Enhancing Fish Populations: Adding Habitats Beats Adding Fish
Robert Arlinghaus
Leibniz Institute of Freshwater Ecology and Inland Fisheries, Germany
Winning article: Ecosystem-based management outperforms species-focused stocking for enhancing fish populations (Science, 2023)
“To find robust answers, we needed to perform a rigorous whole-ecosystem management experiment to compare outcomes of environment-oriented vs. fish species-oriented management practices.”
Many natural ecosystems are experiencing significant degradation, necessitating urgent rehabilitation efforts. This issue is particularly salient in freshwater environments - rivers, lakes, and wetlands - where biodiversity loss is occurring at an even greater rate than in terrestrial environments. Among the most critically endangered vertebrates are freshwater fishes, whose decline has far-reaching consequences not only for aquatic ecosystems but also for inland fisheries and the communities that depend on them for sustenance and economic stability. Effective conservation and restoration actions are needed to reverse this biodiversity decline and restore the integrity of freshwater ecosystems and the services they provide to humanity.
One promising approach to addressing these challenges is ecosystem-based management, which aims to improve or restore key ecological processes, habitats, and species interactions rather than focusing on removing single stressors or supporting individual species that are heavily exploited. This holistic approach recognizes interdependence within ecosystems, aiming to foster resilience and long-term sustainability. However, ecosystem-based management is costly. Policymakers, tasked with balancing ecological integrity and economic feasibility, often hesitate to invest in ecosystem-based management until robust scientific evidence demonstrates its effectiveness. Yet, generating such evidence presents significant challenges - experimenting with entire ecosystems in a replicable fashion is not trivial, and rarely done.
For 20 years, my research team and I have been working on solving fisheries and fish conservation issues in freshwater ecosystems in partnership with stakeholders. I was frustrated for a long time that most research on how to sustain and improve natural fish populations was not scientifically robust because it relied on case studies, non-experimental observations, or solely mathematical modelling. In response, many fisheries stakeholders and decision-makers at local and regional scales were quick to take cognitive shortcuts that fueled reliance on easy fixes to support fish populations. Especially the single-species oriented practice of fish stocking has developed as the cornerstone of inland fisheries management globally. Billions of individual fish, many of which are artificially bred in hatcheries, are released annually in lakes, rivers, and coastal environments to support fish populations or fisheries catch. As a fisheries ecologist my suspicion has always been that many of these supplementation programs will fail as a result of strong population regulation and maladaptation of hatchery fish in the wild. But communicating this hypothesis to fisheries stakeholders was answered with doubt.
To find robust answers, we needed to perform a rigorous whole-ecosystem management experiment to compare outcomes of environment-oriented vs. fish species-oriented management practices. To generate actionable insight and acceptance by rights-holders, these experiments needed to be replicated and conducted at the scale of exploited ecosystems in transdisciplinary, stakeholder-inclusive settings. We took advantage of the excellent situation that fishing rights in German inland fisheries are private property. Local fisheries management is typically organized by recreational angler communities who form angling clubs and associations. In this setting, the angling clubs have sovereignty in deciding which management action to take locally. To guide decision-making, I partnered with my former Ph.D. student Thomas Klefoth (Fig. 2), who at the time was working as a fisheries biologist in a large angler association in Lower Saxony, Germany, the Anglerverband Niedersachsen (AVN). Together with the AVN and led by Thomas as key partner, we designed a radical experiment to examine the biodiversity outcomes of two variants of ecosystem-oriented conservation actions - dead wood introduction and shallow water zone creation- compared with traditional fish stocking of five species in lake fisheries. In partnership with dozens of angling clubs, we raised more than 2 million € and completed a large-scale, transdisciplinary, stakeholder-inclusive, whole-ecosystem manipulative experiment replicated in 20 lakes over a period of six years, sampling about 160,000 individual fish and many other taxa. Using a before-after-control-impact design the study showed that ecosystem-based management, particularly the creation of shallow water zones, sustainably increased lake fish abundance and fish recruitment, and also helped the abundance and diversity of other organism groups. By contrast, the standard practice of fish stocking, which the anglers so greatly believed in, completely failed. The addition of dead wood as a structural element showed only lake-specific effects on fish abundance.
This work convincingly and decisively demonstrated, through experimental and causal evidence, that ecosystem-based management outperforms species-focused conservation action in lakes – adding habitats beats adding fish! What makes this outcome so exciting is that the entire experimental intervention was designed in collaboration with local stakeholders in communities of practice—those who owned the lakes and surrounding land. In this position, the stakeholder had a strong self-interest in finding answers and in experimenting with different management actions. The whole process was characterized by knowledge co-production, where dozens of angling clubs and their managers and literally thousands of anglers worked together to implement and experiment at realistic scales over multiple years. Perhaps the most rewarding aspect was witnessing the emergence of collective action. We chose to work in small, artificially created pit lakes, where we focused on improving a limiting habitat – the shallow water zone, a critical hotspot for biological production in the otherwise steeply sloped lakes. To our great surprise, the addition of just a small percentage of additional shallow habitat had massive biological effects on a whole lake scale.
Small lakes, both in terms of number and area, are the dominant lake type globally. A key takeaway from our work is that local environmental actions, driven by civil society actors, have the potential to fundamentally enhance biodiversity conservation. These actions are scalable if policymakers create the right conditions for pro-environmental action by local fish users, such as minimizing bureaucratic obstacles to modifying and restoring habitats. Beyond the local level, our paper holds the potential for transformative change in environmental policy, aiming to restore biosphere integrity—an essential planetary boundary that has already been crossed.
Figure 1: Population dynamic mechanisms emerging from ecosystem-based habitat management through creating shallow littoral zones and coarse woody habitat addition, and from species-focused stocking management.
Additionally, our study offers important implications for land systems, freshwater, climate change, and biogeochemical flows, all of which are critical planetary boundaries. This is because our work shows the intimate connection of freshwater ecosystem functioning and the services lakes provide, and how biodiversity conservation depends on functioning aquatic-terrestrial linkages. Our work also shows that one should not shy away from experiments at natural system scale or from working closely with communities of practice because only through robust, active, adaptive management experiments can we create a knowledge base on which to motivate large-scale restoration action from the bottom-up.
My suspicion that fish stocking often fails was confirmed. 40,000 individually marked fish and years of work in multiple lakes were needed to provide firm answers. In response, many angler communities in Germany are shifting away from fish releases to habitat restoration, which shows that co-designing and co-testing management solutions, if they work, have immense potential to alter how ecosystems are managed locally. Our work is also an inspiration for conservation biologists and fisheries scientists to use the great power of whole ecosystem experimentation to resolve some of the largest controversies in how to manage and sustain natural resources and natural populations of animals despite heavy exploitation, and in light of rapid environmental and social change. Our findings also serve as a boost to those who have been advocating for ecosystem-based management for decades. The key to success is addressing the critical environmental bottlenecks relevant to closing the life cycle of the target organisms. Ecological knowledge is essential to make informed decisions.
Ecosystem restoration on a large scale is necessary to move ecosystem integrity back within its planetary boundary. Our work provides the scientific foundation on how to achieve this target.
Figure 2: Thomas Klefoth and a bunch of anglers in action.