Marine Research Findings of the VECTORS Project

This website provides access to the research results of the VECTORS project, which can be used to support marine management decisions, policies and governance as well as future research and investment. VECTORS was a large scale project that brought together more than 200 expert researchers from 16 different countries. It examined the significant changes taking place in European seas, their causes, and the impacts they will have on society.

Spatio-temporal differences in early life stage survival of Baltic cod

Recruitment variability of marine fish is influenced by reproductive potential of the stock and the survival of early life stages, mediated by environmental conditions of both a physical (water temperature, salinity and oxygen conditions, ocean currents) and a biological nature (i.e. food, predators). To disentangle the effects of different drivers on recruitment variability, a spatially and temporally explicit understanding of the early life stage dynamics is required. The objectives of our investigations included assessing how the spatio-temporal variability of the ambient hydrography affects the suitability of different spawning areas for successful egg survival of western Baltic cod. Further, we investigated the temporal and spatial origin of juvenile survivors of eastern cod, as larval survival is probably a critical life stage for cod recruitment. These results contribute to understanding of recruitment processes of Baltic cod and can assist in spatial management measures, e.g., placement of Marine Protected Areas.


Favourable conditions for western Baltic cod occur during the late spawning season in April/May. Survival chances are highest in the Kattegat, intermediate in the Great Belt, and low in the Sound, Kiel Bay and Mecklenburg Bay. Unsuitable time periods and habitats are characterised by water temperatures below survival threshold.

The objectives of this study include performing long-term assessments of (i) the transport of western Baltic cod early life stages from spawning to hatching areas and their survival success along drift routes, (ii) the retention and dispersal of early life stages originating from different spawning grounds, (iii) the contribution of different spawning grounds for early life stage survival in relation to the variability in environmental conditions, and (iv) the impact of the timing of spawning on early life stage survival. The approach used was based on a hydrodynamic model, coupled with an Individual Based Model considering western Baltic cod egg stages as well as the yolk-sac larval stage. We combine observations, process knowledge, and numerical modelling, for simulating the dynamics of the western Baltic cod stock. The model was used to provide predictions of the survival success of proportions of eggs and yolk-sac larvae, characterised by losses due to bottom contact or by ambient water temperatures falling below a critical survival threshold. However, the resulting impact on realised recruitment to the stock seems to be limited. Albeit that recruitment was significantly correlated with habitat suitability during the main spawning season in the Kiel/Mecklenburg Bays, the signal was not strong and was not consistent across areas, indicating that environmental conditions during spawning alone do not determine the relative year class strength. This suggests that other factors such as subsequent drift, destination, and related survival of eggs, yolk-sac, and feeding larvae have strong influences on year class strength.


Spatial and temporal differences in larval survival of Baltic cod were demonstrated, which are likely related to food availability. The offspring with high survival probability to enhance the subsequent recruitment are spawned in locations outside the Marine Protected Area, especially in the northwestern part of the Bornholm Basin.

The spatio-temporal origin of surviving juvenile Baltic cod was investigated by coupling age information from otolith microstructure and hydrodynamic modelling, which allowed backtracking of drift routes in time and space. The suitability of hydrodynamic modelling for drift simulations of early life stages was validated by comparing model simulations with the catch distribution from a survey targeting pelagic juveniles, and mortality rates and hatch date distributions of pelagic and demersal juveniles were estimated. Hatch dates and hatch locations of juvenile survivors showed distinct patterns which did not agree with the abundance and spatial distribution of eggs, suggesting marked spatio-temporal differences in larval survival. The good agreement of the spatio-temporal origin of survivors from this field investigation with previous modeling studies on the survival chances of early-stage larvae and with general spatio-temporal patterns of larval prey availability suggests that differences in survival are related to food availability during the early larval stage. The back-calculated survivor hatch locations were generally at the edges of the spawning area in the Bornholm Basin, especially in the North and east of Bornholm Island. Most favourable feeding conditions for larvae seem to occur in the early spawning season, whereas later hatched larvae may profit from increasing abundances of nauplii of T. longicornis and Acartia spp., but only if they are hatched in or rapidly transported to shallower coastal areas. These analyses can provide valuable information with respect to the design and implementation of closed areas or seasons to ensure undisturbed spawning and high survival of offspring.

Lead Author:

Hans-Harald Hinrichsen, Bastian Huwer and Karin Hüssy & Margit Eero
Danmarks Tekniske Universitet (DTU-Aqua)
Date of research: September 2014

Related articles:

Population dynamics of sprat in the Baltic Sea 

Cod, recruitment, temperature and zooplankton

Fish distributions and spatial management measures 

The drivers of a common sole population 

Vital rates of fish larvae 

Baltic cod recruitment and predation

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This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 266445
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