The ichthyofaunal composition of the Elbe Estuary: an analysis in space and time

A stow net (90 m(2) opening) was used, at regular intervals between July 19 89 and June 1993, to collect fish over ca. 4 h on both the flood and ebb ti des at five sites distributed at regularly spaced distances throughout the Elbe Estuary. Overall, 53.4% of the species were marine, and some of these were abundant, e.g. Clupea harengus, Pleuronectes flesus, Pomatoschistus mi nutus, Merlangius merlangus and Sprattus sprattus. However, in terms of num ber of individuals, such species contributed only 9.1%, compared with 90.0% by anadromous species, which was predominantly due to exceptionally high c atches of Osmerus eperlanus. The ichthyofaunal compositions on the flood an d ebb tides were similar, even downstream where salinities change markedly during each tidal cycle. Although this is probably due in part to groups of fish being swept first upstream on the flood tide and then back downstream on the ebb tide, several species clearly tended to occupy particular regio ns of the estuary. Each year the species composition changed sequentially f rom the most downstream site (max. salinity = ca. 31.4 parts per thousand,) to the most upstream site (max. salinity = ca. 1.5 parts per thousand), ma inly as a result of a sequential decline in the number and abundance of mar ine species and a progressive increase in the contributions of both anadrom ous and freshwater species. However, the marine species diagnostic of diffe rences in the compositions among regions varied between years. Thus, for ex ample, the diagnostic species for the downstream sites were C. harengus and M. merlangus in 1990/1991 and S. sprattus in 1992/1993, reflecting differe nces in the recruitment strengths of these marine species in the two years. The ichthyofaunal composition exhibited pronounced annual cyclical changes in each region and thus occurred irrespective of whether or not the salini ty in a region underwent pronounced changes during the year. This cyclicity was attributable to intra-annual variations in the times of recruitment of certain marine species, particularly in the downstream region, and to seas onal migrations of diadromous species into each region, as well as to the m ovements of freshwater species into the upstream region in winter and early spring.