Studies on downstream migrating silver-phase European eels (Anguilla anguilla) in hydropower-regulated rivers
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Within the last 50 years, the European eel has gone from one of the largest freshwater fisheries resources throughout its range to being listed as critically endangered. A variety of factors have been implicated in this decline, but the exact causes remain poorly understood. However, during their downstream spawning migrations from rivers and lakes to the ocean, silver eels are exposed to considerable interference from in-channel structures and the biomass of eels successfully escaping from European rivers has greatly diminished. As a result of this decline, the European Union introduced Regulation EC No. 1100/2007. The key goal of this regulation is to enable, with a high probability, the escapement to the sea of at least 40% of the silver eel biomass that would exist under pristine conditions. All Member States with natural eel habitats were required to establish eel management plans (EMPs) that outlined measures to reduce anthropogenic mortality and to develop monitoring programmes to assess compliance with management targets. In this study, two mitigation measures aimed at reducing hydropower mortality were assessed. In the hydropower regulated rivers Shannon and Erne, eels are captured upstream of dams and transported downstream to river sections with good seaward connectivity. The efficiencies of these ‘trap and transport’ (T&T) programmes were evaluated. It was found that 630 tonnes of eels have been captured and released to date (2009 – 2019) and the quantities released have proven important for reducing anthropogenic mortality rates in line with the EU regulation. However, clear differences exist between the efficiencies of the Erne and Shannon programmes and it was found that the effectiveness of T&T as a mitigation measure is directly linked to the number of hydropower stations bypassed. Trap and transport is only intended as an interim solution while non-intrusive alternative methods are developed. An underwater strobe light array was evaluated in the lower River Shannon to determine its potential to deflect silver eels during their downstream migrations. Eels displayed strong negative phototaxis, with strobe lights reducing the biomass of eels passing a river section by 80.3%. The results of this study suggest that light arrays could be used in future to prevent eels from entering turbines, guide eels to safe bypass routes or to increase the capture efficiency of trap and transport operations. Monitoring programmes are required to assess the effectiveness of EMP measures and to assess compliance with the EU escapement target (40%). This requires that silver eel production (i.e. population size) and escapement are calculated annually. These values have traditionally been quantified using fisheries catch data. However, catch data is only considered robust where fishing is more or less constant throughout the migration period. Unfortunately, gaps frequently occur due to unfavourable environmental conditions or the implementation of fisheries regulations. This prompted the development of alternative monitoring protocols to quantify eel migrations. An acoustic camera (DIDSON) was used to observe eels swimming downstream of an eel fishing site on the R. Erne. A highly significant relationship was observed between nightly DIDSON eel counts and catch at the fishing site (R2 = 0.968, p < 0.001) and will enable fisheries-independent estimation of spawner biomass in future. The acoustic camera was also used to investigate eel route selection at a water regulating structure in the lower R. Shannon. It was found that the proportion of eels migrating via alternative migration routes was strongly related to the proportion of total flow diverted to each route (R2 = 0.827, p < 0.001). Improved knowledge of route selection will assist with the calculation of spawner escapement but also with the development of conservation strategies and the detection of mortality hotspots. Generalised Additive Models were used to model daily catches of silver eels in the Shannon and Erne based on a variety of environmental variables. Final models were used to retrospectively complete discontinuous catch records and increased estimates of production by 9.3% on the Shannon and by 2.8 – 3% on the Erne, compared to estimates of production based on incomplete catch records alone. Finally, the ability of sequentially placed nets to generate quantitative estimates of silver eel production were assessed at a lake and riverine site. It was found that this method was an appropriate sampling protocol for the riverine site, but when applied to lake populations, estimates were highly biased relative to population estimates based on mark-recapture experiments. It is essential that data collection embraces both fisheries and non-fisheries data sources and the monitoring protocols developed in this thesis will facilitate the calculation of production and escapement in future.
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