Large Epibenthic Invertebrates in the Bras D’or Lakes

Abstract

The distribution of large epibenthic invertebrates (lobster and crabs, bivalve molluscs and echinoderms) in the Bras d’Or Lakes is reviewed, and possible limiting factors are identified. The review is based on published and unpublished studies, including recent trawl surveys directed at fish, and trapping studies directed at American lobster Homarus americanus and green crab Carcinus maenas. The reduced salinities within the Lakes probably limit the distribution of several species (rock crab Cancer irroratus, sea scallop Placopecten magellanicus and possibly American lobster), particularly during the more sensitive larval period. Lobsters and eastern oysters Crassostrea virginica serve to illustrate the multiple factors limiting epibenthic invertebrate distribution within the Bras d’Or Lakes. Lobsters are less abundant within the Bras d’Or Lakes than on the outer coast of Cape Breton Island. Possible reasons are the reduced salinity and limited cobble bottom substrate in the Bras d’Or Lakes, coupled with low food availability and low egg production. Low egg production may be the result of overfishing of lobsters in the past. The life history and physiology of the eastern oyster appears to be well suited to the areas of the Lakes with warm summer temperatures. The oyster populations in the Bras d’Or Lakes are limited by natural predators (e.g. starfish and green crab), competitors (e.g. blue mussel Mytilus edulis and M. trossulus), and overfishing. The green crab, a new arrival to the Bras d’Or Lakes, will likely have negative effects on bivalves such as oysters, but the overall effect of green crab on the Bras d’Or Lakes food web is difficult to predict. Recent trawl surveys indicate both sea urchins Strongylocentrotus droebachiensis and starfish are present in considerable abundance, but little is known about their ecological roles in the Bras d’Or Lakes.La distribution des grands invertébrés épibenthique (les homards et les crabes, les mollusques bivalves et les échinodermes) dans les lacs du Bras d’Or est examinée et les coefficients possiblement limitatifs sont identifies. La revue est basée sur des études publiées et non-publiées englobant les plus récentesétudes sur la pêche au chalut dirigées vers les poissons et les études sur la pêche aux casiers dirigées vers les homards américains Homarus americanus et les crabes verts Carcinus maenas. Salinités réduites dans les lacs du Bras d’Or limitent probablement la distribution de quelques espèces crabes roches Cancer irroratus, pétoncle géant Placopecten magellanicus et possiblement le homard américain, en particulier, pendent l’époque sensible du larvaire. Les homards et les huîtres de l’Est Crossostrea virginica montrent plusieurs facteurs coefficients limitatifs de la distribution des invertébrés épibenthique dan les lacs du Bras d’Or. Les homards sont moins abondants ici que sur la côte extérieure de L’Ille du Cap Breton. Des explications possibles sont la réduction de l’eau saline du pavé rond limite dans le substratum de lacs, ainsi que la pauvre disponibilité de mangé et la production basse des oeufs. Cette dernière est peut-être le résultat d’un trop grand prise de homards au passè. L’histoire et la physiologie des huîtres semblent être bien adaptés aux lieux des lacs de Bras d’Or, qui ont des temperatures chaudes dan l’été. La population des huîtres dans les lacs est limitée par des proies natures ( ie étoiles de mer et les crabes verts) compétiteurs ( ie. Moules bleus Mytilus edulis et M. trossulus) et une trop grande prise de poissons. Le crabe vert, une arrivée nouvelle dans les lacs du Bras d’Or va sans doute avoir des impacts négatifs sur les bivalues comme les huîtres, mais leurs impacts en general sur la chaîne nutritive est difficile à prédire. Les études les plus recents sur la pêche au chalut montrent qu’il y a ungrand nombre d’oursins de mer Strongylocentrotus droebachiensis et des étoiles de mer, mais on ne connait pas quel est leur rôle écologique dans les lacs du Bras d’Or.

Salinity tolerance in the early larval stages of Carcinus maenas (Decapoda, Brachyura), a recent invader of the Bras d'Or lakes, Nova Scotia, Canada

“…Since then, the range expansion of this species has been mainly northward and has been correlated to periodic increases in sea surface temperatures (Berill, 1982). C. maenas was first sighted in Canada in the Minas Basin in 1953 (MacPhail et al, 1955) and was found in the Bras d'Or Lakes in Cape Breton, Nova Scotia some time between 1992 and 1995 (Tremblay, 2002). In its native range, as well as in other invaded regions, C. maenas has been considered responsible for significant impacts on epibenthic and infaunal species, such as bivalves, other molluscs, and crustaceans, through predation, competition, and burrowing activities (Grosholz & Ruiz, 1996;reviewed in Behrens Yamada, 2001).…”

“…Water circulation in the Lakes is typical of estuaries, consisting of a subsurface inflow of seawater from the Atlantic Ocean and a surface outflow of less saline water, which in turn results from the inflow of freshwater through numerous small rivers and brooks. As a result, temperature and salinity vary significantly in the Lakes and depend on depth, location, and season (Petrie & Bugden, 2002;Tremblay, 2002). In spring, average salinity is 22, with salinity at depth reaching 25 and surface salinities rarely exceeding 21.…”

“…Despite these low and fluctuating salinities, high abundances of adult and juvenile C. maenas, as well as of larvae, have been recorded in the Lakes (Tremblay, 2002;Cameron & Metaxas, 2005). The presence of adult C. maenas in the Lakes is not surprising, because they are euryhaline and can survive at salinities as low as 4.…”

Overview of the Ecology of the Bras d’Or Lakes with Emphasis on the Fish

“…Some invertebrates fished in the Bras d'Or Lakes are lobsters, oysters, scallops and rock crab. Lobsters are fished in most rocky areas throughout the Lakes but present landings are poor, which may be a result of overfishing or because of poor larval survival resulting from reduced salinity (Tremblay, 2002). Scallop and rock crab fishing is limited to a small area at the north end of the Great Bras d'Or Channel immediately adjacent to Sydney Bight.…”

“…Oysters, once common throughout the Bras d'Or Lakes, have been seriously depleted on public beds due to overharvesting (Dennis, 2001) and are now found in quantity only at lease sites, where they form the basis of an important and growing aquaculture industry. The presence of the green crab, a recently introduced non-native species, is of great concern since it is known to feed voraciously on juvenile bivalve molluscs such as clams and oysters (Tremblay, 2002). Rainbow trout and salmon are also farmed in the Lakes.…”