Atlantic cod and snow crab predator-prey size relationship in the Gulf of St. Lawrence, Canada

Atlantic cod Gadus morhua stomach contents (n = 30 973, including 28 377 non-empty stomachs) and morphometric measurements on live snow crab Chionoecetes opilio and cod were examined to assess the predator-prey relationship between these 2 species. The most common snow crab instars found in cod stomachs were III and V (similar to 6 to 8 and similar to 12 to 16 mm carapace width [CW], respectively) in the northern Gulf of St. Lawrence (GSL) and VI and VII (similar to 17 to 23 and similar to 23 to 31 mm CW, respectively) in the southern GSL. A significant positive relationship was found between cod length and the largest and smallest CW of snow crab ingested by cod. Positive relationships were also found between gape width and body length in cod and between 3 measures of size (maximum span, width at rest, length at rest) and CW in snow crab. Snow crab length at rest was closely related to cod gape width, suggesting that the largest snow crab ingested by cod must be attacked from the side. There appears to be a plateau at 65.1 mm in the relationship between maximum snow crab CW and cod length, caused by the absence of large (adolescent and adult) male snow crab in cod stomachs. Other studies have found recently moulted, soft-shell snow crabs in cod stomachs, but this appears to be rare. Thus, snow crabs are susceptible to predation by cod mostly for the first 4 yr of postsettlement in the GSL. Any effect of cod predation on the snowcrab fishery would be felt 6 to 11 yr later, given growth models established for the GSL.