The return of fish to the Mersey estuary

Abstract

Changes in the diversity of fish populations have been monitored during a period of improving water quality in the Mersey estuary. Historically the Mersey estuary supported a wide range of locally important fisheries, particularly for salmon, mullet, sturgeon, eels and smelt (sparling). With the advent of the Industrial Revolution, increased quantities of sewage and industrial wastes discharged to the Mersey from the Manchester area dramatically reduced the water quality and led to the depletion of fish stocks in the upper estuary. By the time the Manchester Ship Canal opened in 1894 the R. Mersey and its tributaries were heavily polluted, salmon were effectively eradicated and the fisheries in the upper estuary severely limited. By 1920 the commercially important fisheries, for shrimp and flounder, had been displaced downstream to the middle part of the estuary. The dramatic increase in industrial development around the estuary itself in the 1940s caused further deteriorations such that throughout the 1950s and early 1960s anoxic conditions in the estuary were common and fish were reportedly absent from the upper estuary for much of the time. At this time major efforts were made to improve the river and its tributaries upstream of the estuary, and within a decade the quality of river water had improved markedly ( Fig. 1). This led to some improvement in estuarine water quality such that, for example, by 1976 less than 2% of samples from the upper estuary contained no detectable dissolved oxygen compared to nearly 20% of samples taken in 1963 (Fig. 2).In 1976 the North West Water Authority instituted a regular monitoring programme by collecting fish from two industrial intake screens at Stanlow and Runcorn on the Manchester Ship Canal, which has intermittent influxes of water from the estuary. Surveys in the middle and upper estuary with a 2-m beam trawl began in 1981, and electrofishing surveys of tributaries/streams around the estuary shortly afterwards. Within one year the intake screen monitoring programme recorded 19 different species of fish of which four were freshwater species, the remainder typically estuarine or marine. By 1987 the total had risen to 40 species, ten of which were freshwater (Fig. 1). The number of different species observed in any one year ranged from nine to 17. Most of the species were encountered rarely, 33 species being seen on fewer than 5% of occasions and only three species observed on more than 50% of sampling occasions when fish were taken. This pattern was confirmed by the results of the beam trawling which produced a total of 14 different species, a maximum of seven in any one year. Both the commercial intake screen and beam trawl samples were dominated by the same species, ' whitebait ' (predominately sprat, Sprattus sprattus, with lesser numbers of young herring, CIupea harengus) and sand-goby, Pomatoschistus minutus. All species caught by beam trawling had previously been recorded from the intake screens (Figs 3,4). Other species including codling, salmon and grey mullet a...

Temporal abundance patterns and growth of juvenile herring and sprat from the Thames estuary 1977–1992

“…The environment and ecology of European and British estuaries have been linked closely with the history of human activity and altered significantly by recent urban and industrial use (Clyde (Henderson & Hamilton, 1986), Mersey (Wilson et al, 1988), Tyne (Pomfret et al, 1988), Elbe (Thiel et al, 1995) and Zeeschelde (Maes et al, 1998)). The history of the Thames, in particular, provides a classic example of possible human impacts on estuarine ecosystems.…”

Construction of a historical Water Pollution Index for the Mersey Basin

“…From the 1940s to the 1960s the Mersey Basin underwent substantial industrial expansion (NRA 1995; Wilson et al 1988). All the evidence acknowledges that this led to a reduction in the water quality.…”

“…However there is some debate over the quality of the water during this period. Some evidence reports that during the 1950s and 1960s the water was frequently anoxic, and fish were absent from the upper part of the estuary (Wilson et al 1988). Jones (2000) suggests that whilst anecdotal evidence describes the river as ‘dead’ in the late 1950s, this is an exaggeration.…”

“…The 1973 Water Act created the North West Water Authority (NWW) which concentrated on sewage disposal improvements (NRA 1995). The efforts of the 1970s to improve river conditions resulted in a 30 per cent reduction in the industrial pollution load (Wilson et al 1988; Alexander 1982).…”

Fish populations as indicators of long-term changes in UK estuaries

“…The degree of estuarine contamination can be considered as either acute, whereby changes in the fish population occur through death or emigration, or chronic when deterioration in fish health and fecundity can lead to a general reduction in fish community numbers and species richness (ELL1OTT et al 1988). Heavy pollution of estuarine environments can cause declines in fish abundance and diversity, ultimately to a point where a viable fish population can no longer be maintained (HEND~RSON & HAMILVON 1986;WILSON et al 1988). Subsequent improvements in water quality have resulted in increases in the abundance and species richness of a fish community (W~AR~E et al 1984).…”