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.

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The microstructure of the central part of the sagittal otoliths of 55 myctophid species belonging to 27 genera of lanternfishes was compared by means of light and scanning electron microscopy. Multiple primordia were found in the nuclei of all otoliths. In most species and genera a symmetrical pattern of accessory primordia (AP) was observed: they were located along the same growth increment, which indicates simultaneous formation. A clustered pattern of AP was found only in species belonging to the tribe Gymnoscopelini: AP occurred at several growth increments, which indicates that they developed sequentially. The growth increments formed after the formation of clustered AP revealed a sectorial otolith structure, i.e., growth increments were not continuous but separated by radial discontinuities. The pattern and time of formation of AP were found to influence the relationship between otolith diameter and fish length in the Myctophidae. The formation of numerous AP concurrently with transformation of the larva led to a dissociation of fish growth from otolith growth. When AP appeared simultaneously but before transformation, the allometric relationship between otolith size and fish length was not disrupted by this process. Sequential formation of AP considerably before transformation, which occurred only in the Gymnoscopelini, led to an isometric relationship between fish size and otolith size. The potential importance of the AP pattern as a distinguishing character for myctophid larvae is considered to be greatest in the Gymnoscopelini, as these growth centers, in the form of external protrusions, were evident over a wide range of sizes from small larvae to early juveniles.

Thirty-three collections of Gymnoscopelus nicholsi were made between 1976 and 1981 in the South Atlantic using both krill and bottom trawls. In the Antarctic waters the main part of the stock consisted of adult fish 3-7 years old, characterized by a benthopelagic mode of life. Fish of year-class I were observed only in the vicinity of South Georgia, South Sandwich Islands and the Scotia Sea; year-class I1 individuals were absent from Antarctic samples. In the notal zone of the western South Atlantic, fish aged 1-5 years were found. The existence of only one population in the region is a possible explanation of these findings: this population is divided into a pelagic stock ofjuveniles and sub-adults which occur in the offshore waters of the western South Atlantic and benthopelagic stocks of adults distributed over the slope regions off Argentina, South Georgia and Antarctic archipelagos.The computed values of K (von Bertalanffy growth) were 0.48, 0.41 and estimated total annual instantaneous mortality rates (Z) were 0.45, 0.67, and 0.70 for South Georgia, South Shetlands and notal zone of western South Atlantic stocks, respectively.