Population stability of the sea snail at the southern edge of its range

The sea snail Liparis liparis population in the Bristol Channel on the Atlantic coast of Britain is close to the south-western extremity of its range and yet it is one of the more abundant species present. Using a 17-year time series of monthly samples collected at Hinkley Point power station in Bridgwater Bay, Somerset, U.K., the coefficient of variation of annual abundance of sea snail was estimated as 70·48%. Analysis of variance of the six replicate time series showed that 94% of the total variance was attributable to between-year variation in local abundance and only 6% to sampling error. A comparison of the coefficients of variation in the annual number of captures of all the common fish found that the sea snail had one of the more stable populations in the estuary and bottom-living fish tended to show the lowest variability in population number. Arguments based on either the assumed maladaptation of populations at the periphery of their geographical range or the rapid response of short-lived animals to change would have predicted sea snail to have one of the more variable populations. As might be expected for a fish adapted to cooler, more northern and eastern waters, sea snail winter abundance was negatively correlated with seawater temperature. Using the unusual oscillation in water temperature observed during the winter of 1996/199 7 , it is argued that the correlation of sea snail abundance with temperature reflects changes in distribution rather than mortality, with the fish avoiding inshore waters in warm winters. It is hypothesized that demographic plasticity and biotic interactions such as parasite and pathogen attack can result in fish populations showing neither latitudinal nor longitudinal trends of increasing population variability from the centre towards the periphery of the range.