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The restoration of aquatic ecosystems has always been ECON's particlar area of expertise and was initially the basis for the company's foundation 16 years ago. The initial focus was on the restoration of the Norfolk Broads, where ECON pioneered the use of biomanipulation as a tool for restoration, work that is still
continuing and yielding results today (Case Study 1). Other lake related areas of expertise include fish/zooplankton/macrophyte interactions and the impacts of Coot Fulica atra
on macrophyte populations.
ECON also specialise in riverine restoration and were employed to undertake the Feasibility Study (Phase 1) of the River Restoration Project, the aim of which is to review, describe and assess current restoration measures and techniques (Case Study 2).
ECON have also worked for the National Rivers Authority (now part of the Environment Agency) to establish a national rationale for river restoration to further conservation, improve fisheries and promote recreation.
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Case Study 1:
The Biomanipulation of Ormesby Broad, Norfolk, UK
Biomanipulation is defined as the "re-structuring of the biological community to achieve a desirable response". In lakes this is generally the reduction of troublesome algae and a return to clear water and a diverse submerged flora and fauna.
In the Norfolk Broads biomanipulation centres on the removal of zooplanktivorous fish such as small roach Rutilus rutilus or benthivorous fish such as adult bream Abramis brama. Fortunately, the Broads do not contain many Carp Cyprinus carpio, which can be particularly detrimental in other lakes.
Removal of zooplanktivorous fish allows large populations of algal-eating Daphnia and other cladoceran zooplankton to develop, which graze out unwanted algae. By contrast benthivorous fish may promote algal growth through the release of nutrients as they feed amongst the bottom sediments. They may also uproot sumbmerged aquatic plants, which would otherwise compete with algae. This process also stirs up and resuspends bottom sediments, thus increasing increasing lake turbidity. They also produce nutrient rich wastes. The biomanipulation process also involves enhancing the populations of predators (piscivorous) fish such as pike Esox lucius or perch Perca fluviatilis, which prey on smaller fish and previent populations of unwanted species reaching undesirable levels.
The picture below shows and algal dominated lake (inset) and a lake after biomanipulation (main). It also shows the changes in fish community structure from roach & bream dominance to a more diverse community including rudd, perch and tench.
The restoration of Ormesby Broad (55 ha), which started in 1995, was undertaken by a partnership comprised of the Broads Authority, Environment Agency and Essex & Suffolk Water (the lake owners) through the EU Life Programme. The lake was in fairly good condition, as a result of its isolation from a river-borne nutrient supply, and occasionally supported a reasonable submerged flora, so there was no need for expensive nutrient control and biomanipulation was used as a single restoration measure.
The biomanipulation strategy involved removing wintering aggregations of zooplanktivorous fish, then introducing artificial, removable spawning substrate so that any eggs laid by the remaining individuals could be removed and thus prevent recruitment. In the initial removal (spring 1995) eight tonnes of fish (>300, 000 individuals) were captured and stocked into other waters (mainly the formerly connected Rollesby Broad, but also the Thurne system that had suffered a major fish kill following saline incursion). An operation was mounted in spring 1995 to remove the large adult bream remaining in the broad. A "scare line" (150m of rope with weighted brightly-coloured kite tails attached to it - Perrow et al. 1998) towed between boats, was used to drive fish into a waiting seine net. The largest single catch was 206 individuals (522kg) (Holzer et al 1997).
Further small-scale top-up removals in the following winters was also undertaken along with specific efforts to tackle a particular problem, for example bream. Bream managed to recruit in the summer of 1995, and an estimated c. 1.5 million fry, which had aggregated near the barrier, were transferred to Rollesby (scooped up by hand-nets after concentration with electrofishing).
Overall, the strategy has been highly successful. Annual fish surveys showed a decline in the stock of 73 per cent after one year and more than 99 per cent after two. In 2000, the fish stock was still low with a density of <0.1 ind.per square metre) and a biomass around 4 g per square metre (40 kg per ha). The fish community quickly shifted from roach/bream dominance to a more diverse mixture of species, with recruitment of tench and rudd from the small residual populations. Rudd is now the numerically dominant species. The recruitment of young pike appears to be reducing, although this may be expected to increase as the populations of other potential prey species continue to build. Whilst the growth of perch is extremely good and the proportion of potentially piscivorous individuals is increasing, the population has yet to recover to pre-manipulation levels. At the moment, with a low stock and good growing conditions, Ormesby now supports specimen fish of a range of species (bream over 4.5kg, tench to 3.2kg, pike to nearly 12kg and eel to 2.3kg, supplemented by roach and perch over 0.5kg) popular with anglers.
The favourable shifts in the fish community are linked to the five-fold increase (comparing the mean from five years before to six years after) in the cover of submerged plants, coincident with an improvement in the light climate, since biomanipulation began. In some years fine-leaved Potamogeton species cover the majority of the lake, before suceeding to other species in late summer and autumn. Preliminary analysis suggests plants responded so rapidly as a result of the removal of benthivorous bream in particular (Tomlinson et al. 2002).
Despite all the positive signs, the long-term restoration of Ormesby is not yet guaranteed. Management of the fish stock must be continued in the short term to ensure favourable conditions are maintained and steps must be taken to ensure a stable fish community is attained as quickly as possible. Ways to improve the recruitment of piscivores are under consideration.
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Case study 2:
River Rehabilitation study of the River Wensum, Norfolk, UK
The basic aim of the project was to review information on the factors potentially affeting fish populations in the River Wensum, a SSSI and once one of the best roach fisheries in the country, and if necessary to put forward a restoration strategy.
Anecdotal (catch records) and scientific (fisheries monitoring data) indicated that there had been major declines in species such as roach, dace, perch and trout in the Wensum from 1940s to the present. In contrast Chub, introduced in the 1960s was increasing. Other introduced fish, barbel and greyling, were also found to be in decline.
The cause of the decline for roach, dace and barbel, and probably perch and trout was thought to be the failure to recruit, which appeared to be linked to the recent changes in form and function of the River Wensum.
These included:
- Increased abstraction
- Declines in water quality
- Declines in habitat diversity
The most important impacts on the fishery were:
- Sedimentation on spawning gravels
- An increase in potential concentrations of unionised ammonia (potentially fatal)
- An increase in nutrient levels
- Reduction in the number and quality of nurseries for larvae and fry
- Reduction in the diversity of habitat for older fish
The water quality issues were potentially resolvable through the implementation of phosphate stripping at two major sewage treatment works, so the restoration focused on habitat quality. Strategy development was based around identifying the 25 optimum sites for restoration. Sites were selected based on a matrix incorporating various criteria such as proximity to existing areas of high value, favourable land use, and landowner support. These sites were then prioritized using a version of the 'Leitbuild' planning procedure.
The resultant 25 schemes ranged from tree-planting at eroding banks (Downstream of Taverham Mill) to creating littoral margins (Fakenham and Swanton Morley); installation of 'riffles' and bank re-profiling (e.g. over a 2.75 km at Bintree); re-connection of defunct meander loops (at Hellhoughton, Billingford and Attlebridge); re-excavating and re-routing flow into the former channel (Castle Farm); connection of the river with floodplain features such as dykes (Morton, Ringland and Drayton), lakes (Starmoor and Sennowe Park) and reed-beds (Guist and Drayton); by-passing mills (Great Ryburgh and Taverham) to the development of major floodplain wetland at Taverham Mill and Drayton. The degree of technical intervention required, technical difficulty and cost thus varied considerably. The latter was thought to range from £1-100k.
In addition to improving conservation and fisheries value the schemes were also predicted to make positive contributions to the geomorphological stability and flood defence standards of the river. Priority was automatically given to four schemes that had qualified for funding under a separate bid for funds under the European Union's 5b framework, the remainder were selected through preparation of matrices summarising the likely benefit to fisheries and conservation value.
It is suggested that the overall approach adopted during this study may be used as a template and applied to other catchments of a range of river types. However, we would recommend consulting landowners from the start of the process, and would also emphasize the importance of involving a geomorphologist from the early stages too. The importance of rigorous monitoring of implemented schemes was also emphasised. Fish need to be monitored for a minimum of 5 years. In fact some fish are so long lived that 20 years may be required in order to determine actual population change rather than simple changes in distribution. Habitat monitoring using a combination of geomorphological and ecological techniques was also recommended in order to determine the habitat selection of fish.
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