Ecosystem Indicators

Noteworthy (pdf)

We include information here that is relevant to ecosystem considerations of fisheries managers, but does not fit our typical indicator format. Information included here is often new or a one-time event.

Invasive Green Crab in Southeast Alaska

Invasive European green crab Carcinus maenas, hereafter referred to as invasive green crab, were ob- served in Alaska for the first time, on Annette Island in Southeast Alaska; their presence was confirmed, July 2022. The observations were the result of early detection monitoring for the aquatic invasive species by the Metlakatla Indian Community’s Fish and Wildlife department, in anticipation of the species’ con- tinued northward expansion originating from San Francisco Bay. NOAA Fisheries, Alaska Region began supporting this effort in 2020 after adult green crabs were found in Skidegate Inlet, Haida Gwaii, British Columbia). As a new invasive species in the Gulf of Alaska, the potential impact of green crabs at population scales is still negligible, but they have the potential to disrupt local, intertidal systems.

Green crab are found in rocky nearshore and shallow seagrass habitat, primarily in protected bays and estuaries. They live 4–6 years (northeastern Pacific population) and reach a maximum carapace width of 100 mm (Young and Elliott, 2020). Their larval phase lasts up to 90 days (Ens et al., 2022). Green crab are generalist predators of shellfish, including clams and juvenile Dungeness crabs. Larger crabs (e.g., red rock crabs, Cancer spp.) are key predators of green crabs. El Nin˜o events and marine heatwave conditions facilitate range expansion of Green crabs on the eastern Pacific coast (Yamada and Gillespie, 2008), and favorable recruitment events are linked to previous warm winters, allowing for greater larval survival and reproductive success.

The primary ecological impacts of invasive green crab are through shellfish predation and eelgrass habitat disruption which alters nearshore nutrient cycling. Green crabs uproot eelgrass beds while searching for food, and consume plant roots. Eelgrass habitat in Alaska is important for juvenile groundfish, including walleye pollock (Gadus chalcogrammus), Pacific cod (Gadus macrocephalus), rock sole (Lepidopsetta bilineata), yellowfin sole (Limanda aspera), juvenile rockfish (Sebastes spp.), and juvenile salmon (Oncorhynchus spp.), and forage fish (including Pacific herring, Clupea pallasi, capelin, Mallotus villosus, surf smelt, Hypomesus pretiosus, Pacific sand lance, Ammodytes hexapterus, and Pacific sandfish, Trichodon trichodon) (as summarized in Johnson et al., 2003; Harris et al., 2012; Mundy, 2005). Grosholz et al. (2011) summarize how the Invasion of green crabs has been linked to declines in shellfish populations in eastern North America, including the soft-shell clam (Mya arenaria) fishery in New England and eastern Canada, bay mussels (Mytilus edulis), Manila clams (Venerupis philippinarum), bay scallops (Argopecten irradians), and hard-shell clams (Mercenaria mercenaria).

There are no published reports of green crab directly impacting groundfish populations. However, given their documented impacts in other regions, it will be important to monitor the direct and indirect effects on the broader marine ecosystem as they expand into the Gulf.

Contributed by Bridget Ferriss¹ and Tammy Davis²

¹Resource Ecology and Fisheries Management, Alaska Fisheries Science Center, NOAA Fisheries

²Invasive Species Program, Alaska Department of Fish and Game, Juneau, Alaska

Contact: bridget.ferriss@noaa.gov