Long-term trends in the estuarine abundance of Nilssonʹs pipefish (Syngnathus rostellatus Nilsson)

Data on the relative abundance of Syngnathus rostellatus Nilsson sampled over 16 years from a power station on the Thames estuary were used to investigate linkages between estuarine environmental and seasonal variables and fluctuations in pipefish abundance using multiple regression. Temperature was the dominant influence on abundance and, when combined with salinity, defined a two-dimensional niche space in which 50.3% of all pipefish were sampled. Abundance was non-linearly related to temperature, decreasing at lower and higher temperatures and increasing in the range of 11–17 C. Trend and seasonal factors also significantly influenced abundance. Regular patterns of seasonal occurrences (spring/autumn) correlated with an estimated temperature preferendum of 14–14.2 C. Comparison of estimated thermal niche limits with estuarine and coastal sea surface temperatures suggested intra-annual estuarine abundance fluctuations were tied to a facultative exploitation of warmer estuarine waters. Long-term trends in annual abundance further indicated a facultative use of the estuary as water conditions allowed. Populations increased as organic pollution in the Thames decreased, but decreased in the early 1990s coincident with droughtinduced increases in water temperature.