This study is the first comprehensive documentation of the geographical range of Anguillicola crassus in its host, the European eel Anguilla anguilla, in the Republic of Ireland. The prevalence and intensity of infections across 234 sites and 93 river basins in Ireland comprising rivers, lakes and transitional waters (estuaries) were analysed. While only 32% of the river basins were affected by this nematode, they correspond to 74% of the total wetted area. Significant differences in infection levels among water body types were found with lakes and transitional waters yielding the highest values, which can be attributed to the proportions of juvenile (total length, L(T) < 300 mm) A. anguilla caught. There were no significant differences in infection levels between water body types for adult A. anguilla or between sexes for any water body type. Prevalence was significantly lower in juvenile compared with adult A. anguilla captured in rivers and a positive correlation between infection levels and host size-classes was found. Future efforts should focus on monitoring the spread of A. crassus infections and assessing the swimbladder health of A. anguilla in Ireland.
The European eel (Anguilla anguilla) has undergone an unprecedented population decline since the 1980s, with current recruitment levels fluctuating from 3 to 15% of historical levels for the last 20 years. Monitoring of glass eels and elvers as 0 + recruitment is an essential step in helping to understand the trend in recruitment and to better quantify the current recruitment time series. Two locations within the Shannon estuary on the west coast of Ireland were monitored for glass eel recruitment from January to April in 2017 and 2018. This study used a generalised linear mixed model to examine a range of environmental variables impacting on glass eel abundance in transitional waters. Results found that water temperature and moon phase were the most important variables. Tidal height and cloud cover also influenced the abundance of glass eels but to a lesser extent. This study found that focussing survey efforts on nights around the full moon when water temperatures exceed 5℃ will allow a catch which is representative of the population in an estuary. Glass eel monitoring needs a long-term sampling plan in order to account for annual fluctuations apparent in glass eel recruitment.
Traditionally, alkalinity has been widely used in order to determine the level of acid sensitivity, while pH values were often viewed as an indicator from sensitivity to impact. These methods of assigning acidity classifications are problematic, due to large temporal changes in flow conditions and various geological types, which influence the results. The Sodium Dominance Index (SDI) is essentially a ratio of cations, namely, the concentration of Na + , divided by the sum of the concentrations of Na + , Ca 2+ , and Mg 2+ expressed as a percentage. Values of >60 SDI denoted sites 'at high risk' of acidification and were therefore classified acid sensitive. From a total of 248 sites (65 of which were forested) in Ireland, base flow sample results showed significant correlations between SDI, alkalinity and pH levels. Significant relationships for SDI to differing underlying geologies were also noted. The data suggested no effect of coniferous forest on the SDI results. It was found that SDI did not differ significantly between base and elevated flow conditions. It was concluded that a dilution effect of calcium (partially balanced by inputs of sodium and magnesium during storm events) along with a depletion of carbonate sources from soils resulted in small fluctuations in the Index between flow conditions.
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