The European green crab is spreading rapidly along the Pacific Coast, threatening ecosystems and aquaculture. In this webinar, three experts will share new research on how we can better understand and manage this invasive species.

• Abby Keller (UC Berkeley) will explore how decision theory can guide when to try to remove green crabs—or when to adapt to their presence—as populations grow beyond control.
• Mary Fisher (UC Davis) will present research from Willapa Bay, WA, using DNA metabarcoding tools to uncover what green crabs are eating and how their diet differs between aquaculture sites and natural habitats.
• Rikke Jeppesen (Central Coast Regional Water Quality Control Board) will share findings on how native sea otters may help keep green crab populations in check in California estuaries.

This webinar is a great opportunity for anyone interested in invasive species, coastal ecosystems, and science-based management. This webinar is being jointly hosted by NAISMA and Sea Grant.

The transition from resistance to acceptance: managing a marine invasive species in a changing world
Presented by Abby Keller
Often the pace of European green crab spread and establishment outpaces resources available for population suppression. When should a manager “resist” change by removing green crab or “accept” change by investing resources in adaptation? We use concepts from decision theory to develop a framework for navigating management decisions in this resist-accept paradigm, highlighting scenarios when population control can no longer shape the invasion trajectory.

What can diet DNA tell us about predation impacts of an invasive marine species?
Presented by Mary Fisher
The invasive European green crab (Carcinus maenas) has recently increased in abundance and expanded its distribution in the US Pacific Northwest, generating concern for estuarine ecosystems and aquaculture production. However, regionally-specific information on the trophic impacts of invasive green crab is very limited. We used diet DNA metabarcoding (“diet DNA”) to compare the stomach contents of green crabs collected on clam aquaculture beds versus intertidal sloughs in Willapa Bay, Washington, providing the first in-depth description of European green crab diet at a crucial time for regional management. We first identified 54 putative prey items using DNA metabarcoding of stomach content samples from 61 green crabs. Arthropods were the most frequently detected prey, with the native hairy shore crab (Hemigrapsus oregonensis) the single most common prey item. We then compared diet composition across sites using prey presence/absence and an index of species-specific relative abundance. We found that the stomach contents of crabs collected from clam aquaculture beds were significantly different from the stomach contents of crabs collected at intertidal sloughs. For eight prey species, we also calibrated diet DNA data to quantitatively compare DNA abundance between prey taxa, to describe an ‘average’ green crab diet at an intertidal slough versus a clam aquaculture bed. In addition to providing timely information on green crab diet, our research demonstrates the novel application of a recently developed model for more quantitative DNA metabarcoding. This represents another step in the ongoing evolution of DNA-based diet analysis towards producing the quantitative data necessary for modeling invasive species impacts.
Associated paper (open access): Fisher et al. 2024. Invasive European green crab (Carcinus maenas) predation in a Washington State estuary revealed with DNA metabarcoding.https://doi.org/10.1371/journal.pone.0302518

Recovering population of the southern sea otter suppresses the green crab, a global marine invader
Presented by Rikke Jeppesen
Understanding the role of apex predators on ecosystems is essential for designing effective conservation strategies. Supporting recovery of apex predators can have many benefits; one that has been rarely examined is control of invasive prey. We investigated whether a recovering apex predator, the southern sea otter (Enhydra lutris nereis), can exert local control over a global marine invader, the green crab (Carcinus maenas). We determined that southern sea otters in Elkhorn Slough estuary in California can consume large numbers of invasive green crabs and found strong negative relationships in space and time between otter and green crab abundance. Green crabs persisted at highest abundance in this estuary at sites with artificial tidal restriction that were not accessible to otters. Green crab abundance remained lower in this estuary than in all other estuaries in the region, which lack resident sea otters. Conservation organizations and agencies have invested heavily in recovery of southern sea otters, increasing their numbers in this estuary. Restoration of natural tidal exchange, lost marshes, and seagrass beds further support sea otter populations. We have demonstrated that these investments in top predator recovery and habitat restoration have reduced the impacts of a global invader. Our investigation highlights that investment in recovery of top predators can increase beneficial food web interactions and resilience of the entire ecosystem.

Speakers:

Abby Keller is a PhD candidate in Environmental Science, Policy, and Management at the University of California, Berkeley. She studies decision analysis and quantitative ecology.

Mary Fisher is a postdoctural scholar at the Coastal and Marine Sciences Institute at the University of California, Davis. She is an interdisciplinary marine scientist interested in how adaptation to climate change affects coupled human-natural systems. Her research has drawn on quantitative and qualitative techniques to explore trade-offs and unexpected consequences associated with climate adaptation in US West Coast fisheries.

Rikke Jeppesen has a PhD in Ecology and Evolutionary Biology from University of California Santa Cruz. She is currently an Environmental Scientist at the Central Coast Regional Water Quality Control Board, San Luis Obispo. Her green crab studies were conducted at Elkhorn Slough National Estuarine Research Reserve where she studied invasive invertebrates, while also working on water quality research and large-scale salt marsh restoration projects.