Marine Ökologie

Local and global patterns in fisheries-induced evolution

PI: Jan Dierking,  c-PI: Thorsten Reusch ; with the help of project technician Hendrik Schulz, Master student Laura Elsbernd


This project, funded by the Future Ocean Cluster of Excellence  Research topic “Ocean Resources”, is interested in two little explored but potentially very important aspects of fisheries induced evolution (FIE), namely (1) the role of environmental selection relative to fishing induced selection, and (2) its global (economic) consequences. In this context, FIE refers to (genetically based) shifts towards earlier reproduction and smaller size caused by “Fishing out the big ones” and increased mortality rates from fishing (Figure 1). Such trends have now been observed in many fish stocks, and provide some of the best examples of rapid evolution within just a few generations.


Figure 1 Two mature cod males (45 cm and 24 cm) caught during the May 2012 Bornholm Basin fisheries cruise of our group. Average size at maturity has shown a declining trend in this stock



To address (1), we are exploiting a unique long-term (1986-today) integrated data-series of the Baltic Sea Bornholm Basin cod stock, arguably one of the most interesting cod populations worldwide from an evolutionary and ecological perspective.[1]
This series has been collected by our group and Danish collaborators from the DTU Aqua over the past 27 years, much of it on GEOMAR research vessels including the RV “Alkor ”, and comprises >60.000 individual fish data sets including otoliths, egg and larval abundances, and fine-scale oceanographic (salinity, oxygen, temperature) and foodweb information. The Bornholm Basin stock has experienced heavy fishing that should in principle lead to FIE, but the peculiar environment of this basin, with its combination of low salinity (à low water density) and anoxic deep water layers on the spawning grounds, can lead to complete loss of eggs with low buoyancy. Selection should then favor large females, which have larger eggs with higher buoyancy

[1] Why is this the case? The Bornholm Basin cod stock has evolved in and persists in the Baltic Sea, a shallow marginal sea with only a narrow connection to the North Sea and open ocean. Due to this and other particularities, the Baltic is an excellent model for questions related to climate change and extreme environmental conditions. Notably, oxygen minimum zones in bottom layers occur naturally (and are expected to increase further), and partial CO2 pressures regularly reach levels that are considered worst case scenarios for oceans well beyond the year 2100. All this means that cod in this area is dealing with conditions that oceanic species may be facing only 100 years from now.


How is Bornholm Basin cod faring under these two strong, opposing selection pressures ?

  Figure 2 Potential selection pressures on Bornholm Basin cod. Fishing pressure should select for small size and early maturity, whereas high mortality of eggs of small individuals (which have lower buoyancy and sink into anoxic layers, red diagonal lines) should favor large size

Specific questions include whether the highly variable survival of eggs of all but the largest females (related to anoxia) induces temporal changes in effective population size Ne (i.e., the number of individuals contributing to reproduction), and whether egg buoyancy acts as factor enforcing reproductive isolation between populations (in collaboration with oceanographer Hans-Harald Hinrichsen  and fisheries biologists Christoph Petereit  and Burkhard von Dewitz, EV). As initial step, we have made the potential of the sample- and dataset accessible (à database completion; optimization of DNA yields from otoliths; population biology of samples to confirm temporal stability of the Bornholm Basin stock using SNPs (collaboration with Einar Nielsen’s group, DTU Aqua  Silkeborg) and microsatellites, continuation of cruises to extend the data series (Figure 3)


Figure 3 Impressions from our Bornholm Basin cruises (fisheries and oceanographic work) with RV Alkor conducted at three different times per year to expand the cod long-term data series.



Regarding (2), in a more global approach, together with economists Martin Quaas  and Till Requate  (Department of Economics, CAU) and co-workers, we are contributing to two manuscripts assessing consequences of FIE, “Economic costs of fisheries-induced evolution” and “Status of the world fish stocks in 2048”.