Putting together the pieces of the Fraser River seascape.
By Lia Chalifour
Estuaries in the Northeast Pacific are being lost at an unprecedented rate, and the Fraser River estuary is no exception. More than 70% of the estuarine habitat in the Fraser has been converted or altered, reducing seascape connectivity and estuarine productivity (Waldichuk 1985, Sutherland et al. 2013), with further major development proposals currently under review.
This is the context in which myself (University of Victoria PhD Candidate, Lia Chalifour), Dave Scott and Misty MacDuffee (Raincoast Conservation Foundation Biologists), Dr. Josie Iacarella (Research Scientist at the DFO Institute of Ocean Sciences), Dr. Tara Martin (UBC), and Dr. Julia Baum (UVic) spent two years studying fish in the Fraser River estuary to see how the loss of different habitats may impact them.
In 2019, we published our findings in the journal Marine Ecology Progress Series. Our work demonstrates the importance of habitat connectivity to fish biodiversity and estuarine productivity at a time of increased development pressures.
Fish that live in estuaries may move up the river, through the estuary, and back and forth into marine waters - foraging, breeding, and hiding from predators, depending on their needs. Some of them will prefer fresher water, and some prefer saltier water. Some are adapted to hide in tall eelgrass, and others lay flat and camouflage into the sand. Because of this variation within and among fish species, multiple habitat types are required to conserve overall fish biodiversity and abundance.
Focusing on a single habitat or species risks underestimating the value brought to the system by each component. In the case of the Fraser, focusing solely on habitat remediation for eelgrass and ignoring the ongoing loss of marsh could lead to further declines in salmon populations.
We tested for species-specific habitat use by sampling fishes in three interconnected estuarine habitats (brackish marsh, eelgrass, sand flat), across seasons and temperature regimes. We quantified fish species richness, community distinctness and catches (of Chinook and chum salmon, other migratory fishes, and resident fishes), in the Pacific Northwest’s Fraser River estuary.
The heavily urbanized Fraser River estuary was once the world’s most productive salmon basin.
We started this work out of concern that development in the estuary was continuing, yet research on the potential impacts of this development to the fish communities using the estuary was clearly lacking. In the 1970s and 80s, some really great work was done that showed that salmon moved through the estuary at different rates, with some potentially using the estuary to grow during early life stages (Levy and Northcote 1979 and 1982). Even during that time, biologists raised concerns about the impacts of estuarine habitat loss on these fish communities (Levings 1985, Waldichuk 1985).
Faced with multiple new development proposals for the estuary, we teamed up to investigate the fish communities still using the existing habitat, and the variations in that use within the estuary, in order to inform better management of this critically important ecosystem.
Habitat loss in the Fraser River estuary
The Fraser is a vast river system that drains roughly a quarter of BC. Beginning high in the peaks of the Rocky Mountains and working its way through BC's 16 biogeoclimatic zones to the Salish Sea and Pacific Ocean. It is the largest single contributor of freshwater to the Salish Sea, and drives primary production and nutrient cycling, currents, and temperature regimes in this system.
Juvenile salmon have been recorded feeding on terrestrial insects caught in the Fraser River plume way out in the middle of the Strait of Georgia (Barraclough and Phillips 1978). The silty plume of the Fraser can be seen clearly from space as it pushes freshwater nutrients and glacial till across the Strait of Georgia and laps against the shores of the Gulf Islands.
The Fraser River estuary has had a long history of development since Europeans arrived, including dyking and filling in areas of marsh and wetlands to build houses and farm the land, forcing the river to take a unified path to create a deeper channel for navigation, and building long jetties through the estuary to the drop-off at the Salish Sea to facilitate large ferries and cargo ships docking and connecting to the mainland. All of these changes, in addition to invasive species, rising and warming waters, and pollution, have impacts on the fish communities living in the nearshore environment.
I have heard that Captain Cook missed the Fraser River when he was looking for a passage inland, because the channels were so small and meandering through the marshlands that it did not appear to be passable.
Today, the Fraser has been dyked and channeled into largely two routes: the South (main) arm and the North arm, and is home to Canada’s most active port with very large cargo carriers passing through daily. We were dwarfed by these massive ships in our research vessel, and can only imagine what it must be like sharing the waters with them as a small fish. Forcing much of the flow of the Fraser through the South arm via dykes and jetties is meant to facilitate pulling the glacial till out to sea and maintain deeper passageways, but there is so much silt that travels down the river that regular dredging is a must to allow for these ships to travel inland. As a juvenile salmon or small fish, the currents from the river in these major channels are formidable. Still, we managed to find thousands of fish breaking away from these river highways and using the backwater habitats provided by the marsh, eelgrass beds, and shallow sand flats across the estuary.
Eelgrass, marsh, sand flats, & connectivity
We found that no matter the season or the year, eelgrass tended to support about three quarters of the estuary’s fish. Similarly, two thirds of the Fraser River salmon were caught in marsh habitat, and the Chinook that rear in the estuary particularly favoured the marsh. No one habitat explained the full fish community.
All habitats are important, and their connectivity is crucial for a functioning ecosystem.
Despite many empty nets, we did make some very important catches in the other habitats. Marsh in particular, consistently supported the greatest catches of Pacific salmon species. Juvenile Chinook that enter the estuary as fry are probably more comfortable in the less saline, more turbid and calm waters of the marsh - they provide shelter during a time when these small fish need to hide from predators and grow, before they are able to transition into their marine-ready form [smolts]. Wild salmon throughout BC have experienced overall declines in recent years (BC Wild Salmon Advisory Council 2019), and Chinook have been one of the hardest-hit species. These are also the primary prey of the endemic Southern Resident Killer Whale population, which have consequently been assessed as Endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC).
Overall, eelgrass habitat supported the greatest fish species richness (n = 37) and catches (37,402 fish), exceeding that of either the marsh (19 species, 7,154 fish) or sand flat (22 species, 6,697 fish). However, the majority of salmon were caught in the marsh (61%). This supports the premise that connected seascapes of different habitat types maintain greater biodiversity and productivity, and we suggest that estuaries be managed as such.
This Euler diagram shows the distribution of fish biodiversity in each habitat over two years of surveys in the Fraser River estuary. Eelgrass supported 15 unique species and 22 shared species, marsh had 8 unique species and 11 shared, and sand flat had 1 unique species and 21 shared among habitats. Fish species that were only caught in the marsh included Coho salmon, and the highest catch incidences for salmon were in the marsh.
Project partners
Thank you to our project partners Pacific Institute for Climate Solutions, Marine Environmental Observation Prediction & Response Network, Pacific Salmon Foundation, and Raincoast Conservation Foundation.
References
Levy DA, Northcote TG, Birch GJ (1979) Juvenile salmon utilization of tidal channels in the Fraser River estuary, British Columbia, no. 23, 0315-3029. Westwater Research Centre, Vancouver
Levy DA, Northcote TG (1982) Juvenile salmon residency in a marsh area of the Fraser River estuary. Can J Fish Aquat Sci 39:270–276
Levings CD (1985) Juvenile salmonid use of habitats altered by a coal port in the Fraser River estuary, British Columbia. Mar Pollut Bull 16:248–254
Waldichuk M (1985) Robert’s Bank coal port, British Columbia: how did the environmental impact assessment fare? Mar Pollut Bull 16:179–185
Barraclough WE, Phillips AC (1978) Distribution of juvenile salmon in the southern Strait of Georgia during the period April to July 1966-1969. Fish Mar Serv Tech Rep 826: 47 pp
BC Wild Salmon Advisory Council (2019) Recommendations for a made-in-B.C. wild salmon strategy. Victoria, BC
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