Dramatic decline and limited recovery of a green crab (Carcinus maenas) population in the Minas Basin, Canada after the summer of 2013

Abstract

This paper reports the results of a ten-year monitoring program of an Atlantic Canadian population of green crabs, Carcinus maenas, in the Minas Basin of the Bay of Fundy. Intertidal densities, sex and reproductive ratios, juvenile recruitment, subtidal catch-per-unit-effort (CPUE), and sizes of crabs in this population were recorded from 2008 to 2017. In 2013 intertidal densities, mean crab sizes, subtidal CPUE, and proportions of crabs mature and reproducing all dramatically decreased to all-time lows, and large crabs virtually disappeared from the population. From 2014 to 2017 the population partially recovered but remained in an altered state. Potential causes of interannual changes to this population were investigated by correlating intertidal densities to 67 seasonal environmental variables. Crab densities in a given year were best explained by potential settlement, mean and variability of autumn, spring, and summer wind speeds, winter and spring temperature, and spring precipitation. However, potential roles of other factors (e.g., summer temperatures, North Atlantic Oscillation index) could not be ruled out. Changes in abundances of other species in the region, particularly predators and prey of green crabs, have also been observed and present possible alternative causative agents that should be investigated. Populations of other marine species in the Gulf of Maine-Bay of Fundy region within which the Minas Basin is situated have also been reported to have undergone dramatic changes in and after 2013, suggesting the occurrence of some oceanographic event or regime shift in the region. Declines to the monitored crab population in this study may have resulted from this same 2013 event. These observations have implications for recruitment to marine populations in this region.

Life History and Population Dynamics of Green Crabs (<em>Carcinus maenas</em>)

“…In some locations they migrate into deeper water during the cold winter months but in other areas many appear to remain in the shallow subtidal throughout the year, although numbers are reduced [113,125]. Following a colder than normal winter the density of the population is reduced [113,126,127] and many crabs can be found dead inside their Spartina bank burrows (M. Fregeau, Salem State U., pers com), evidence that they did not migrate. During the summer months females tend to be found in higher salinity subtidal areas except when they congregate in shallow water to mate.…”