SLU news
New chemical method reveals the age of eastern Baltic Sea cod
Published: 01 September 2020
An emaciated eastern Baltic Sea cod made the cover of the latest issue of the Journal of Fish Biology. The article is part of Yvette Heimbrand's dissertation and was featured by the editor-in-chief Michel Kaiser in the journal "Between JFB Covers" editorial. The published study compares the traditional age estimation method, using cod ear bones (otoliths) with a new, chemical method.
The cover also shows a photo of a cod otolith with corresponding "chemical maps" showing the concentration of various trace elements. To age fish, the number of annual growth zones in the otoliths, found in the fish's head is usually counted. Similar to a tree, annual growth zones are formed throughout the life of the fish. The cod in the eastern Baltic Sea, however, is considered very difficult to age determine due to visually vague and low contrasting growth zones. The lack of reliable age and growth information has for decades been problematic for fisheries management when giving scientific advice for the eastern Baltic Sea cod stock and for setting fishing quotas, says Yvette Heimbrand, PhD student at the Department of Aquatic Resources (SLU Aqua).
Therefore a new method for determining the age of cod was developed and tested by interpreting seasonal variations in trace elemental concentrations in cod otoliths. Three age readers tested the new chemical method to determine the age of the same cod otoliths as sex age readers with many years of experience of the traditional method of counting visual growth zones. Although this was the first time that the chemical method was tested, results showed a higher agreement between age readers than the traditional method. The estimates of the traditional method gave both higher and more variable ages than the chemical method.
The trace elements that were best suited for age determination were magnesium and phosphorus , both reflecting somatic growth and metabolic activity. . Also manganese, which increases when dissolved oxygen drops in summer/fall, often provided a seasonal signal of hypoxia. The cover shows higher concentrations of manganese in yellow and orange during summer hypoxia and two blue bands corresponding to winters, when the oxygen recovered. This study shows that the new chemical method is a promising alternative for fish stocks that are difficult to age, such as the cod stock in the eastern Baltic Sea, says Yvette Heimbrand.
Link to the article: Seeking the true time: Exploring otolith chemistry as an age‐determination tool https://onlinelibrary.wiley.com/doi/full/10.1111/jfb.14422
Link to the editor-in-chief Michel Kaisers "Between the JFB Covers editorial”: Otolith microchemistry helps to unlock the chronology of age determination https://onlinelibrary.wiley.com/doi/10.1111/jfb.14470
Contact:
Yvette Heimbrand, PhD
Department of Aquatic Resources, SLU
yvette.heimbrand@slu.se, -46(0)10-478 41 26