We arrived at our first site following a bone-shaking and bumpy trip down a logging road to Tarundl Creek. Already, we were greeted by a Haida Gwaii black bear a hundred yards away, clearly disinterested. The stench of the chum salmon (Oncorhynchus keta) was immediate and unforgiving, and it only took a few minutes of walking towards the river to find a partially decayed carcass amidst the sword ferns. The chum salmon were spawning from late September to early October, and the bears wasted no time in embracing this pulse subsidy of nutrients. They often catch a fish, drag it to shore, take a bite (typically from the brain, ovaries, or dorsal muscles), and move on. Bears have been found to account for up to 70% of salmon transfer in these systems and catch on average 13 salmon each day, or 74% of all salmon returning to the stream. In our fifty-meter riverside transect that stretched twenty meters into the forest, we easily found 41 salmon carcasses in various stages of decay. We flagged them, set up aerial and terrestrial insect traps, and spent the next three weeks returning to the site to assess their states. In the following weeks, we flagged a total of 86 fish carcasses in our short transect.
Our flagged salmon transformed quickly. The fish we initially deemed as “freshies” rapidly lost their titles. The gills of the salmon were an ideal home for maggot eggs, which burst and colonize the body in days. Beetles including the bright orange-backed Nicrophorus investigator could be spotted once you flip over the salmon. The smell was not your first clue of a nearby carcass – the flies were. Studies have found up to sixty terrestrial invertebrate species on decaying salmon in British Columbia, including parasitic wasps that predate on the larvae that feed directly on the carcasses! In the damp environment of the riparian system, the salmon arrived and were consumed quickly. The nutrient input of the salmon on these systems can be assessed through the tracking of Nitrogen-15 isotopes, but we instead took a look at the benthic invertebrate communities relative to other points in the stream. Our benthic analysis identified an abundance of Ephemeroptera, which act as scrapers in the streambed. This brought up the question of whether the riparian or aquatic systems that salmon visit are donor-controlled by the rich influx of nutrients they bring each year. While nutrients from salmon may increase the growth rate and abundance of flora, there appears to be no change in species diversity in the riparian area. That said, when looking at the piles of dozens of salmon carcasses that become entrained in rivers by the end of the spawning season, it is evident that the aquatic nutrient influx is major.
Sample of decayed salmon by river
In a typical river system, the initial external (“allochtonous”) nutrient input is often litterfall, a source of organic matter that is broken down by shredders and becomes entrained in the system. Unlike most fish, salmon are upstream vectors. They use olfactory senses to return to their natal streams, spawn, and quickly die afterwards, providing an input of coarse particulate matter and nutrients to the upstream system. While nearly all Pacific salmon are anadromous (meaning they return upstream to spawn) and semelparous (meaning they only reproduce once), there is immense variation in behaviour amongst species and subspecies. The five Pacific salmon species have thousands of populations that can exhibit phenotypic and behavioural differences! The marine residence time of salmon may vary from 18 months in the case of Pink Salmon (Oncorhynchus gorbuscha) to up to 6 years for Coho Salmon (Oncorhynchus kisutch) and Chinook Salmon (Oncorhynchus tshawytscha). The movement and success of each year’s stock is heavily influenced by the coldwater nutrient upwelling associated with strong Aleutian low-pressure systems. In Canada, populations of salmon are organized into Conservation Units under the Wild Salmon Policy to help identify their conservation status.
On numerous instances in our field excursions, we encountered the staff and volunteers of a local salmon hatchery, the Hecate Strait Streamkeepers Society. The Streamkeepers embody one of the many efforts occurring in British Columbia to preserve and restore crucial salmon runs. A day spent volunteering with the Streamkeepers in early October entailed trapping and collecting female coho returning to spawn. This can be accomplished by setting up a tangle net across the banks, and slapping the water with large sticks in the direction of the net. This frightens the fish and directs their movements toward the net. The eggs are harvested from these females, meticulously raised in hatcheries, and later released.
Streamkeepers in the river catching salmon
Logging, road construction, and associated culverts have had a profound impact on the geomorphology of streams and the movement capability of salmon. In response to this, the concept of the Riparian Fish Forest project was formed by John Broadhead. A series of maps was created, documenting the presence of salmon amidst the thousands of lakes and rivers in Haida Gwaii, logging history, and riparian habitats. This included 1782 fish surveys, identifying 14 resident salmon species, and over 15 000 geographic barriers to fish movement. This project extensively contributed to the protection of riparian salmon habitats across Haida Gwaii.
We returned to the study site at Tarundl four times over three weeks to watch the journey of the decaying salmon. Between these assessments, we explored new marine-terrestrial interfaces (MTIs) across the archipelago to observe other ecosystem interactions with the salmon subsidy. There are 150 known predators of salmon, and one of the most abundant families are the gulls. The Deena River is home to the largest salmon run in the Skidegate inlet. Here, we observed hundreds of gulls (Larus spp.) of numerous species amidst dozens of bald eagles (Haliaeetus leucocephalus) on the estuary banks. The gulls congregate in masses during their fall migration and have consumed up to 26% of all the salmon carcass biomass and 19% of eggs in studied systems. The opportunistic consumption of large amounts of salmon by gulls have been identified in some areas with limited salmon recovery as a threat. In California where salmonid populations are on a decline and gull populations are steadily increasing, predation rates of gulls for steelhead salmon are only 30% but critically endanger salmon as a result of habitat degradation. In this case, gull management strategies are a component of salmon recovery. Nonetheless, gulls act as an important biological vector for nutrient transfer. These gulls uptake this subsidy of nutrients and redeposit it into the terrestrial system through guano, with thousands of kilograms of nutrient-rich feces dispersed in the weeks following salmon spawning.
While we estimated gull populations amidst the hundreds of white birds in flight, a black bear emerged from the treeline. For ten minutes, we watched the bear lumber through the stream and lazily catch salmon in all their abundance. The carcasses at our feet, the salmon splashing in the water, the blinding upheaval of gull flocks into the air, the bald eagles atop the trees, and the bear swiping at the stream painted the perfect picture of salmon in the MTI.
Article written by Hashveenah Manoharan
“I would like to acknowledge and extend my deepest gratitude to the Haida and all other residents of Haida Gwaii, the Haida Gwaii Institute and Dr. Scott Wallace for welcoming me and enabling this experience”
Spawning river