The recovery of sea otter (Enyhdra lutris) populations near Canada’s Vancouver Island is a conservation success story – but coastal communities and fisheries are feeling the pinch as they must now compete with them for their favourite shellfish.
In the long-term, however, scientists have calculated that the otters’ impact on ecosystems could bring substantially greater financial rewards, offering a modelling framework to evaluate long-term ecological changes driven by such apex predators.
They report in the journal Science that benefits of sea otter recovery – healthier kelp forests, greater carbon storage, higher fish catches and ecotourism – could exceed more than C$50 million per year if managed well.
This could outweigh the losses to crab, clam and urchin fisheries by as much as $46 million annually despite their short-term pain, says lead author Edward Gregr from the University of British Columbia.
The good news will hopefully save the otters yet again.
“Since their re-introduction to parts of the Pacific Northwest in the early 1970s, sea otter populations have been expanding at a rate as high as 18% a year,” Gregr says.
“Over the years, this has led to increasing economic impacts for coastal invertebrate fisheries … and increasing animosity from fishers. Otters were starting to be called rats of the sea, and there was likely no small amount of illegal killing to keep them away from lucrative fishing grounds.”
The researchers recognised an opportunity to conduct a natural experiment and compare ecosystems with and without the endearing marine mammals, which had been hunted to near extinction for their fur.
They combined field work, trophic dynamics and economic data in a social-economic model and predicted the value of the four diverse ecosystem services with otter populations at full capacity – estimated at 5000 critters – or no otters.
Results showed that sea otters could generate 37% greater ecosystem biomass per year through recovered kelp forests, which sea urchins had virtually mowed down.
Restored habitat for finfish, such as rockfish, greenlings and salmon, would increase their value by $9.4 million. Increased carbon sequestration – which reduces ocean acidification – was valued at $2.2 million and ecotourism by $42 million.
This well exceeds the $7.3 million loss predicted through loss of geoduck clam, crab and urchin fisheries, which could be compensated by the gains. The researchers suggest this loss could also, to some degree, be mitigated in other ways.
For instance, highly valued seafood such as geoduck clam and Dungeness crab could attract higher prices as supply outpaces demand. The crabs also have habitats beyond the reach of otters, so could be sourced in deeper waters.
The model’s estimates don’t factor in broader benefits of kelp forests for biodiversity and ecosystem resilience, and other species that could help fisheries diversify.
While the study doesn’t offer solutions, it does provide a framework for stakeholders to manage otter conservation with everyone’s interests at heart.
“The immediate value of the results could contribute to broadening the understanding of fishers, communities and managers about the range of changes,” says Gregr. “It’s not just otters vs shellfish.”
Noting that costs and benefits will be shared unequally among established commercial activities and will cause inequities between communities and cultures, he hopes their integrated model will improve mutual understanding.
In the longer term, the model is “transformational”, according to a related commentary by James Estes and Lilian Carswell from the University of California, US, with the framework it offers for evaluating costs and benefits of apex predators on other ecosystems, a common source of confusion and conflict.
“The analysis by Gregre et al. should spark a new era of ecological-economic research that can be used by natural resource policy-makers and managers to make and defend more rational, equitable and far-sighted decisions affecting predators.”
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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