Recruitment success of roach varied dramatically between 1979 and 1982 in Alderfen Broad, a small lake in eastern England. When fry were abundant (in 1979 and 198 1, but not in 1980 or 1982) the summer zooplankton became sparse and was dominated by copepods and rotifers. In years of good recruitment, as each of the preferred cladoceran prey species entered the diet of underyearling roach, its density dropped dramatically. Mean size of Daphnia hyalina and Ceriodaphnia quadrangula also declined significantly during these seasons. The results of enclosure experiments indicate that the link between roach recruitment and zooplankton dynamics is causal. Older roach also feed to a significant extent on zooplankton, but the 0+ age group exerted the greatest influence. The abundant underyearling roach in years of good recruitment showed poor growth, as a result of depression of their prey populations. Older fish also grew poorly in these years and were then less fecund in the following season. The evidence indicates that there may be a 2-year cycle in roach recruitment, and we describe the unusual circumstances in Alderfen which may be responsible.

1. Accurate, cost-effective monitoring of fish is required to assess the quality of lakes under the European Water Framework Directive. Recent studies have shown that environmental DNA (eDNA) metabarcoding is an effective and non-invasive method, which can provide semi-quantitative information about fish communities in large lakes.2. This study further investigated the potential of fish-based eDNA metabarcoding as a tool for lake assessment by collecting and analysing water samples from eight Welsh lakes and six meres in Cheshire, England, with well-described fish faunas.Water samples (N = 252) were assayed using two mitochondrial DNA regions (Cytb and 12S rRNA).3. eDNA sampling indicated the presence of very similar species in the lakes compared to those expected on the basis of existing and historical information. Firstly, 24 species were detected, with a total of 111 species occurrences in the lakes studied using eDNA. Secondly, there was a significant positive correlation between expected faunas and eDNA data in terms of confidence of species occurrence (Spearman's r = 0.74, df = 109, p < 0.001). Thirdly, eDNA data can estimate relative abundance with the standard five-level classification scale ('DAFOR').Lastly, four ecological fish communities were characterized using eDNA data which agree with the predefined lake types according to environmental characteristics. Synthesis and applications.There are some limitations when using conventional captured-based methods for surveying species richness and relative abundance, such as morphological identification bias, difficulties in recording small-bodied, rare and/or elusive species and destructive impacts on the environment. This study provides further evidence that environmental DNA metabarcoding outperforms other captured-based survey techniques in a wide range of lake types for community-level analysis whether in species detection, relative abundance estimate using the standard five-level classification scale or characterization ecological fish communities. Therefore, the fish-based environmental DNA metabarcoding, a non-invasive genetic method, has great potential as an assessment tool for lake quality under the European Water Framework Directive.

The EU water framework directive (WFD) provides a template for sustainable water management across Europe. The WFD requires the development of procedures to ensure appropriate mitigation of anthropogenic impacts on river ecosystems resulting from water abstraction and impoundments. It is widely acknowledged that alterations to flow regime impact on riverine ecosystems. As a result, hydromorphology, which includes the hydrological regime, is embedded within the WFD as a supporting element to achieve good ecological status (GES). Environmental flow releases from impoundments such as reservoir dams will need to be implemented to mitigate impacts from their construction and operation. This paper outlines the process involved in the analysis of available scientific information and the development of guidance criteria for the setting of environmental flow release regimes for UK rivers. The paper describes two methods-developed by round-table expert knowledge and discussions and supported by available data-for implementation of the WFD for rivers subject to impoundments. The first is a method for preliminary assessment of a water body to determine if it is likely to fail to achieve GES because of changes to the flow regime (indexed by simple flow regime statistics) in systems where appropriate biological assessment methods are limited or currently unavailable. The second is a method for defining an environmental flow regime release based on the requirements of riverine ecological communities and indicator organisms for basic elements (building blocks) of the natural flow regime.

The behavioural basis of prey selection by underyearling bream (Abramis brama (L.)) and roach (Rutilus rutilus (L.)) SUMMARY. A clear difference in the ability lo escape from fish predators exists between members of the Cladocera and Copepoda. The results of our laboratory studies have shown that underyearling roach and bream both found copepods more difficult to capture than cladocerans. However, bream were far more efficient than roach at catching the more elusive copepod prey. The basis for this difference was the greater strike ability of bream, most likely related to its more protrusible mouth. In their natural environment the two species of fish exhibited food resource partitioning with planktonic Cladocera predominating in roach guts and copepods and non-planktonic Cladocera composing the vast tnajority of the gut contents of bream. Differences in diet are partly due to their contrasting attack abilities. Additional variation may arise because of subtle differences in the timing and location of foraging.

The sampling of environmental DNA (eDNA) coupled with cost-efficient and ever-advancing sequencing technology is propelling changes in biodiversity monitoring within aquatic ecosystems. Despite the increasing number of eDNA metabarcoding approaches, the ability to quantify species biomass and abundance in natural systems is still not fully understood. Previous studies have shown positive but sometimes weak correlations between abundance estimates from eDNA metabarcoding data and from conventional capture methods. As both methods have independent biases a lack of concordance is difficult to interpret. Here we tested whether read counts from eDNA metabarcoding provide accurate quantitative estimates of the absolute abundance of fish in holding ponds with known fish biomass and number of individuals. Environmental DNA samples were collected from two fishery ponds with high fish density and broad species diversity. In one pond, two different DNA capture strategies (on-site filtration with enclosed filters and three different preservation buffers versus lab filtration using open filters) were used to evaluate their performance in relation to fish community composition and biomass/abundance estimates. Fish species read counts were significantly correlated with both biomass and abundance, and this result, together with information on fish diversity, was repeatable when open or enclosed filters with different preservation buffers were used. This research demonstrates that eDNA metabarcoding provides accurate qualitative and quantitative information on fish communities in small ponds, and results are consistent between different methods of DNA capture. This method flexibility will be beneficial for future eDNA-based fish monitoring and their integration into fisheries management.