Otoliths make the difference

Background and Objective

In the Baltic Sea, the fisheries management of cod is based on two stocks, cod of the western Baltic in the ICES areas 22 to 24 (Kiel Bight/ Mecklenburg Bight, Arkona Sea) and cod of the ICES areas 25 to 32 (Bornholm Sea and all areas east- and northwards). The area 24 (Arkona Sea) is a mixing area between Western and Eastern cod. The question is: What is the proportion of cod from area 22 and 25 in area 24; and how much cod is actually originates from area 24? The mixing proportions in area 22 and 25 are also unclear. This ambiguity results in considerable uncertainty in the stock assessments, particularly when cod from one area “spill over” into other areas, thereby tampering with the stock assessment of the adjacent areas.

To improve the management, we have to re-sort the renegades to their original stock. Presently, this is not readily possible. But we use a number of different modern techniques to find these renegades out. The aim is to assign individual cod to specific areas of origin.

Approach

We use different methods for stock separation, such as genetic signatures, life history traits, spawning time, growth and maturity, parasite infestation etc. Some of these methods are fairly expensive and depend on the discovery of the right markers, i.e. the characteristic signal within the background noise. We are specialized in the analysis of otoliths, the ear stones of fishes, which are used in age determination. We use shape analysis and investigate chemical signatures in otoliths (and in the muscles). While a fish grows, the chemical characteristics of the surrounding aquatic environment are archived in the otoliths, located in the inner ear of the fish. The analysis of the chemical composition along the growth axis of an otolith can help to reconstruct the life history of a fish. Our institute has a micromill for high-resolution milling to recover powder for chemical and isotopic analysis. For instance, we investigate the ratios of the stable isotopes of ¹³C, ¹⁵N, ¹⁸O or ³⁴S in otoliths to draw conclusions on the area of origin of a fish or movement patterns between different water bodies.

Results

Several papers with contributions from OF staff have already been published on „stock separation of Baltic cod“ in the international literature.