Cod, a primary fishery species in the Baltic Sea since prehistoric times, has recently experienced marked declines in size and condition. This is due to a combination of worsening water quality conditions from hypoxia intensification, reduction in prey availability even as cod densities have risen, and increased occurrence of parasites and disease. One of the net results of this has been a decrease in formation of clear annual growth rings in otoliths of Eastern Baltic Cod. Otoliths (ear-stones) are part of the hearing and balance system in fishes, and grow incrementally as a fish grows. In seasonal environments, growth bands are laid down, much like tree rings. But in Eastern Baltic cod it has become increasingly difficult to "read" otoliths and determine ages. This is a serious problem, because age is a key variable used in stock assessment for the management of exploited populations, including quota-setting. In 2014 the situation became so dire that the stock assessment was not approved for this stock. Solutions are clearly needed.

We have been testing a new method to age cod, using otolith microchemistry in conjunction with optical imagery of otoliths. A suite of elements are measured on microscopic transects across an otolith from initial formation (birth) to the outer edge (death), and are compared to visible features. Initial trials with “readable” otoliths are promising, and take advantage of not only those elements that are related to somatic growth, but also those that relate to use of different habitats. With funding from the Swedish Research Council Formas, we are expanding this study, to document which combinations of elements are most useful and robust. We will then apply this to cod in the Eastern Baltic in different geographic areas and at different time periods, in order to test whether readability improves in similar or different ways as a function of different stressors. Finally, we will test the generality of the method on a second species, Baltic herring, which also experience readability problems.

Our intent is to develop a “best practices” methodology for aging cod and other Baltic fishes. In addition to publishing and presenting our results in scientific fora, we will work closely with stakeholder groups in Baltic fishery and ecosystem management.

Participants: Karin Limburg (PI, ESF and SLU); Michele Casini (co-PI, SLU); Karin Hüssy (co-PI, DTU), and Yvette Heimbrand (PhD student, SLU)