Understanding Eastern Baltic Cod

Tuesday 20 Sep 16


Fritz Köster
Director of institute
DTU Aqua
+45 35 88 30 00
Director Fritz Köster, DTU Aqua, gives opening plenary lecture on the processes that influence the dynamics of fish stocks at the ICES Annual Science Conference 2016 in Riga

The ICES Annual Science Conference 2016 opened om Monday September 19th, 2016. Director Fritz Köster, DTU Aqua gave the opening lecture, "Understanding the processes behind fish stock dynamics: Where are we?", on the scientific knowledge base needed for explaining historical and projected fish stock dynamics. Gadoids was used an examples, such as the anticipated successful recovery and subsequent failure to assess the status of one of the most well-studied groundfish stocks – the eastern Baltic cod.

Fritz Köster began by defining terms and stock traits involved with assessments before moving on to present examples that conveyed the status of science in understanding the myriad factors that play into a fish stock's status. Ranging from recruitment and spawning to growth and nutrition to natural and fisheries induced mortality on one level to climate and human pressures on another – and are vital in establishing reference points and forecasting as management becomes longer term.

Resolving stock structure and migratory behaviour requires information at smaller spatial and temporal scales than present stock assessments cover and requires studying various linkages between the environment and individual behaviour, Köster explained. The example of changing distribution, in Greenlandic cod in the early 1990s was used to demonstrate the importance of considering the biological drivers of a stock prior to assessment. This was also held up as inspiration for the current example of Baltic cod.

The quality of information about stock structure and spatial dynamics has improved vastly since then, based on the use of modern technology in genetics and tagging. These have contributed to new insight into migratory behaviour, spawning site fidelity, group cohesion and the potential for recolonization of areas characterized by stock depletion.

Köster outlined the case of the Canadian Gulf of St. Lawrence, looking at what hampered the recovery of the cod stocks after declines in the early 1990s accompanied by a collapse of its size and age structures. It was found that poor growth and nutritional condition, declining reproductive success and increased natural mortality acted in concert rather than one single process acting alone.

Baltic cod example

A parallel was drawn with the eastern Baltic cod, with a similar question of stock recovery being asked and explored. Whilst the spawning components reduced and growth and condition declined, recruitment went up. The influences behind this increase in recruitment were identified; however, Köster stated, the relative importance of these processes are unclear and warrant further investigation.

Another dimension to the Baltic cod example is the availability of fish and benthic prey and hydrography, with declining oxygen conditions and rising temperatures all negatively affecting the cod's nutritional condition and growth. Köster also examined the relationship between nutritional condition and natural mortality and the impact of a growing seal population on the fish stock. Historical developments in the 20th centrury have seen periods of lower nutritional condition and growth as well as higher natural mortality before.

"The processes impacting fish stocks are so complex that singular assessors, single institutes, and single countries are unable to manage and it needs international collaboration, which is ICES' strengths," said Köster in a press release sent out by ICES, stressing that whilst process knowledge has advanced, there are still many areas that require studying important processes simultaneously in a combination of empirical analyses, controlled laboratory experiments, field experiments, and modelling.

"Apart from dedicated process studies, integrated regional monitoring systems and sustained access to samples, data and information across North Atlantic ecosystems are key, with ICES being central in coordinating these activities."

The ICES Annual Science Conference 2016, 19–23 September 2016 in Riga, is attended by over 600 scientists, policy makers, organization representatives, and students from all over the world.

By Simon Cooper, ICES and Line Reeh