Description: Based on an extensive review of available knowledge, several sources of information have been identified as potentially useful in fish stock predictions. They refer to a variety of biological processes of importance for stock dynamics, e.g. growth, maturation and egg production as well as early and juvenile life stage survival and subsequent recruitment, for which examples are given. Environmental variables impacting these processes were derived, ranging from simple statistical exploratory analyses to complex process studies for various stocks in different sea areas. Causal relationships are understood to varying degrees and in several cases the identified variables may only be taken as proxies for processes not investigated in detail yet. Besides the explanatory power of relevant variables, their predictability and related predictive time frames are of major importance for a potential application in stock predictions. These criteria in particular may hamper implementation in the foreseeable future in several cases. However, the information may still be highly relevant for a) hindcasting stock developments that are so far not fully understood, b) defining stock projection scenarios for simulation of different fishery management strategies under varying environmental conditions and considering species interactions and c) elucidating areas of future research to further enhance our predictive capabilities.

Description: The recruitment processes of Baltic cod and sprat were analysed and critical periods were identified by addressing the major impact factors on individual early life history stages separately and relating observed abundance data between successive stages. For cod, recruitment appeared to be dependent on egg survival, with low oxygen concentration in dwelling depths and predation by clupeids as the major causes for egg mortality. Surviving egg production and larval abundance were weakly correlated, whereas larval abundance was significantly related to year class strength. This indicated that the period between the late egg and the early larval stage is critical for cod recruitment. A potential variable identified to affect this life stage was prey availability for larvae. For sprat, early and late egg stage production as well as late egg stage production and larval abundance were significantly related. However, year class strength was largely independent of larval abundance. Thus, the period between the late larval and early juvenile stage appeared to be critical for sprat recruitment. Potential variables identified to affect this life stage were ambient temperature and wind stress. Environmental factors showing statistically significant covariance with the survival of one of these critical life stages were incorporated into stock-recruitment models for individual spawning areas separately and for the Central Baltic combined.

Description: The distribution of herring larvae in relation to environmental conditions and the occurrence of possible prey and predator organisms was studied during a 4-day period on a permanent station in the northern North Sea in September 1999. The vertical distribution of herring larvae was sampled in 20-m intervals by means of a multiple-closing net. To resolve the small-scale patchiness of herring larvae and planktonic prey and predator organisms, a towed in-situ video system was used, the Ichthyoplankton Recorder. A diel vertical migration of herring larvae was observed with different intensities depending on their body length. Small larvae (〈10 mm) were concentrated in the upper water layers during daytime and were distributed more homogenously during night time. Large larvae (〉16 mm) showed the highest abundances in the upper water layers during the day and were concentrated in deeper water layers during the night. The presented results appear to be relevant for individual-based modelling of the fate of larval herring populations. La distribución a pequeña escala y la migración vertical de larvas de arenque del Mar del Norte (Clupea harengus, Teleostei: Clupeidea) en relación con factores abióticos y bióticos. – Se han estudiado la distribución de las larvas de arenque en relación a las condiciones ambientales y la incidencia de posibles presas y organismos depredadores a lo largo de un periodo de 4 días en una estación permanente situada en el Mar del Norte en septiembre de 1999. La distribución vertical de las larvas de arenque fue muestreada en intervalos de 20 m mediante una red de apertura múltiple (multiple opening/closing net). Para determinar a pequeña escala la heterogeneidad en la distribución de las larvas de arenque, así como de las presas y depredadores planctónicos, se utilizó un sistema de vídeo in-situ (The Ichthyoplankton Recorder). Se observó una migración vertical nictimeral con diferentes intensidades dependiendo de la longitud corporal de las larvas. Las larvas pequeñas (〈10 mm) se concentraban en las capas superiores de la columna de agua durante el día y se distribuían más homogéneamente durante la noche. Las larvas más grandes (〉16 mm) mostraban su mayor abundancia en las capas superiores de la columna de agua durante el día y se concentraban en capas de agua más profundas durante la noche. Los resultados presentados en este trabajo pueden ser relevantes para la modelización (individual based modeling) del destino de las poblaciones de larvas del arenque.