Dennis Ensing, Agri-Food and Biosciences Institute, and Hugo Maxwell, Marine Institute, are co-chairs of ICES Working Group on Science to Support Conservation, Restoration and Management of Diadromous Species (WGDIAD). In this feature, they present the challenges faced by diadromous fish species.
Some of the world's most iconic fish species are diadromous fish; the Atlantic salmon, the Beluga sturgeon, and the European eel. Diadromous fish need to migrate between freshwater and saltwater to complete their lifecycle. Either they have to move into freshwater from saltwater to spawn, or the reverse, moving to the sea after living in freshwater to produce the next generation. Unfortunately, these are also some of the most endangered fish species in the world. Despite efforts, including those by ICES scientists, the conservation status of many of these species in the North Atlantic is not improving.
What issues do these species face, and what new paths can the scientific community take to try to improve the current situation?
The conservation status of most diadromous fish species is not good, to say the least. Some species, such as the Chinese paddlefish, are now extinct. Others, like the European sturgeon, are critically endangered. Even diadromous fish species that are still being fished are doing poorly. Many local populations have been extirpated (locally extinct), such as the Atlantic salmon populations of the River Thames which disappeared in the 1830s. Remaining populations have seen numbers fall, often below safe biological limits. The number of juvenile European eels entering Lake IJssel in the Netherlands has dropped to 1% of what it was in 1980. Atlantic salmon also show marked declines in numbers today relative to 1980s, with scientists in ICES network estimating the number of adult salmon in the Atlantic Ocean in 2019 at around three million, compared to almost eight million in the mid-1980s.
What causes these dramatic declines?
There are many factors at play, but the answer is simple: the impact of humans on the habitats these fish live in. What aggravates the issue is that diadromous fish, in contrast to non-diadromous species, need to use a wide range of different habitats to successfully complete their lifecycle from egg to spawning.
Take the spawning migration European eels make to their place of birth in the Sargasso Sea, more than 5, 000 kilometres from the European rivers and lakes where they have lived for most of their lives. As they begin their journey in the river's upper reaches, the eels have to overcome obstacles like weirs and hydroelectric dams where many will be crushed in the hydroelectric turbines. As they continue, they will have to contend with navigation locks for shipping, water pollution from industry and agriculture, and the fishermen who set their nets to catch eels.
In the lower stretches of main rivers, the problems only increase with water removal for human use, the channelization of the riverbed, and other alterations to the natural flow of rivers. Even entering the sea is a precarious undertaking for diadromous fish. At this vulnerable stage in their journey, where the environment changes from freshwater to saltwater and the temperature from warm to cold, the fish must adapt their bodies and any disturbance of this process can weaken and disorientate fish, making them easy prey for predators.
Once at sea, the problems are not over. Here, many eels end up in fishermen's nets. And the hardships of the freshwater migration often leave invisible scars: organ damage from a turbine blade strike or pollution, depleted energy reserves due to the navigating the multitude of obstacles. All contributing towards reduced chances of survival. While all diadromous fish do not make migrations as long as the European eel, one thing they have in common is having to move through those dangerous estuaries.
And let's not forget one of the biggest and most persistent current threats to the survival of any fish species, climate change. It can affect the ocean lifecycle of diadromous fish by changing ocean currents, water temperatures, food availability, and other factors these fish have adapted to over thousands of years. When these elements are disrupted by climate change the number of diadromous fish dying at sea will increase.
It's not all bad news
There are positive diadromous fish stories. Many populations of diadromous fish might not be doing well, but they are hanging on. They are adapting to new conditions, but natural selection always comes at the cost of higher mortality.
Some species are moving into new habitats. Previously polluted rivers have seen the spontaneous return of diadromous fish after improvements to water quality and habitat, like the Mersey in England.
Rather than addressing single issues, in Denmark, an ecosystem-based approach to diadromous fish conservation is taken. Rivers that looked like canals have been completely rewilded and now flow like natural rivers, estuaries have had extensive wetlands reintroduced, dams have been completely removed where possible, pollution reduced, and spawning habitats restored. One of the most striking results is a complete revival of the Atlantic salmon population in these rivers. Once reduced to just double figures of returning adults, they now annually see more than ten times that number appear. These projects did not come cheap, but it shows it can be done. It is quite simple: a healthy river will support healthy fish populations. And not just fish, but all other plants and animals that share these habitats. It's the “Field of Dreams Hypothesis": If you build it, they will come.
International Year of the Salmon
Outside the North Atlantic, the North Pacific Ocean also has a wealth of diadromous fish species. It will not come as a surprise that these Pacific species are equally struggling with the effects of climate change and human impacts on their freshwater and saltwater habitats.
This was one of the conclusions from the recent International Year of the Salmon (IYS), an initiative to bring people together to share and develop knowledge more effectively, raise awareness, and take action in support of all salmon species. Scientists working under this banner found that the different species they were researching on opposite sides of the globe showed remarkable similarities in declining numbers, growth problems in the ocean, and the mass mortality of salmon at sea. Surely this must be caused by the same globally acting processes. And if so, similar solutions might also apply.
The IYS has left a lasting legacy among salmon scientists: the realisation that more collaboration between the Pacific and Atlantic is needed, on all diadromous fish species. We need to learn from each other's methods in estimating the numbers of fish, to find better ways to link our large biological and oceanic data sets to look for similarities and differences in the reaction of fish populations to changing environment. We need to better understand the changing Pacific and Atlantic ecosystems that diadromous fish live in, need to forge permanent links, in form of a scientific network, between the scientists from these two ocean basins.
From the Pacific, scientists from the North Pacific Anadromous Fish Commission (NPAFC) have teamed up with their Atlantic colleagues in ICES Working Group on Science to Support Conservation, Restoration and Management of Diadromous Species (WGDIAD) to establish such a network of scientists from the Pacific and the Atlantic working on diadromous fish. Because what applies to fish applies to fisheries scientists too: If you build it, they will come.
The Working Group on Science to Support Conservation, Restoration and Management of Diadromous Species (WGDIAD) provides an international forum for the coordination of work on diadromous species. Their work addresses many of ICES scientific priorities - Ecosystem science, Impacts of human activities, Observation and exploration.
Discover all seven interrelated scientific priorities and how our network will address them in our Science Plan: “Marine ecosystem and sustainability science for the 2020s and beyond".
Leaping salmon. Image: Natural History Photo Agency.