Comment on “Early arrival of spring-spawning Atlantic herring Clupea harengus at their spawning ground in the Kiel Fjord, western Baltic, relates to increasing winter seawater temperature” by Ory et al. (2024)

Abstract

In their recent paper, Ory et al. (2024) used a unique set of multidecadal commercial catch data from Kiel Fjord (western Baltic Sea) to investigate the effects of a changing temperature regime on the timing of herring spawning migration. Using a subset from the past 13 years (2008–2020) with detailed information (weekly catches in the Kiel Fjord), they derived a normal function to describe the seasonal course of immigration intensity as a bell-shaped curve with a single peak in the middle of the season. Using this function, they then estimated the seasonal immigration patterns of herring for all 37 previous years (1971–2007), for which only the total, initial, and terminal catch was available along the season. We perceive that the authors have not considered the behavioral changes and fluctuations that are likely to occur given the large time span of the data series. We believe that the use of actual recorded data (start and end of catches in each season) would provide a better basis for investigating temporal shifts in migration patterns than using seasonal curves estimated in this way.

Nevertheless, the results in this article are consistent with previous studies on spring-spawning herring in the western Baltic. Polte et al. (2021), for example, have already documented an earlier spawning activity in the main spawning area of this metapopulation, Greifswald Bay (Bekkevold et al., 2023), due to increasingly mild winters. Thus, the findings of Ory et al. are a valuable addition, which also supports the notion that climate change effects on important fish species are a current problem and not an abstract future scenario.

This work received funding from the European Union (EU-Data Collection Framework DCF), the Federal Ministry of Education and Research of Germany (BMBF) through the projects SpaCeParti, (BMBF grant 03F0914E) and balt_ADAPT (BMBF grant 03F0863D), and the “Unsere Besten promovieren in Rostock” scholarship of the University of Rostock.