Relationship between ocean ecosystem indicators and year class strength of the invasive European green crab (Carcinus maenas)

Abstract The annual abundance of the non-native European green crab, Carcinus maenas, in Oregon estuaries varies greatly with ocean conditions. Average numbers were high following the 1997–1998 El Nino, decreased and remained low ( 2 per trap) following the extended anomalous warming in 2014–2016. The year class strength of young crabs is strongly linked to ocean indicators during their planktonic larval development. Many of the same physical and biological ecosystem indicators used in salmon forecasting are also correlated with green crabs, but in the opposite direction. While cold ocean conditions benefit salmon, warm ocean indicators are positively linked to green crab year class strength. Among the best indicators for green crab year class strength are winter water temperatures, the sign of the Pacific Decadal Oscillation index, the day of physical and biological spring transitions, and negative biomass anomalies of northern copepods. These correlations suggest that green crabs need (1) warm winters (temperature > 10 °C), which enable larvae to complete their development in the near-shore, (2) strong northward flow of coastal waters during winter, which allows larvae to be transported from established populations to the south and (3) coastal circulation patterns that keep larvae close to shore, where they can be carried by wind and tidal currents into estuaries to settle. By using a relatively simple stoplight approach of ranking indicators, we were able to explain 69% of the inter-annual variability in green crab year class strength, while a quantitative metric of a combination of indicators explained 64% of the variability. Recruitment in 2018 and 2019 exceeded what was expected from the suite of ocean indicators. We discuss the possible role of additional larval sources, from the north or from local estuaries, that may have contributed to the increased recruitment during these years. If breeding populations of green crabs in Oregon and Washington continue to build, the relationships between ocean conditions and recruitment we have developed based solely on larval sources from the south could be greatly underestimating recruitment in the future.