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Clam recruitment in Haíɫzaqv Territory is in decline and the invasive green crab may be to blame

The entrance to Gale Creek was impacted by an oil spill in 2016. Oil spill impacts, paired with the European green crab invasion, may be contributing to clam population impacts.

In 2016, the Nathan E. Stewart oil spill led to the release of over 100,000 litres of marine diesel oil and other petroleum products into the entrance of Gale Creek. Gale was home to thriving bivalve populations, important commercially, ecologically, and a source of food and income for the community of Bella Bella and the Haíɫzaqv Nation. But since the spill, we've seen fewer juveniles clams, and adult clams of lower health and quality.

My PhD project initially started as an exploration of the potential ongoing impacts of the oil spill on clams in the area (and contrasting this with clam populations from areas outside of the spill zone). But it took a turn when it seemed that a new threat was on the horizon.

The spill coincided with invasion of the European green crab, Carcinus maenas, an invasive species that was introduced to the Central Coast of British Columbia sometime in the past 15 years. It wasn't until around the time of the spill that the crabs took hold of the beaches across Gale creek.

The European green crab, Carcinus maenas, wreaks havoc on intertidal and subtidal ecosystems, owing to it's lack of natural predators on the west coast of Canada.

Green crabs are voracious predators, eating everything in sight, including their own kind. They also reproduce quickly and in high densities. This doesn't bode well for animals that are generally stationary and that take longer to reach maturity, like intertidal bivalves.

I had the opportunity to head to Bella Bella to investigate the extent that the green crabs are targeting manila clam populations. Manila clams, Ruditapes philippinarum, are an important commercial species and abundant across the west coast. And because they are a commercial species, that means we can source clam seed (cultured clams averaging 5-15 mm, dependent on the species) super easily for this study.

The manila clam are abundant along the west coast and are important commercially.

Open cages to allow green crab in.

But first, we had to deploy cages into the intertidal environment (the zone that is exposed to air at low tide, and covered by water at high tide) where manila clam like to hang out. To test the effects of the green crab on manila clam, we constructed two cage types: open and closed. The open ones had openings on the sides that would allow the green crab to enter and exit at their leisure. But the closed ones were fully closed, preventing any green crabs from getting in.

We then seeded the cages with manila clam seed; these were small juveniles ranging between 3 and 20 mm (averaging about 9.5 mm). And then we waited to see what happened!

Closed cages to prevent green crab predation.

After 24 hours, a group of us came back out to see what happened to the clams. One of the four quadrants was excavated, sieved using fine mesh sieves and brought back to the lab where we counted the number of clams and measured them. And we knew just how many clams went into the cages so we could easily know how many remained.

What we saw was convincing - the green crabs were coming in and reducing clam populations. After 24 hours, we managed to recover, on average, 66% of the clams we put into the closed cages, and only 12% from the opened cages! After 17 days, we recovered 54% from the closed cages, and just 4% of the clams from the opened cages.

And it wasn't just abundances that were being affected - mean clam lengths were declining following green crab predation, meaning that the crabs were likely targeting the larger clams first.

Aerial view of the 24 cages (12 opened, 12 closed) within the intertidal zone.

We're seeing some substantial effects in short duration on the manila clam juvenile populations. This summer and fall, we'll be looking into size class distributions (densities of adult and juvenile clams) at beaches with and without green crabs, to see whether or not there is this missing size class among uncaged populations. And if this missing size class exists, it doesn't bode well for the manila clam populations of the future. This will be key information needed to better focus restoration efforts and in the fight against the green crab invasion.

Only time (and monitoring) will tell just how much the green crab will affect manila (and native) clam populations. But for now, we'll continue investigating how the green crabs are impacting clam populations and see about stopping the invasion in its tracks.

Written by Tyler Black.

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