Distribution and prevalence of Anguillicola crassus in eels from the tidal Thames catchmentb

Data gathered between 1988 and 1992 document the spread of the parasitic nematode Anguillicola crassus among eels in the tidal Thames catchment. Eel samples revealed a parasite prevalence ranging between 12 and 32% with a variation in intensity of infection of between one and five nematodes per infected host. Differences in the salinity regime between sampling points may be linked to the range of levels of infection in eels because of the saline tolerance limits of parasite developmental stages. The euryhaline teleost, the smelt ( Osmerus eperlanus ) found throughout the tidal river has been shown by others to be able to transfer nematode larval stages experimentally to large eels. Smelt found in the tidal Thames thus could possibly act as a further intermediate host to the eel population. The results support the theories proposed by previous workers that the parasite originally entered the tidal Thames via the commercial trade in live eels.     

A long-term study of population characteristics and downstream migrations of the European eel Anguilla anguilla (L.) and the effects of a migration barrier in the Girnock Burn, north-east Scotland

Downstream migrations and population characteristics of eels Anguilla anguilla were studied between 1967–1982 and 2002–2005 using a fish trap and electrofishing in the Girnock Burn, a small oligotrophic upland sub‐catchment of the River Dee, north‐east Scotland, 70 km from the tidal limit. In limited mark‐recapture studies, 9% of eels were recaptured up to three times and 97% of all recaptures were made at the same electrofishing site. The recaptured eels had a low mean growth rate of c. 13 mm year⁻¹. Smaller eels appeared to show preferences for shallower habitats with small boulder and gravel–sand substrata. Trap catches exhibited seasonal modes in total length at 140–180 mm in late spring, and 320–340 mm in early autumn, probably relating to water temperatures and discharges. From other studies, it is inferred that the spring mode comprised sexually undifferentiated nomadic eels and the autumn mode differentiated males beginning their spawning migration. Large female eels were rare. The fish trap appears to have formed a major barrier to upstream migration since its construction in 1966. In‐stream density has decreased significantly since then from 16 to three eels 100 m⁻², biomass from 260 to 78 g 100 m⁻² and emigrants from 700 to 100 individuals year⁻¹. Emigrants have comprised c. 5% of the standing stock year⁻¹ since the 1970s. The proportion of larger differentiated eels in the Girnock Burn has, however, remained relatively constant and escapement has been c. 100–200 (probably male) eels year⁻¹ since the late 1960s. Evidence, including that from other northerly British rivers, is reviewed to assess the possible impacts of Europe‐wide declines in glass eel recruitment since the 1980s. It is recommended that the data series be maintained, plus further sex determination and ageing studies. Installation of an upstream trap to capture immigrants and studies of recolonization are proposed.     

Accelerating rates of freshwater invasions in the catchment of the River Thames

We identify a total of 96 freshwater non-indigenous species established in the River Thames catchment, England; of which 55 % were introduced intentionally. Our analysis shows that 53 % of the species became established in the last 50 years and invasion rates have significantly increased since 1800. Analysis of shipping activity and population size in the catchment area revealed a positive correlation with non-indigenous species discovery, suggesting that globalisation has facilitated many species invasions. Our calculated modern (post 1961) invasion rates reveal that one non-indigenous species is discovered every 50 weeks, despite legislation aiming to prevent introductions, making the Thames catchment among the most highly invaded freshwater systems in the world. Although sympatric invaders are expected to interact and alter one another’s impact, most studies focus on individual conspicuous invaders. Our results indicate that it is essential to comprehend multiple invasions in future work.     

Juvenile Chinese mitten crabs (<Emphasis Type="Italic">Eriocheir sinensis</Emphasis>) in the Thames estuary: distribution,movement and possible interactions with the native crab <Emphasis Type="Italic">Carcinus maenas</Emphasis>

The Chinese mitten crab, Eriocheir sinensis is a successful invader. Whilst non-breeding adult E. sinensis have been associated with the destruction of riverbanks, little is known about the ecology of this species in its invaded areas. This is especially true of the juveniles which are a key migratory stage. Intertidal surveys along the Thames estuary indicated an increase in the population since the 1990s. Juvenile E. sinensis were abundant in the sampled upper tidal region of the Thames, refuging under boulders in the intertidal at low tide. Seasonal differences in sampled populations were observed, with a significantly lower abundance of crabs found during winter compared to summer. Mark-recaptures indicated movement in the intertidal occurring during high tide, with an influx of new crabs evident after a single tidal cycle. Endogenous rhythms were also apparent, with peaks in activity occurring corresponding with night-time high tide conditions. E. sinensis juveniles can successfully exclude similar sized native Carcinus maenas from shelters in the laboratory, regardless of which species originally inhabited the shelter. This may have implications for native estuarine Carcinus populations, which can rely on such intertidal shelters for refuges.     

Recent recruitment trends of juvenile eels in tributaries of the River Thames

Over the past 25 years it has become apparent that European eel recruitment has declined despite increased awareness and the development of protective legislation at local, national and European levels. A number of factors in both the freshwater and marine environment have been identified as potentially causing the decline, but there is little evidence how they specifically affect eel populations. There has been very little monitoring of European eel populations and therefore few robust data sets to allow long-term analysis. Eels were monitored at three sites in the Thames catchment between 2005 and 2009. At one site (River Roding) the catch per unit effort declined significantly after 2007, whilst at the other sites (River Thames and River Darent) catches were low throughout the period of the survey. Comparison of eel abundances at the River Darent caught in the present study with those caught in the mid 1980s would indicate that recruitment has declined by over 99%. The study highlights the lack of data relating to eel recruitment in the River Thames, and the UK as a whole, and supports both improved monitoring and the adoption of a precautionary approach in their management, irrespective of the causal factors for the decline.     

Sources,distribution and temporal variability of trace metals in the Thames Estuary

Water and intertidal sediment samples were collected from 28 stations along the Thames Estuary, from the tidal limit to the outer estuary. Surveys were conducted in 1997, 1999 and 2001 to assess spatial distributions and short-term variability. Highest concentrations of trace metals in water coincided with high turbidity in the mid-estuarine region, although the particle-associated fraction varied from 22% (As) to 95% (Pb). Theoretical dilution line (TDL) plots showed that dissolved metals were largely derived from a combination of diffuse (sediment desorption) and localised point sources (outfalls, industry). Dissolved Cu and Zn both exceeded environmental quality standard (EQS) levels during the survey period. The majority of sediment metals showed common distributional patterns, with increasing concentrations upstream. This increase was greatest for pollutant metals: Ag, Cd, Cu, Hg, Pb and Zn. Partial extraction of sediment metals with 1 M HCl showed that >50% of Ag, Cd, Cu, Mn, Pb and Zn were potentially bioavailable and that the proportion of bioavailable sediment metals also increased upstream. The majority of sediment metal concentrations exceeded Interim Sediment Quality Guideline threshold effects levels (TELs) over much of the estuary. Sediment concentrations of Cu, Hg, Pb and Zn also exceeded probable effects levels (PELs) at many estuarine sites. Despite improvements in recent years, the Thames Estuary remains chronically contaminated with a range of metals.     

Modelling of unexploited and exploited populations of eels, Anguilla anguilla (L.), in the Thames Estuary

Eels were fyke-netted in different zones of the Thames Estuary at various times of year and subsamples sexed, aged and stomachcontentsexamined. Instantaneous mortality coefficients (Z) were calculated for each 3 cm length class for eels > 30 cm and used to construct a compartmental model to assess effects of natural and fishing mortality and migration on different life stages of males and females. Growth rates were good and fishing mortality effects low and masked by effects of emigration/immigration (and, possibly, local migrations). Density-dependent mortality appeared to be having minor effects. Males dominate freshwater sites, females the more saline zone and possible implications of food availability and density are discussed. It is concluded the stock is not being over-exploited but that elver recruitment is relatively low. Means of increasing exploitation are discussed, as are ways of increasing production of females to enhance elver recruitment to the Thames and other European rivers.     

Colonization, larval survival and epidemiology of the nematode Anguillicola crassus, parasitic in the eel, Anguilla anguilla, in Britain

In late 1987, immature Anguillicola crassus were reported for the first time in Britain from eels from two river systems. By late 1988, gravid adults were present in a number of rivers in the east of England in two discrete centres of distribution: one in East Anglia correlated with the route taken by lorries exporting eels to the continent, and one in the R. Thames correlated with the import of eels to London. The parasite was firmly established in the R. Trent, where prevalence levels reached 100% in some places. Laboratory investigations showed that adult parasites and their eggs remained viable even after infected eels had been maintained for 4 weeks in 100% sea water. Hatching of eggs declined with increasing salinity, but was not totally inhibited by sea water. Survival and infectivity of freeliving second stage larvae were maximal in natural fresh water (95% survival for 4 months, and 50% still infective to copepod intermediate hosts after 70 days), but declined in alkaline water and with increased salinity. Nevertheless, in 100% sea water, 50% of larvae were still infective after 8 days. Specificity to the intermediate host was low, and eels of all sizes could be infected. These characteristics, plus a high reproductive potential, give the parasite exceptional colonization potential and ability, enabling it to survive natural movements of eels from catchment to catchment and to increase rapidly within a new locality. The ability of free-living larvae to adhere to the substratum and survive in sea water enables them to survive in eel-transport lorries from which they will not readily be removed by flushing, the normal cleansing procedure. It is concluded that there were two separate introductions of the parasite to Britain; via the eel import trade through London, and, totally unexpectedly, via the eel export trade in lorries traversing East Anglia. The parasite is now firmly established in Britain and will continue to spread by natural movements of eels but especially by human-assisted movements of infected eels for stocking and market. This latter practice is recognized as a major factor in introducing and disseminating fish parasites.     

Aspects of the respiratory physiology of Tubifex tubifex in relation to its ecology

An ecological survey of the London Basin of the Thames was undertaken in an attempt to define the limits of distribution of tubificids and to correlate their distribution with the oxygen concentration and the salinity.
The oxygen consumption of Tubifex tubifex has been measured at various oxygen concentrations to define precisely the critical level.
Measurements of oxygen consumption at various salinities have also been carried out, since salinity varies considerably over those parts of the estuary where tubificids are found. Salinity tolerance experiments were carried out in an attempt to determine the reason for the downstream limit of London Bridge.     

Recolonization pattern of the polychaete Lanice conchilega on an intertidal sand flat following the severe winter of 1995/96

The recolonization process of Lanice conchilega was studied on an intertidal sand flat in the northern Wadden Sea after an intertidal population had been wiped out during the severe winter of 1995/96. After very low recruitment in the first two years, the population fully recovered in the third year after its destruction. By then, the distribution pattern closely resembled the pattern before the severe winter with >95% of all Lanice occurring below –0.5 m mean tidal level and dominance of the density categories 6–50 individuals (ind) m^–2 and 51–200 ind m^–2. Lanice larvae have been observed to attach preferentially on the tubes of adults. However, the high recruitment in the third year was also seen in areas devoid of adult Lanice, indicating that settlement facilitation by adults was of little importance for the recolonization process. Instead, we suggest that the observed recruitment pattern was a consequence of a large-scale decline in the metapopulation in the tidal basin and the onshore coastal subtidal which resulted in insufficient larval supply onto the tidal flats and low recruitment in the first two years following the severe winter. Received in revised form: 7 May 2001 Electronic Publication     

Cryptic lineages in a small frog: the post-glacial history of the spring peeper,Pseudacris crucifer (Anura: Hylidae)

The spring peeper (Pseudacris crucifer) is believed to have been a primary herpetological invader of eastern North America following the most recent period of glacial retreat. We examined the phylogeographic pattern and population structure of P. crucifer to determine whether the distribution of haplotypic variants reflect post-Pleistocene recolonization dynamics. A number of geographically isolated evolutionary lineages were supported by both maximum parsimony and neighbor-joining analyses, and by coalescence approaches applied to mtDNA. South-western Ontario represents a high level of genotypic diversity (pi) due to the presence of two divergent lineages. The geographic distribution of these lineages are interpreted as reflecting post-glacial recolonization dynamics from separate, isolated refugia during the late Pleistocene that have come into secondary contact in SW Ontario. The phylogenetic placement of haplotypes from the range of P. crucifer bartramiana (Florida and South Carolina) does not allow for monophyly of P. crucifer crucifer, and therefore the bartramiana subspecies designation does not reflect a separate evolutionary lineage.     

The fate of Escherichia coli through water treatment and in distribution

The removal of E. coli bacteria at each stage of water treatment is presented, showing how the filtration stages contribute most to reduction of bacterial numbers in the water. At treatment works without filtration stages, the emphasis is put on catchment management to limit contamination of the raw water and ensure that the numbers of viable E. coli in the source water remain low. Routine monitoring of the raw water provides data on seasonal trends in numbers of E. coli bacteria and allows effective management of supply. In the UK there is no evidence that E. coli grows in the water distribution system, whether in the planktonic stage or within biofilms (O'Neill et al. 1997). The detection of E. coli in the distribution system is rare and prompts a thorough investigation. Repeat samples are taken from the point which originally failed, along with a number of hydraulically linked samples including samples from hydrants. The response to the detection of E. coli is discussed. A series of experiments carried out on a pilot pipe system is briefly described and the results discussed in relation to the routine samples taken in the Thames Water Supply area.     

A comparison of freshwater and marine/estuarine strains of Pomphorhynchus laevis occurring sympatrically in flounder, Platichthys flesus, in the tidal Thames

Collections of flounder, Platichthys flesus, at two sites on the tidal River Thames in 1994 and 1995 have, for the first time, revealed the sympatric occurrence of the freshwater and marine/estuarine strains of the acanthocephalan parasite Pomphorhynchus laevis. This natural co-occurrence of the strains has been employed to compare infection levels and a range of parasite attributes of the two strains under conditions of sympatry. At both Lots Road (Chelsea) and Tilbury the marine/estuarine strain was present at far higher infection levels than the freshwater form. In a detailed comparison of worms from Tilbury flounder, a range of differences was revealed between the two strains. In single strain infections in individual fish, freshwater and marine/estuarine worms had distinct but overlapping gut microhabitat use patterns, with the former having a central intestinal bias and the latter a bias for the posterior region of the gut. In mixed strain infections, niche contraction occurred so that no overlap occurred. Freshwater worms were larger and had more eggs, more ovarian balls, and a higher percentage of fully developed eggs than the marine/estuarine worms. These differences are thought to reflect intrinsic, presumably genetically determined, differences between the two strains as they occurred in the same fish host species collected at the same place and time. Apparent differences in strain reproductive potential in flounder in the tidal Thames are discussed in the context of previous studies and the intermediate host segment of the parasite life cycle.     

Migrations of elvers and juvenile European eels, Anguilla anguilla L., in the River Thames

The up-river migration of eels in the R. Thames has been studied at a number of locations since 1985. The run occurs between April and October, with the majority of eels moving over a distinct period averaging 47 days in May–June. The timing of the run varies and its commencement appears to be mainly dependent on water temperature. The number of unpigmented elvers migrating from the estuary to fresh water is variable and most appear to spend at least one year in the estuary. The average size and age of migrants increases with distance upstream. Migrants leaving the estuary were mainly 1–3 river-year (<14 cm) pigmented juveniles, though individuals of up to 45 cm were encountered. At a site 15 km from the estuary the majority of migrants were between 4 and 8 river-year eels (20–30 cm). Mark–recapture studies showed variable tendencies to display migratory behaviour among individuals, with some taking up to 2 years to reappear in a trap when released a few hundred metres below it. The implications of the results with respect to the commercial fishery in the estuary, populations in the catchment, and the contribution of Thames eels to international stocks and recruitment are discussed.     

Age of European silver eels during a period of declining abundance in Norway

The European eel (Anguilla anguilla) is critically endangered throughout its range. Knowledge about age distribution of future spawners (silver eels) is essential to monitor the status and contribute to the recovery of this species. Determination of age in anguillid eels is challenging, especially in eels from the northern part of the distribution area where growth is slow and age at maturation can be up to 30 years or more. Eels from the river Imsa in Norway have been monitored since 1975, and this reference time series has been used to assess the stock at the European level. Population dynamics in this catchment were analyzed during the late 1980s by estimating ages on whole cleared otoliths. However, techniques for revealing annual increments on otoliths have evolved over the years sometimes yielding significant differences in age estimates. In this study, the historical otolith data were reanalyzed using a grinding and polishing method rather than reading the whole otolith. The new age estimates were considerably higher than the previous ones, sometimes by up to 29 years. Since the 1980s, mean age of silver eels only slightly increased (from 19 to 21 years in the 2010s). This was mainly due to the disappearance of younger silver eels (<15 years) in the 2010s. The new age estimates agreed with the steep decline in recruitment which occurred in the late 1980s in the Imsa catchment. Mean growth (30 mm/year, min–max: 16–64 mm/year) has not changed since the 1980s, although density in the catchment has decreased. Revealing and reading age of slow‐growing eels remain a challenge but adding a measure of otolith reading uncertainty may improve age data collection and contribute to recovery measures for this species.     

Environmental factors affecting migration of the European eel in the Rivers Severn and Avon, England

Studies were conducted during 1991–1993 on environmental factors affecting the upstream migration of eels in the Rivers Severn and Avon, England. Migrants (> 156 000 pigmented elvers and > 189 000 juveniles) were trapped as they attempted to ascend weir or sluice barriers. Multiple regression models were developed to compare catches per trap per night (C) with data for various key environmental parameters at seven sites, from the tidal limit to a maximum of 42.5 km upstream. The key stimulus for migration of both elvers and juveniles at the tidal limit was water temperature, with some weaker monthly influences related to seasonal temperature increases. Smaller annual influences probably related to earlier glass eel recruitment into the lower estuary. A weak early tidal effect was demonstrated only once, in 1993 in the Severn. Temperature also exerted significant effects on C of juvenile eels at the tidal limit and in the non-tidal rivers, although effects weakened with distance upstream. Year, month, river flows and whether traps were mounted on weirs or sluices made only small contributions at a few sites. Distance between traps also contributed to combined data for upper Severn sites. The threshold temperature in all cases was 14–16°C, with low to zero catches below 10–11°C, catch maxima being achieved above 18–20°C. The implications of strong temperature-dependence of migration in relation to stock recruitment and management are discussed. Special reference is made to recent decreases in recruitment of eels to Europe and N. America and possible long-term effects of global warming.     

Tidal and diel timing of river entry by adult Atlantic salmon returning to the Aberdeenshire Dee, Scotland

The timing of salmon migration from the estuary of the Aberdeenshire Dee into the river in relation to tidal phase and time of day was studied by combined acoustic and radio-tracking of individual fish and by analysing records of untagged fish from a resistivity fish counter 0.8 km upstream from the tidal limit. Up-estuary movements that led to river entry were predominantly nocturnal and tended to occur during the ebb tide. Penetration into the non-tidal reaches of the river also tended to occur at night, but the timing of salmon movements was no longer significantly associated with tidal phase. The tracking data suggested that the reduction in the strength of the association between salmon migration and tidal phase resulted from variability in rates of progress from the estuary to the river. This variability may have been random, or related to changes in migratory behaviour during entry to fresh water. A simulation of upstream progress by groups of salmon illustrated the decay of the relationship between salmon movements and tidal phase and the advance of the average tidal phase of observed salmon movements with increasing distance of the observation point from where movements were initiated. The magnitude of these effects depended on the average rate of upstream progress and variability in the rate of progress, being greatest when upstream progress was slow and variable. These results highlight a limitation of point observations of migration with regard to identifying environmental stimuli for migration and quantifying their effects.     

Ecological Restoration of Headwaters in a Rural Landscape (Normandy,France): A Passive Approach Taking Hedge Networks into Account for Riparian Tree Recruitment

The safeguard of riparian ecosystems is a major field of study in the understanding and maintenance of the ecological health of rivers. Vegetation communities found on these ecotones ensure essential functions such as limitation of river bank erosion and protection of rivers from pollutants. The aim of our study is to investigate the potential for natural regeneration of trees on river banks after passive restoration. We have also studied the influence of landscape on recolonization through the analysis of the influence of hedge networks. Our study takes place on headwaters in Normandy (France) on Vallée‐Aux‐Berges, a stream, which has been passively restored for the last 6 years. As passive restoration removes stresses (heavy trampling and grazing) caused by cattle on river banks, we expect it to help the growth of natural plant communities. The condition of this stream—from the start of restoration work to the present—is compared to another one in the same catchment considered to be ecologically healthy. Our results suggest that passive restoration leads to an increase in tree cover on river banks and contributes to the improvement of the banks' physical integrity. Landscape structure seems to be a major factor for this recolonization: the more the stream is surrounded by hedge networks, the more the recolonization by trees on banks is effective. These results indicate that the influence of landscape structure should be considered in future restoration management in similar headwaters.     

Shift of grey seal subspecies boundaries in response to climate,culling and conservation

Identifying the processes that drive changes in the abundance and distribution of natural populations is a central theme in ecology and evolution. Many species of marine mammals have experienced dramatic changes in abundance and distribution due to climatic fluctuations and anthropogenic impacts. However, thanks to conservation efforts, some of these species have shown remarkable population recovery and are now recolonizing their former ranges. Here, we use zooarchaeological, demographic and genetic data to examine processes of colonization, local extinction and recolonization of the two northern European grey seal subspecies inhabiting the Baltic Sea and North Sea. The zooarchaeological and genetic data suggest that the two subspecies diverged shortly after the formation of the Baltic Sea approximately 4200 years bp , probably through a gradual shift to different breeding habitats and phenologies. By comparing genetic data from 19th century pre‐extinction material with that from seals currently recolonizing their past range, we observed a marked spatiotemporal shift in subspecies boundaries, with increasing encroachment of North Sea seals on areas previously occupied by the Baltic Sea subspecies. Further, both demographic and genetic data indicate that the two subspecies have begun to overlap geographically and are hybridizing in a narrow contact zone. Our findings provide new insights into the processes of colonization, extinction and recolonization and have important implications for the management of grey seals across northern Europe.