatched. After larvae had begun to feed, their distributions moved closer to the water's surface. Since larvae are negatively buoyant relative to the density of water in the upper layers of the Baltic, this migration requires active swimming. Hence the hydrographic structure of the water column in the Baltic likely imposes a modest metabolic cost on larvae. We also investigated factors determining the vertical hstribution of feeding larvae. The distribution of these larvae was poorly correlated with prey abundance (i.e. concentration of copepod stages). However, distributions were correlated with prey availability as estimated by combining measures of light-dependent larval feeding incidence with the measured prey concentrations. Our observations suggest that a vertical migration among Baltic cod larvae is necessary for 2 reasons. This migration enables larvae to obtain suitable feeding conditions, and to avoid mortality that could be induced by exposure to the low oxygen conditions typical for the sub-halocline layer.