Eastern Baltic cod—New knowledge on growth and mortality

Project resumé
The project “Eastern Baltic cod - new knowledge on growth and mortality is a way to improved
management advice” focused on development and implementation of approaches and methods to
describe growth and natural mortality of Eastern Baltic (EB) cod, to improve the knowledge on stock
status and thereby the basis for fisheries management advice. The project was funded by the
European Maritime and Fisheries Fund and the Danish Fisheries Agency.
The different activities and methods investigated in this project supplement one another and enable to
improve the present stock assessment in short term, and also contribute to development of more longer
term solutions to ensure solid scientific basis for fisheries management advice in future.
The ongoing tagging program is one of such activities directed to developing long term solutions to
growth of EB cod. Good cooperation with fishing industry is essential for the success of the tagging
program to provide the necessary information. Therefore, the present project has supported the tagging
program by focusing on cooperation and communication with the fishing industry to establish best
possible procedures for tagging and ensure return of the recaptured tagged cod.
Molecular-genetic analyses have been conducted that show low expression of growth hormones in EB
cod. Such advanced technics have hitherto not been used in stock assessment context, and the pilot
analyses conducted in this project therefore provide a basis for greater utilization of such innovative
approaches and techniques in fisheries management context in future.
Analyses of the otolith daily rings, together with length distributions in the stock allowed quantifying the
reduction in growth of smaller cod in later years. We also found a connection between growth and
nutritional condition of fish that suggests that growth of EB cod likely has declined in line with reduced
condition. Reduced growth of EB cod in later years is also explainable in a bioenergetics model that
was developed to investigate the biological hypothesis related to condition, feeding and maturation.
Reduced size at maturation, which was histologically confirmed is another factor that is expected to
lower growth. Overall, all of the available investigations exploring cod growth suggest a reduced growth
of EB cod in later years compared to the period in early 2000s or in the 1990s.
Several factors possibly contributing to an increased natural mortality were investigated as well. Seal
predation was found to be important locally, though not explaining a high natural mortality of the entire
EB cod stock. Analyses of stomach data show that cannibalism related mortality was high in years
around 2010, but substantially declined in later years to a low level. Stomach data in combination with
bioenergetic modelling suggest that mortality related to low feeding level and low nutritional condition
occurs. Overall, natural mortality has likely increased in 2000s. Survey index describing stock
development has also been improved.
Stock status in relation to proxy reference points has been estimated and applied in ICES advice using
production model (SPICT), where specific version has been developed for EB cod to accommodate the
change in production in later years. The progress with understanding growth and natural mortality of
EB cod has allowed also for development of a quantitative assessment model (Stock Synthesis).
Overall, the results of the project have contributed to increased understanding of the stock status, which
has enabled establishment of the benchmark process for EB cod in ICES, with the benchmark
assessment meeting scheduled for early 2019.
The work conducted in the project has links to Horizon 2020 PANDORA project that in combination with
another EMFF project will address seal parasite infestation, which is another factor potentially causing
reduced growth and mortality of EB cod.