Green crabs (Carcinus maenas) as the grim reaper: Destruction ofeelgrass beds in Nova Scotia

摘要

Following the first appearance of invasive Carcinus maenas in the southern Gulf of St. Lawrence in the early 1990s, 385,000 crabs km super(-2) occurred in Antigonish Harbour in 2000. Eelgrass (Zostera marina) which had large stable populations in the estuary for the previous 60 years declined by 95% in 2001, and many parts of the harbour were devoid of living eelgrass. This decline also occurred in adjoining estuaries where large numbers of crabs also occurred, but did not take place in estuaries further west and north where green crabs had not yet invaded, or numbers were low. In July-August 2002, shoot density of eelgrass at an experimental site in Tracadie Harbour with high crab numbers declined by 75% when a control harbour with few crabs showed increasing shoot density. A field experiment using 1.5 m super(2) cages, with and without green crabs, suggested that C. maenas was responsible for the loss of eelgrass. Telephone interviews with knowledgeable observers around Nova Scotia also suggested a correlation between declining eelgrass populations and increasing numbers of crabs during the previous ten years. The impact of green crabs on eelgrass is associated with their foraging activity in which they produce pits up to 30 cm wide and 15 cm deep. During foraging, eelgrass shoots are dislodged by crab digging, or the shoot bases are shredded, causing release of whole shoots into the drift. Since 2001, green crab numbers have been in decline in Antigonish Harbour, and there has been a substantial recovery of Z. marina.