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How Sea-Run Fish Connect Ecosystems

Our conceptual model investigates the ecological role of sea-run fish and the connections they make among different ecosystems.

New England/Mid-Atlantic

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A color image taken underwater showing multiple sea run fish species making their way upstream.
Atlantic salmon and river herrings in the Milford dam fish passage on the Penobscot River in Maine. Credit: Penobscot River Restoration Trust/Cheryl Daigle

Sea-run fish migrate between freshwater, estuarine, and marine habitats during their life cycle. As they migrate, these fishes provide important ecological services and connect ecosystems. Most of these species are born in rivers, use freshwater nurseries before they migrate to sea, and return to the river to reproduce. The exception is the American eel which is born at sea, migrates in rivers to grow, and returns to sea for its reproduction.

The abundance of these fish in the Northwest Atlantic is at, or near, all-time lows for most species. A number of stressors have led to the decline: dams, overfishing, pollution, and climate change. With fewer of these fish, researchers believe that the ecosystem services these species once provided have substantially diminished.

For example, at historic abundance levels alewives provided an important source of energy-rich forage for nearshore fish species in the spring. This lost connection may be partially responsible for the current low abundance of other commercial species in nearshore areas in the Gulf of Maine. Other examples of impaired ecosystem services include:

  • Transport of marine-derived nutrients to freshwater ecosystems by species such as Atlantic salmon
  • Removal of fine sediment and higher abundance of aquatic insects in river habitats with sea lamprey spawning
  • Predator-prey roles in freshwater systems by species such as rainbow smelt

To repair these connections, we urgently need to look at sea-run fish populations as a community and consider the ecosystems they inhabit altogether. We need to consider their combined interactions and the important roles they play in the healthy functioning across the varied ecosystems (freshwater, estuarine, or marine ecosystems) they inhabit. They also face similar challenges that can be solved simultaneously. This would improve our large-scale approach to research and management, and provide information to better communicate the important roles of these species.

Our goal is to improve ecosystem services delivery and ecosystem resilience to increase the productivity of the sea-run fish community.

The Complex Ecology of Sea-Run Fishes

This illustration depicts a conceptual watershed and coastal region to show how fish migration pathways make important connections between marine habitats through ecosystem services. There are mountains on the left with freshwater lakes and rivers that flow down to an estuary at the center and then to the high seas on the right. The flowing waters are color-coded from light to dark blue to indicate lakes, upper rivers, middle rivers, lower rivers, coastal rivers, and estuary waters.
The Diadromous Watershed-Ocean Continuum (DWOC) is a visual representation of how sea-run fish provide essential ecosystem connections through ecosystem services. Credit: NOAA Fisheries

Our conceptual model, the Diadromous Watersheds-Ocean Continuum (DWOC) synthesizes the key roles of sea-run species as a community to highlight their importance for ecosystems and how they connect them by delivering important ecosystem services. It provides a framework to work collaboratively to make progress toward ecosystem-based management for sea-run fishes. These connections vary in space and time via the migrations paths and habitats occupied by diadromous fishes along the watershed-ocean continuum: from lakes and upper river areas to coastal, estuaries and high sea areas, across regional and international borders.

The Northwest Atlantic coast hosts a diverse sea-run community. We’re focusing on:

Study Methods

We use ecosystem services to summarize the information on the diadromous community. Ecosystem services are separated into three categories: cultural, maintenance and regulatory, and provisioning.

Cultural

A color image. Kids use long-handled fish mesh nets to capture alewifes migrating through a fish passage on a stone dam. It is summer and sunny. The water is running fast and is very foamy.
Kids learning about alewife migration at a fish passage. Credit: NOAA Fisheries/Sarah Bailey

Cultural services are non-material benefits people obtain from nature. These include:

  • Spiritual experiences
  • Recreation, including recreational fisheries
  • Aesthetic enjoyment
  • Physical and mental health
  • Scientific data
  • Educational opportunities

Maintenance and Regulation

Image
A color image. A snake-like, dark colored sea lamprey on a rocky ocean bottom.
Habitat conditioning - a sea lamprey moving rocks to build its nest. Credit: Audrey Turcotte/Boston College.

Maintenance and regulation services regulate the ecological balance and provide the basis for other services by:

  • Habitat formation and conditioning for different species
  • Retaining biodiversity
  • Cycling nutrients
  • Predatory interactions and food web control

Provisioning

A color image. Whole and fileted salmon are displayed for sale on a stainless steel counter in light colored plastic bins, some with a covering of crushed ice.
A salmon market in Greenland. Credit: NOAA Fisheries/Timothy Sheehan

Provision services include:

  • Subsistence and commercial fisheries harvests
  • Raw materials to produce medicines, biofuels, pharmaceuticals, and other goods

Using an ecosystem services approach allows us to use simple language to describe individual or cumulative ecosystem benefits and ecological connections these species provide. Ecosystem services delivered by fish are influenced by the fish’s seasonal abundance, their migration patterns and timing, and the location of the habitats they use along the diadromous watersheds-ocean continuum, which ranges from freshwater to estuarine and marine environments. Through these ecosystem services, sea-run fish also connect terrestrial and aerial ecosystems.

Preliminary Results

Identifying Research Gaps

Image
A large snake-like fish curled up in a fine mesh sampling net.
For many species, like the American eel shown here, we need to improve the knowledge on the ecosystem services they deliver. Credit: NOAA Fisheries/Valerie Ouellet

Approaching the sea-run fish community through DWOC provides insight on research gaps we need to address to support management priorities that will increase sea-run fish productivity, ecosystem resilience, and ecosystem services delivery. These research gaps include:

  • Better characterization of the ecosystem services by different species (for example, little is known about the roles of rainbow smelt or American eel)
  • A better understanding of species interactions within and outside the sea-run fish community, such as predation or protection from predation
  • A deeper understanding of the effect of climate change and other human activities on habitats

A Scientific Framework for Ecosystem-Based Fisheries Management Approaches

We have highlighted the need for more interdisciplinary research, spanning from headwaters to the ocean and including biology, physiology, geomorphology, climate science, and socio-economics. This way, the ecosystems’ interdependence and the full life cycle of the species can be properly considered.

A Path Forward

Using DWOC, we developed a step-by-step approach to progress toward ecosystem-based management for the diadromous community. This approach takes into account social connections between people and ecosystems that influence sentiments toward sea-run fish. Our framework steps are:

  • Step 1: Examine what factors affect the spatial structure of the sea-run fish community across all life stages, population abundances, and population and community diversity to achieve our principal goal: to improve ecosystem services delivery, ecosystem resilience,and increase diadromous community productivity.
  • Step 2: Evaluate the roots of the problem and explore a multi-solution approach while researching the ramifications of the different solutions along the ecosystem continuum.
  • Step 3: Define management objectives, with quantitative targets of diadromous fish abundance, diversity, and distribution to support ecosystem services delivery.
  • Step 4: Define management actions coordinated across species, life-cycle stages, and ecosystems, encompassing specific governance boundaries to increase the productivity of all diadromous fishes

In parallel an adaptive management approach needs to be put in place to allow re-evaluation of the root problems, adjustment to management priorities and actions, and monitoring as progress is made.

Keys to Success

Clear and transparent communication between all parties involved is key during this entire process. Another key element is to understand the connections between people and sea-run fish in the affected area so we can estimate the expected level of social and community support for management actions. Finally, while planning management actions we need to consider the fact that habitats have been transformed and continue to be transformed due to human activities, which also impact fish species. Additionally, climate change affects all aspects of the sea-run fish community and habitats across spatial scales.

It can be challenging to work at the scale of multiple ecosystems. Our approach shows how advances can be made at smaller scales and on local issues by starting on a smaller suite of ecosystem services for instance, while embedding these steps into that large-scale holistic understanding of the diadromous communities and ecosystem connections. This also to incrementally make progress toward ecosystem-based management of the sea-run fish community.

Study Team

Principal Investigator

Project Team

References

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Last updated by Northeast Fisheries Science Center on 03/04/2025