Oysters once had an essential role in the ecology of estuaries. But putatively in response to many factors, such as invasive species, oysters are declining in number and impact throughout their historical range, including Great Bay Estuary (GBE) in New Hampshire USA. The European green crab (Carcinus maenas) has become a successful invasive species in GBE as it is now naturalized. The green crab can adapt easily to high temperatures and as temperature is rising rapidly in this region, it is becoming more prominent in GBE concurrent with declining settlement rates of oysters. The green crab is a potential oyster predator, which could wreak havoc economically and ecologically in GBE. To explore the abundance and distribution of green crabs in GBE, the occurrence of green crabs was studied in GBE from April-November using 3 trapezoid crab traps deployed at each of 4 sites throughout the GBE. These sites include 2 wild oyster reefs near Nannie Island and Lamprey River in lower GBE and 2 oyster farms near Cedar Point and Fox Point in upper GBE. Each week green crabs collected at each of the four sites were counted, weighed, measured, and sexed.
Results/Conclusions
Initial analysis of the 2021 field season data found that green crabs were most abundant, on average, at oyster farms whereas the wild reef areas had few to no green crabs. Highest catch per unit effort (CPUE) throughout the season took place at the farmed site near Cedar Point between the weeks of late July to mid-August. Nannie Island, a wild reef site in the lower bay, had on average the second highest CPUE effort throughout the field season, third highest CPUE on average was the other farm site at Fox Point in the upper bay, last was Lamprey River on average. A wide range of green crab carapace widths were observed each week, ranging from 67±6SD mm at Nannie Island to 69±9SD mm at Lamprey River. The average mass of a green crab over the entire season ranged from 90±23SD g at Fox Point to 98±24SD g at Cedar Point. An initial analysis using a generalized linear model indicated a significant effect of site on CPUE (p-value < 0.05), except Lamprey River relative to the other sites. Our study finds green crabs are distributed throughout GBE, they congregate in certain areas, particularly the farmed areas in the upper bay.
