Changes in the wildfowl populations wintering on the Severn Estuary

Monthly winter counts of seven major wildfowl species show that, since 1960, Bewick's swan, shelduck, wigeon, teal and pintail have significantly increased in both the Severn and Britain as a whole; European White-fronted geese have shown no trend on the Severn but have significantly declined in Britain; mallard show no trend on the Severn but have increased in Britain. All except shelduck show a correlation between the trends in Britain and the Severn Estuary, while all but mallard and pintail rank as internationally important on the Severn. In recent years the Severn has assumed a considerable importance for pochard and tufted ducks. The status of many species in the Severn Estuary is dependent on the management of key areas.     

Fish fauna of the Severn Estuary. Are there long-term changes in abundance and species composition and are the recruitment patterns of the main marine species correlated?

Fish were collected from the intake screens of the Oldbury Power Station in the Severn Estuary in each week between early July 1972 and late June 1977 and at least twice monthly between early January 1996 and late June 1999. The annual catches, after adjustment to a common sampling effort, demonstrate that the abundance of fish at Oldbury was far greater in the 1990s than 1970s, mainly due to marked increases in the numbers of certain marine species, such as sand goby, whiting, bass, thin-lipped grey mullet, herring, sprat and Norway pout. These increases may reflect the great improvement that occurred in the water quality of the Severn Estuary between these decades. The only species that declined markedly in abundance was poor cod. Modest declines in flounder and River lamprey paralleled those occurring elsewhere in the UK. The species composition in the two decades also differed, reflecting changes not only in the relative abundances of the various marine estuarine-opportunistic species, which dominated the ichthyofauna, but also in those of the suite of less abundant species in the estuary. The cyclical changes undergone each year by the species composition of the fish fauna of the Severn Estuary reflect sequential intra-annual changes in the relative abundances of species representing each of the marine, diadromous and freshwater categories. New approaches have been developed to test whether or not large sets of correlations between patterns of recruitment amongst abundant marine species (internal correlations), and between those patterns and salinity and water temperature within the estuary (cross-correlations), were significant. The correlation profile analyses found no evidence that the annual recruitment strengths of these species were either intercorrelated, or correlated with either one or a combination of both of the above environmental variables. Yet, the timings of the recruitment of these species into the estuary were intercorrelated, i.e. a slightly earlier or later than normal immigration by one species in a given year was paralleled by the same trend in other species. However, this association in recruitment times could be linked neither to salinity nor water temperature within the estuary, nor to a combination of these two variables. These results indicate that, while the factors that influence the annual recruitment strengths of the juveniles of different marine species vary, inter-annual differences in the phasing of events that regulate spawning times and/or larval dispersal influence, in the same direction, the times when marine species are recruited into the estuary.     

Aspects of the biology and heavy metal accumulation of Ciliata mustela

The feeding habits, rates of growth, fecundity and other aspects of the general biology of five-bearded rockling from the Severn Estuary and the Bristol Channel are reviewed for the period 1974/76. Additionally the changes in heavy metal accumulation during this time have been measured and an attempt has been made to relate them to the diet of the fish.     

Environmental effects of tidal power generating schemes

Summary The Severn Estuary has been proposed as a site for a tidal power generating barrage. The character of the existing environment would be changed but, if the risks of accumulating pollutants and eutrophication can be avoided, the reduction in turbidity and tidal scour could increase species diversity and productivity. The interests of wading birds and wildfowl could be accommodated by special reserve areas. The main unknowns are the hydrographic and sedimentological regimes that would follow construction of a barrage, and prediction of biological changes depends on knowing the physical and chemical nature of the altered environment.     

Variation in populations of Littorina obtusata and L. mariae (Gastropoda) in the Severn Estuary

Populations of both species of flat periwinkle were examined at five discrete sites along the Severn Estuary, and at a marine site in Dyfed. The vertical distribution of the two species showed a clear partitioning of the shore – Littorina obtusata occupied the high/mid shore and L. mariae the low shore area. L. obtusata also extended further up the Severn Estuary than L. mariae. Maximum densities of both species were found in Milford Haven. Biometric investigations revealed intra and inter-specific differences in colour morphs, shell damage and size between populations. The possible influence of differential selection pressures at the various sites is discussed.     

Benthic macrofauna and sediments in the Severn Estuary

In the Severn Estuary and Bristol Channel, a multivariate analysis indicated eight groupings of macrobenthic fauna, each associated with a different physical habitat. Samples were faunistically impoverished, especially in sublittoral areas, but greater species richness was associated with reefs of Sabellaria alveolata , which extended well into the estuary.     

Appraisal of contaminants in sediments of the Inner Bristol Channel and Severn Estuary

In the inner Bristol Channel and Severn Estuary, most contaminated stations were in muddy, depositional areas of the estuary, particularly along the Welsh coast between the Rivers Taff and Usk, and also on the English coast between Avonmouth and Severn Beach. Some deeper areas, dominated by sand and mixed sediments, also showed contaminant concentrations in excess of those predicted from sediment texture, organic matter, and aluminium or iron concentrations.
Concentrations of lead, copper, chromium, nickel and zinc have decreased in the muddy sediments since the 1970s, but only lead has decreased in sand. With the exception of zinc, which is high in the Severn, trace element concentrations were comparable to those from other British estuaries.
Landward of the proposed barrage, contaminants are associated with the sub-estuaries and muddy areas at their confluence with the main estuary. Post-barrage effects on contaminants will be focused in these muddy areas. However, fine sediment deposition on what are now sandy areas would increase their contaminant burden unless calcium build-up from the resulting molluscan fauna has the antagonistic effect of contaminant dilution in the sediment.     

Intertidal zonation of animals and plants on rocky shores in the Bristol Channel and Severn Estuary–the northern shores

The vertical distribution of common species of macroalgae and fauna on rocky shores extending through the length of the estuary is described. In the west, the shores are fully marine and exposed to Atlantic waves. In an eastward direction, greater shelter and an increasing tidal range occur along a gradient of salinity and turbidity. The flora and fauna change along this gradient. No single biological feature signals a transition from the Bristol Channel to the Severn Estuary but four arbitrary regions are recognized: a 'marine' section eastwards to Swansea, a 'transitional' section between Swansea and Cardiff, an 'estuarine' section up to Newnham, and the tidal river Severn.     

Anatomical variability and secondary sexual characteristics in Pomphorhynchus laevis (Müller, 1776) (Acanthocephala)

A brief historical survey demonstrates the differences in opinion concerning the taxonomic status of species within the genus Pomphorhynchus. The anatomy of P. laevis from a single population in the River Severn, England was found to be highly variable. Much of this variability is produced as individuals grow. P. laevis possesses circumbursal and circumpenial rings of receptors on the male copulatory bursa. Specimens from Eire and the R. Severn had identical arrangements of these receptors. A preliminary appraisal of the value of secondary sexual characters as criteria of specific identity in the Acanthocephala is presented. ac]19860721     

The occurrence of Eubothrium fragile (Cestoda: Pseudophyllidae) in twaite shad, Alosa fallax (Lacépède) in the River Severn

The occurrence, size and maturity changes of Eubothrium fragile have been studied in postlarvae, juveniles and adult twaìte shad, Alosa fallax , from several locations in the River Severn. Parasites were only found in adult shad and not in post-larvae or juveniles. No juvenile or recently acquired cestodes were identified as such, but adults were present in shad throughout the whole period of their spawning migration. A large proportion of the parasites were gravid upon arrival in the river and, although eggs were subsequently released into fresh water, there was no loss of cestodes from the fish. It was concluded that E. fragile is a marine species, that the parasites found in adult shad in fresh water were the residue of a marine life cycle and that the eggs released in fresh water were part of the parasite's natural reproductive wastage. The distribution and biology of E. fragile were discussed and it is considered that it is typical of the marine species of the genus.     

The evolution of the fine sediment regime of the Severn Estuary and Bristol Channel

Fine sediment forms tidal flats along most of the Severn Estuary coastline but subtidal deposits are localized, mainly confined to Newport Deep and Bridgwater Bay. The estuary is experiencing a period of sea level rise which has led to coastal mud erosion in recent centuries. Sediment exchanges with the sea and river inputs are negligible compared to the large-scale exchanges within the estuary itself. Measurements of tidal flat level changes, archaeological finds and anthropogenic chemical concentrations confirm that mud flat erosion is long term and typical of the entire estuary. Sea level rise and coast erosion would normally lead to much of this sediment being redeposited inshore at the landward limit of the alluvium. Other than in narrow salt marsh strips, such deposition is prevented in the Severn by the river walls. In cores of subtidal muddy sediment, the number and thickness of sand layers decreases upwards. The top metre or so is entirely mud and exhibits thousands of layers of varying thickness and origin. Radiochemical analysis confirms that mud is accumulating in the subtidal zone of Newport Deep and the seaward periphery of the Bridgwater Bay mud patch and is increasing in proportion to sand. Circumstantial evidence strongly suggests that for at least the last 600 years the dominant pattern has been one of erosion of fine sediment from the coastal margin and its accumulation in subtidal sinks.     

Growth and abundance of O-group herrings, Clupea harengus L., in the Severn Estuary

Collections made from the cooling water intake screens of Power Stations in the Severn Estuary and the Bristol Channel have been used to investigate the biology of O-group herrings in this region. The abundance of herrings at Oldbury, the principal sampling site, was low in most years and in five successive seasons between July 1972 and December 1976, large numbers were caught only between the summer of 1975 and the spring of 1976. The herrings were first observed in July, when many had still not completed metamorphosis, and reached peak numbers in September and October during which period they were clearly growing. Numbers declined in December before rising dramatically in January at which time the length-frequency curves displayed a pronounced bimodality. On the basis of data from other parts of the Bristol Channel and Severn Estuary, it is suggested that this bimodality is due to the influx of some animals from other regions where the growth rate was greater. Although movement is apparently taking place at this time and the numbers decline markedly in subsequent months, the last remnants of the population do not leave the estuary until early May. Post-pelvic scute (K₂) and vertebral counts (VS) of 13.82 and 55.24 respectively, indicate that the Severn Estuary herrings are the product of a spring spawning stock, a view entirely consistent with length and field data.     

Characteristics of the ichthyofaunas of southwestern Australian estuaries,including comparisons with holarctic estuaries and estuaries elsewhere in temperate Australia: A review

Data on the species compositions and the ages, sizes, reproductive biology, habitats and diets of the main species in the ichthyofaunas of seven estuaries in temperate southwestern Australia have been collated. Twenty-two species spawn in these estuaries, of which 21 complete their lifecycles in the estuary. The latter group, which includes several species of atherinids and gobies with short lifecycles, make far greater contributions to the total numbers of fish in the shallows of these estuaries than in those of holarctic estuaries, such as the Severn Estuary in the United Kingdom. This is presumably related in part to far less extreme tidal water movements and the maintenance of relatively high salinities during the dry summers, and thus to more favourable conditions for spawning and larval development. However, since estuaries in southwestern Australia have tended to become closed for periods, there would presumably also have been selection pressures in favour of any members of marine species that were able to spawn in an estuary when that estuary became landlocked. Furthermore, the deep saline waters, under the marked haloclines that form in certain regions during heavy freshwater discharge in winter, act as refugia for certain estuarine species. The contributions of estuarine-spawning species to total fish numbers in the shallows varied markedly from 33 or 34% in two permanently open estuaries to ≥ 95% in an intermittently open estuary, a seasonally closed estuary and a permanently open estuary on the south coast, in which recruitment of the 0 + age class of marine species was poor. The larger estuarine species can live for several years and reach total lengths of ~ 700 mm and some estuarine species move out into deeper waters as they increase in size. Several marine species use southwestern Australian estuaries as nursery areas for protracted periods. However, sudden, marked increases in freshwater discharge in winter and resultant precipitous declines in salinity in the shallows, and in other regions where haloclines are not formed, are frequently accompanied by rapid and pronounced changes in ichthyofaunal composition, partly due to the emigration of certain marine species. In contrast, the ichthyofaunal compositions of macrotidal holarctic estuaries undergo annual, cyclical changes, due largely to the sequential entry of the juveniles of different marine species for short periods. The ichthyofaunal compositions of the narrow entrance channels, wide basins and saline riverine reaches of large, permanently open southwestern Australian estuaries vary, reflecting the marked tendency for some species to be restricted mainly to one or two of these regions. Comparative data indicate that the characteristics determined for ichthyofaunas in southwestern Australian estuaries apply in general to estuaries elsewhere in temperate Australia.     

The diet of juvenile and adult twaite shad Alosa fallax fallax (lacépède) from the rivers Severn and Wye (Britain)

The diet of juvenile and adult twaite shad was studied at a number of freshwater and estuarine sites in the rivers Severn and Wye and in the coastal waters of Cardigan Bay (West Wales). In the Severn estuary and in the freshwater reaches of the Severn and Wye, adult twaite shad consumed little during their pre-spawning migration. In the Severn Estuary post-spawning adults were present during the summer months where they fed actively on mysids, gammarids and shrimps. Mysids dominated the diet of the adults caught in coastal waters during the autumn. The diet of one year old twaite shad was studied in the Severn Estuary. In May the one year olds from the lower estuary consumed mainly gammarids. During the summer months, this age-class moved into the inner estuary where they fed predominantly on mysids.The larvae and juveniles (age 0⁺) fed mainly on chironomid larvae and pupae and Simuliidae larvae, while in fresh water. In the estuary copepods, cladocerans and mysids dominated the diet of the juveniles. There was some suggestion of an increase in prey size during their period of residency in the estuary. The contrasting distribution of the juveniles in two river systems is discussed in relation to the effects of channelization.     

An appraisal of anglers'catches in the River Severn, England

Angling competition catch data were gathered by postal questionnaire during 1975–1986 for three sections of the R. Severn. Angling success and species composition were analysed relative to both water temperature and year. The importance of barbel increased rapidly from 5° C to 15° C then levelled. Roach, chub and gudgeon showed peaks in the ranges 7–12, 10–15 and 9–16° C, respectively. Dace, bleak, eel and bream displayed consistent increases in contribution to catch with rising temperature. Year-to-year variations in angling success differed according to river section and for the most part could be explained by changes in the importance of species. The percentage of anglers with catch during the period June-November ranged from 50 to 91% and catch rates from 66 to 121 g man ¹ h ¹.     

Variation in dog-whelk shells in relation to wave action and crab predation

In Nucella lapillus there is usually a progressive gradation in mean shell shape from short squat individuals on exposed headlands to more elongated ones in shelter. This pattern has been attributed to the differential selective effects of wave action and crab predation, although it would almost certainly be an oversimplification to imagine these as the only significant forces involved, especially near the limits of the species' distribution.
However, whilst this pattern of shell-shape variation with exposure is generally true over most of the animal's range in both Europe and North America, there are exceptions. There are occasional enclaves with much more elongated shells than would normally have been expected. Sometimes these are so abundant, as in the Severn Estuary, that all dog-whelk enclaves are unusually elongated, regardless of the exposure of their habitat.
In other areas, such as south-eastern England, the Solway Firth and the area around the mouth of the Conway the shells are not unusually elongated, but the enclaves show very little variation: a normal sheltered-shore form is seen wherever the species occurs. Shetland samples appear to be a combination of this pattern (predominating) with the 'normal' one showing the full range of shellshape variation.
Nucella appears in the Atlantic fossil record at the end of the Pliocene without any antecedent forms. It is assumed to have colonized from the Pacific and exploited the vacant niche before the onset of the Ice Age. Successive advances and retreats of the ice, with attendant changes in sea level, will have served to break up and recombine those dog-whelk populations that were not eliminated. It is postulated that the modern Nucella lapillus is a combination of several genetically distinct populations, some of which show the full range of variation and may respond to the selective effects of crabs and waves, and some of which do not and show little variation.     

The emergence of insects from a British River warmed by power station cooling-water

As a result of some field observations and laboratory experiments, a number of authors (see text) have suggested that insects living in rivers, even slightly heated by power station cooling water, may have their life-cycles altered sufficiently to lead to the elimination of species owing to early emergence into a lethally cold air, shorter emergence periods, disrupted mating behaviour, or the elimination of necessary chill-periods in winter. If this were so, an important food resource for fishes in some rivers might be lost.Part I of this paper (Langford and Daffern — in press) described the results of an emergence-trapping programme on the River Severn around the cooling water outfalls of Ironbridge A and B Power Stations. This paper describes in detail the emergence pattern and periods of eight species of Ephemeroptera, seven species of Trichoptera and one species of Megaloptera, upstream and downstream of the outfalls during the period July 1969–November 1971. The patterns are analysed in relation to river flows and water temperature.Although t7emperature increases reached 8°C, maximum water temperatures reached 28°C, total degree-hours were up to 24% more during spring, and there was a four-week advance in degree-hour accumulation; there was no evidence of a pattern of early emergence downstream. Emergence of most species occurred over a temperature range of 12°C to 28°C and there was no evidence of the high temperatures in 1970 halting emergence or shortening the emergence period.Captures of all species of Ephemeroptera were obviously influenced by spates, i.e. increased river levels and flows. Evidence suggests that emergence may be suppressed under such conditions. Catches of most Trichoptera species were not as markedly influenced by river flows. Implications are that future flow changes in the middle reaches of the Severn as a result of water transfers could affect emergence patterns of Ephemeroptera.There is no positive indication that temperature changes caused by the cooling-water discharge had any influence on onset or progress of emergence of either Ephemeroptera or Trichoptera. It is suggested that where early emergence has been induced experimentally in the laboratory, constant high temperatures, stable flow conditions and food have been provided. In a natural river system, it is unlikely all these conditions would be maintained consistently enough for lethally early emergence to occur.It is concluded, therefore, that before predictions about the effects of power station cooling water discharges on the emergent insects of rivers are made, based on extrapolation of laboratory data, it is essential that the relevant field conditions and relationships should be considered. Also the natural variability and adaptability of species should be investigated in more detail.     

The benthic ecology of river estuaries entering the Severn Estuary

The fauna of the Rivers Avon, Usk and Wye was similar to that for soft sediment in the Severn Estuary. It was dominated by Nereis diversicolor, Corophium volutator, Aiacoma balthica and oligochaetes. Maximum densities of Corophium and Macoma were higher than in the main estuary, while that of Nereis was similar. Distributions of dominant species showed gradients along river length and across river profile. These are discussed.     

A comparison of the metamorphosing and macrophthalmia stages of the lampreys Lampetra fluviatilis and L. planeri

Comparisons of metamorphosing and macrophthalmia stages of the closely related species, L. fluviatilis (L.) and L. Planeri (Bloch), have shown that these can be distinguished within a few weeks of the onset of metamorphosis by characteristic differences in colouration and body form. Measurements of several body intervals have disclosed differences between the macrophthalmia stages of the two species. A sharp distinction between the blunt teeth of L. planeri and the supposedly sharp teeth of L. fluviatilis has not been confirmed in these early stages, but significant differences have been found in the numbers of teeth in the anterior field of the oral disk and in the lateral and posterior marginal series. For material from several rivers, the range of length of metamorphosing and macrophthalmia stages of L. fluviatilis is83–119 mm (mean 99.3 mm). Weights varied from 0.71-2.5 g (mean 1.51 g). Regression coefficients for weight on length are much lower in the macrophthalmia of L. fluviatilis than in comparable stages of L. planeri. Total oocyte counts on macrophthalmia of L. fluviatilis gave values from8000–20,000 which are in general agreement with egg counts for adults of this species in the river Severn. Observations on the earliest metamorphosing forms have shown that it is not possible at this stage to distinguish the males of the two species by the structure of the testes. The development of the lumen in the adult foregut of L. fluviatilis has been shown to be variable and in some instances the gut does not become patent until early Spring. The possibility has also been raised that, in exceptional cases, a temporary lumen may also be present for a short period in L. planeri. Observations on the foregut and dentition, together with field data, suggest that the climax of downstream migration and the onset of parastic feeding takes place in late March or April.     

Whiting migration in the Bristol Channel: a predator-prey relationship

The relationship between whiting, Merlangius merlangus (L.), and the common shrimp, Crangon crangon L., in the Bristol Channel and Severn Estuary was investigated. The whiting population was found to comprise 0-group juveniles which only remain in the region for about 10 months before moving offshore. While within the estuary, the distribution and abundance of the two species were found to be clearly related. It is hypothesized that the whiting depends on the common shrimp because it is the only abundant prey species in the winter. Both the whiting and shrimp populations were found to not follow any long-term trend in numbers. It is suggested that the whiting population is stable because it is constrained by shrimp abundance which is, in turn, limited by physical constraints. Similar predator-prey relationships may be common to many European estuaries.