On the rebound: removal programs yield local-scale benefits but do not sustainably suppress populations of invasive European green crabs (Carcinus maenas)

Marine invaders, including the green crab (Carcinus maenas), cause widespread disruption in their invaded ranges. In a Marine Protected Area in Atlantic Canada, the concurrent collapse of a protected strain of Irish moss (Chondrus crispus) and establishment of green crabs prompted a removal program in 2016–2018. We evaluated the outcome of that program using surveys within and outside (control) the MPA that preceded (2015 and 2016) and ran parallel to the removal program. We used generalized linear models to assess effects on catch rates and demographics and then compared those results to the outcome of other removal programs using the framework of recruitment and growth overfishing. Green crab abundance increased between 2015 and 2017 despite removal efforts, but the increase of both males and females was suppressed compared to control sites, to 29% and 36% of expected numbers, respectively. Between 2017 and 2018, males became significantly smaller relative to control sites. Although green crab eradication is not achievable, benefits would accrue if recruitment overfishing could suppress populations to levels that could be maintained by modest harvesting. Comparing among studies showed that removal did not reduce populations or that supressed populations could rebound quickly. However, within-year population reductions and shifts to smaller sizes, which are consistent with growth overfishing, show that removal programs may achieve short-term/local benefits but seem unable to sustainably suppress populations. At larger scales, the green crab's status as one of the most resilient invaders is likely to remain.