Researchers are finding that having good neighbors and few large predators makes predatory fish populations more resilient.

A regime change is gradually spreading to the archipelagos of the Swedish Baltic Sea coast, where shallow bays, once dominated by pike and perch, have one by one become dominated by one of their prey species, the three-spined stickleback.

Today, research shows that the ability of pike and perch to maintain their dominance over sticklebacks increases if there are enough pike and perch nearby, which can help the local population resist the intrusion of sticklebacks. The researchers also found that the resilience of pike and perch populations declined with increased predation from gray seals and great cormorants.

Pike and perch populations have experienced a marked decline along the Swedish Baltic Sea coast. One of the main causes of this decline is the dramatic increase in the number of threespine sticklebacks. Stickleback is a small species of fish that reproduces quickly and shows a good appetite for the eggs and larvae of pike and perch.

Through this predation, it eliminated local populations of pike and perch, particularly in the outer archipelago. As the movement spread successively from bay to bay, it became known as “the stickleback wave”. When stickleback comes to dominate an area, it has cascading effects on the rest of the ecosystem, with consequences for water quality and habitat-forming vegetation.

To better understand how the ecosystem functions and how we can maintain strong populations of pike and perch, it is important to determine what factors make an ecosystem resilient or vulnerable to the stickleback surge.

A new study published in Natural communications by researchers from Stockholm University, the Swedish University of Agricultural Sciences (SLU) and the University of Groningen, shows that access to nearby spawning and nursing areas (connectivity) can increase the ability of predatory fish to resist the wave of sticklebacks.

“Larger areas of spawning habitat close enough together mean that predatory fish have ‘neighbors’ that can migrate and support a low population. Small, isolated bays without these neighbors will struggle when stickleback hits,” explains Agnes Olin. , researcher at the Swedish University of Agricultural Sciences and first author of the study.

In this study, researchers analyzed the abundance of juvenile perch, pike and stickleback in a large number of bays along the Swedish Baltic Sea coast to determine which factors determine the relative density of these species .

The most important determining factor in determining whether a certain bay was dominated by stickleback or predatory fish was the quantity of sticklebacks arriving in the bay in the spring to spawn. However, if there were many predatory fish spawning habitats nearby, the likelihood that predatory fish would maintain dominance when sticklebacks arrived increased.

“Even though the importance of connectivity in nature is widely recognized, empirical studies demonstrating whether and how connectivity affects species interactions are still relatively rare. This study fills this gap using a large dataset, collected on a relatively large geographic area, to show empirically how connectivity can affect key ecological processes,” explains Örjan Bodin, professor at the Stockholm Resilience Center at Stockholm University and co-author of the study.

The study also showed that the positive effect of proximity to spawning grounds, i.e. connectivity, disappeared in cases of high local densities of gray seals and great cormorants. These two top predators have increased in the Baltic Sea and can eat a lot of pike and perch in some areas.

This suggests that at particularly high densities of seals and cormorants, predatory fish populations may be suppressed to such a degree that it does not matter whether there are many predatory fish spawning habitats nearby. , since there are no fish anyway.

“This is the first time we can show that gray seals and great cormorants have such a large-scale impact on pike and perch populations along the Baltic Sea coast. If we want to strengthen these fish populations, so it could be important to implement measures that reduce the predation pressure of seals and cormorants in the spawning grounds of perch and pike,” says Ulf Bergström, researcher at the Swedish University of Agricultural Sciences and co-author of the study.

“The study shows that a holistic approach (is important) when it comes to managing ecologically important fish species, such as pike and perch. The management of habitat, fish and their predators, including seals, cormorants and sticklebacks, are all closely linked,” explains Johan Eklöf, professor at Stockholm University and leader of the research project that funded the study.

“Additionally, our results highlight the importance of having a broader spatial perspective: what happens in one bay has ripple effects in other bays as the fish move, so you can’t just look at one bay at a time.” time.”