Spatial, Temporal and Tidal Variation in Crab Populations in the Forth Estuary, Scotland

Abstract

Five species of crabs were recorded in the Forth Estuary, of which Liocarcinus depurator and Cancer pagurus were confined to the lowest reaches where salinities are 24–35%‰. Carcinus maenas, Liocarcinus holsatus and Hyas araneus also occurred in the middle estuary where salinities are 22–31%‰ in summer but often fall to ~12%‰ in winter. A very few L. depurator and Cancer pagurus were confined to the lower estuary and mostly occurred in low tide trawls in summer. Carcinus maenas alone was recorded much further up the estuary where salinities rarely exceed 0–5·7%‰. Carcinus maenas predominated numerically throughout with an overall mean density of 0·0075 m−2 in the middle and lower estuary. Carcinus maenas in the estuary channel measured 16·6–82·9 mm carapace width (CW) with a modal size–class of 50–60 mm. Males were outnumbered by females (0·68:1 overall), and more markedly in summer than winter. Yet males were significantly bigger on average than females. Egg-masses were borne by 7·5% of all trawled females, but by 31·1% in April–May. On adjacent shores, by contrast, the much higher numbers of C. maenas were nearly all <20 mm CW. Low water trawls yielded significantly more C. maenas, but significantly lower proportions of males, than high water trawls. Liocarcinus holsatus was most abundant within the mid-lower estuary basin, with numbers decreasing both upstream and at the estuary mouth. Males outnumbered females by 2·26:1 overall, and most markedly at the extremities of the range. Males were significantly bigger than females, although sizes of both generally increased towards the mouth of the estuary. Only three of the 101 females bore eggs. Low water trawls yielded slightly more L. holsatus than high water trawls. Numbers of H. araneus increased markedly from mid-estuary to the estuary mouth where counts averaged 0·0021 m−2. Males outnumbered females by 2·17:1, and more so among those >40 mm CW. No female bore eggs. Records of the presence of C. maenas, Liocarcinus sp. and Hyas sp. all increased in the estuary from 1985 to 1995 reflecting increases in intertidal faunal diversity and changes in industrial practices since the early 1980s.