Serena Donadi, Agnes Olin, Michele Casini, Johan Eklöf, Mårten Erlandsson, Ronny Fredriksson, Olavi Kaljuste, Niklas Larson, Jonas Nilsson, Ulf Bergström, Reduced predation and competition from herring may have contributed to the increase of three-spined stickleback in the Baltic Sea, ICES Journal of Marine Science, Volume 82, Issue 2, February 2025 https://doi.org/10.1093/icesjms/fsae168
Top-down Control
Predators are an important force in maintaining ecosystem balance. They exert control at the top of the food chain, keeping mesopredator populations in check. Mesopredators are animals found in the middle of the food chain, between primary producers and top predators. For example, a small to medium sized fish on a coral reef would be a mesopredator. They feed on algae or smaller fish, but are prey for sharks or dolphins. Too many of these fish would unbalance the reef, eating all the food in the area. With no food, there’s no fish. This balance is fragile and can break down even from small changes in the environment.
Unbalanced Change
The Baltic Sea is actually experiencing some of these changes right now. A mesopredator known as the three-spined stickleback (Gasterosteus aculeatus) has increased dramatically over the last 20 years. This is best seen in the Central Baltic where it’s 13 times more abundant. This increase is more than likely because of the reduction of the stickleback’s predators in the region, the northern pike (Esox lucius) and European perch (Perca fluviatilis). With reduced predation pressure from these two fishes, the stickleback population has been able to grow unchecked. There is concern this will upset the ecosystem balance by eating fleas and larvae that keep algae in check. If the algae is not controlled, habitat-forming vegetation can’t grow. Therefore, less habitat for fish to forage and use as shelter.
While these interactions are well documented, there is less known about how the three-spined stickleback interacts with offshore fishes such as Atlantic herring (Clupea harengus) and European sprat (Sprattus sprattus). Scientists investigated how herring and sprat interact with stickleback. Specifically they looked for direct competition for food and any predator interactions.
Unknown Interactions
Herring were surprisingly found to be a large predator of stickleback, which had not been documented before. Specifically, stickleback made up 64% of the diet of herring over 22 cm (8.6 in). For herring under 22 cm, stickleback were a much smaller portion, probably focusing more on zooplankton and assorted fish larvae.
This predation means there could be some top-down regulation on stickleback in open water. However, there’s been a significant decline in large herring in the Central Baltic, the same region stickleback populations have exploded. This could mean the predator pressure is missing from this area as well. Similarly, sprat, who would be in competition for similar food resources as stickleback, have declined by nearly 100% in the Bothnian Sea. Another region where stickleback populations have increased. In areas where herring and sprat populations remain steady, stickleback numbers haven’t grown. This indicates that healthy populations of these two fishes have some sort of regulatory role.
Returning Balance
This study uncovered some previously unknown fish interactions between herring, sprat, and stickleback, including large herring predation and sprat competition with stickleback. However, the ecosystems in the Bothnian Sea and Central Baltic have already experienced an unbalanced shift toward stickleback. This is largely due to significant decreases in herring and sprat numbers. Both species have been heavily fished since the 1980s, leading to both stocks being halved. Unfortunately, there’s been minimal recovery, demonstrating the dominance stickleback may have over the ecosystem now.
Future recovery efforts should focus on herring and sprat to reintroduce predation and resource competition to limit stickleback numbers. This study provides useful information on the intricacies of the marine food web and how actions can cascade to unintended consequences. However, we can learn from this event and practice more sustainable fishing, while monitoring fluctuations in local populations of other species for unbalanced shifts.
I am a recent MSc graduate in marine biology from Bangor University, where I studied population dynamics of elasmobranchs off the coast of Wales. My interests lie in ecological data analysis to understand environmental processes and identify natural patterns. However, nothing beats being in the field and interacting directly with the marine life.
