Do herring eat fish?

Until now, herring were perceived as typical plankton feeders, playing a key role in marine ecosystems thanks to their diet based on tiny organisms floating in the water. However, the latest research published in Nature Communications shows that reality might not be as straightforward as previously thought. Could these common fish have a slightly more predatory character?

Herring under scrutiny

The latest discovery was made by an international team of scientists from Uppsala University, led by Jake Goodall. The aim of the study was to understand how the Atlantic herring (Clupea harengus), one of the most numerous fish species in the world, adapts to local environmental conditions. Scientists were particularly interested in the differences between herring populations living in the Atlantic Ocean and those that settled in the brackish waters of the Baltic Sea around 8,000 years ago.

Researchers used advanced whole-genome sequencing techniques to identify genetic differences between populations. Genome sequencing was supplemented with stable isotope analyses of nitrogen (δ15N) and carbon (δ13C), which provide information about the fish’s diet and trophic position in the ecosystem. They also used otolith analysis—ear stones in fish—to understand their migration patterns and preferred habitats.

Data collection also included detailed morphological studies, such as the analysis of gill rakers and body measurements, as well as chemical studies to determine the accumulation of pollutants in fish tissues.

Research findings: a new perspective on herring

The findings were surprising. It turned out that a genetically distinct herring ecotype, locally known as Slåttersill, has developed in the Baltic Sea. It differs significantly from its plankton-feeding relatives. These individuals are larger, grow faster, and have a diet based on fish, primarily three-spined sticklebacks (Gasterosteus aculeatus).

The analysis of gill rakers revealed that Slåttersill is not fully adapted to its new diet. Most individuals showed gill damage, suggesting that the shift to a fish-based diet occurred relatively recently in evolutionary terms.

Genetic analysis indicated that Slåttersill belongs to a new ecotype that evolved in response to the lack of competition from other fish-eating species in the young Baltic Sea. Interestingly, these fish have modified genes related to adaptation to low-salinity environments and limited plankton resources.

Ecological and practical significance

Natural processes can be dynamic and surprising. The fish-based diet of predatory herring, focused on sticklebacks, is not only a curiosity but also a significant and realistic chance to restore balance in the Baltic waters. Sticklebacks, whose population has spiraled out of control, consume the larvae of larger predatory fish such as pike and perch, disrupting the natural cycle in the ecosystem. By filling the gap left by missing predators, herring may help halt this process and rebuild a healthy balance in marine waters.

However, this solution comes with a cost. Predatory herring, though fascinating, represent a small and highly vulnerable population. Overfishing in the Baltic Sea could lead to the loss of this unique group, underscoring the need for a well-thought-out fisheries management policy. Without it, we risk not only the extinction of Slåttersill but also the loss of their positive impact on the entire Baltic ecosystem.

Why does the Baltic Sea favor predatory herring?

The shift to a fish-based diet did not occur in Atlantic herring, and the key difference lies in the specific characteristics of the Baltic Sea. This relatively young sea formed after the last glaciation, functioning as an isolated ecosystem for thousands of years. Few species reached its waters, creating a unique void in the role of fish-eating predators.

In such conditions, herring quickly adapted to their new role. While their Atlantic relatives continued feeding on plankton, the lack of competition in the Baltic allowed for an evolutionary dietary shift. This is an excellent example of how nature abhors a vacuum and how quickly species can change when new opportunities arise.