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Environmental and habitat change can have profound and complex impacts on fish. We examined an unexploited population of European eel (Anguilla anguilla) from a West of Ireland catchment. The population is long-lived and slow-growing compared to many other European eel populations. Von Bertalanffy growth curves showed decadal changes, with a trend towards larger K, and t0 values in both males and females and a smaller L∞ in females. A growth biochronology spanning seven decades (1950s–2010s) was constructed using otolith annual increment measurements. We found evidence of high variability in growth over the course of the time series. A decrease in growth occurred after the early 2000s, potentially driven by habitat and climatic changes. Growth was negatively correlated with early spring and winter temperatures, providing strong evidence that the length of the growing season impacts eel growth. Growth was also positively correlated with summer temperatures and the number of days that exceeded 16˚C (GSL16˚C). The response to temperature was age-dependent; at age one the positive relationship with GSL16˚C was most pronounced and the negative relationship with winter temperatures was not evident. This study demonstrates the impact of climate change and highlights the complexities of eel growth strategies in a changing environment.

Coastal lagoons are a dynamic habitat, with varying marine and freshwater inputs determining the presence and extent of stratification, and the physical and chemical environment of the epi- and hypolimnion. As a result, the biotic assemblages that thrive in such environments are a diverse mix of species, with wide ranges of tolerances. While annual succession and vertical distribution of phytoplankton assemblages in marine and freshwater ecosystems are well documented, few data are available which describe the spatial and temporal variability of phytoplankton in coastal lagoons, even though these are a protected habitat under the European Union's Habitat Directive. In this study, high frequency monitoring (HFM) of chlorophyll fluorescence (ChlF) using a vertical profiling sonde was used to describe the variation in algal biomass over six annual cycles and through the water column of Lough Furnace, a perennially stratified coastal lagoon on the west coast of Ireland. Spot sampling, and species enumeration of phytoplankton were used to confirm the patterns observed in the fluorescence data, which indicated a general pattern of increasing biomass starting in spring, and a deepening of the deep chlorophyll maximum (DCM) over the summer. Typical of a stratified system, the DCM was often situated below the surface mixed layer, in saline hypoxic water. The best predictors of daily maximum ChlF were the depth of the surface mixed layer and daily solar radiation. We conclude that HFM allows an unprecedented glimpse into the substantial variability and complexity of phytoplankton distributions, a key biotic variable.

Long-term data, over four decades, were analysed to examine temporal trends in survival indices and phenotypic characteristics of Atlantic salmon Salmo salar returning to the Burrishoole national salmonid monitored river in Ireland. Before 2007, the marine drift net fishery was the major capture method for salmon in Irish home waters, accounting for over 70% of the commercial catch and targeting mixed stocks from multiple rivers. The authors examined size differences in fish captured in marine and freshwater environments and the impact of closure of this fishery on long-term survival indices and fish size. Return rates to Irish home waters for wild one sea-winter (1SW) and a ranching strain of hatchery-reared 1SW Atlantic salmon stocks showed a declining trend up to the time of closure of the fishery (1985-2006). In contrast, closure of the drift net fishery resulted in the anticipated increase in return rate to fresh water in the short term. Nonetheless, the short-term upward trend was not sustained in the following years: the trend for return rate to fresh water (1985-2017) was found to be neither increasing nor decreasing. Mean return rates to fresh water 10 years pre- and post-closure of the drift net fishery increased from 7.4% to 8.5% for wild 1SW and significantly from 2.4% to 3.7% for ranched 1SW suggesting some benefit had accrued as a consequence of drift net closure. For ranched 1SW salmon, entry into fresh water was found to be occurring earlier, which is likely a phenotypical response to changing climatic conditions. A declining trend in fish length was found in the pre-closure period, followed by a more stable trend post-closure. Similar patterns were observed for fish condition and weight parameters. Significantly, a step change in fish size occurred just before the closure of the Irish drift net fishery in both marine and freshwater habitats, when the average length decreased by 3.8 and 4.6 cm, respectively, between 2005 and 2006. This suggests an environmental effect on the population, rather than a fishery closure effect. Similar trends in fish length were observed in wild 1SW salmon kelts and ranched 2SW salmon in fresh water. The stable but not increasing trends post-closure suggest that conditions at sea may not be improving. These findings show that a clear decline occurred in wild and ranched salmon populations' return rates and lengths, while the drift net fishery was still active. Closure of the fishery did not result in a rebound to pre-exploitation levels of these indicators. Nonetheless, the trends went from declining to stable, suggesting the closure helped mitigate the impact of unfavourable environmental and rearing habitat conditions. These findings, based on four decades of data, highlight the urgency of strengthening monitoring of fisheries populations in face of climate change, so as to guide precautionary management measures that, as this study suggests, may be able to mitigate its impacts.

Humic lakes play a key role in the processing of organic carbon (OC) mobilised from their catchments, but knowledge of OC dynamics in lakes within maritime temperate climates is limited. Climate exerts a significant influence on mechanisms of OC capture, storage, and processing on the wet and cloudy west coast of Ireland. We examined a high-frequency dataset of partial pressure of CO2 (pCO2) in the surface waters of Lough Feeagh collected over 1 year. The annual pattern in pCO2 ranged between 491 and 1169 µatm and was strongly related to allochthonous riverine OC inputs. In contrast to observations in colder climates, a single peak in pCO2 occurred in Lough Feeagh in early September. Generalised additive mixed modelling revealed that 2 variables, inflow water colour concentration (a reliable proxy for DOC concentrations) and lake Schmidt stability, together explained 68% of pCO2 variability. Both the statistical analysis and timing of the peaks in inflow DOC and pCO2 strongly suggested that catchment carbon export drove pCO2 supersaturation in the lake, and hence CO2 emissions. We estimated that between 217 and 370 t CO2-C (0.55–0.94 t/ha) was emitted during the study period. These results highlight the interplay between catchment OC fluxes and climate in determining pCO2 dynamics in maritime temperate lakes.

While winter storms are generally common in western Europe, the rarer summer storms may result in more pronounced impacts on lake physics. Using long-term, high frequency datasets of weather and lake thermal structure from the west of Ireland (2005 to 2017), we quantified the effects of storms on the physical conditions in a monomictic, deep lake close to the Atlantic Ocean. We analysed a total of 227 storms during the stratified (May to September, n = 51) and non-stratified (November to March, n = 176) periods. In winter, as might be expected, changes were distributed over the entire water column, whereas in summer, when the lake was stratified, storms only impacted the smaller volume above the thermocline. During an average summer (May&ndash, September) storm, the lake number dropped by an order of magnitude, the thermocline deepened by an average of 2.8 m, water column stability decreased by an average of 60.4 j m&minus, 2 and the epilimnion temperature decreased by a factor of five compared to the average change in winter (0.5 &deg, C vs. 0.1 &deg, C). Projected increases in summer storm frequency will have important implications for lake physics and biological pathways.

Pass‐through passive integrated transponder (PIT) antennae are often used in river and stream habitats to monitor the movement of aquatic species. Where these habitats are prone to high flows containing suspended debris, traditional pass‐through antennae designs are particularly vulnerable to damage which can be time consuming and expensive to repair and lead to extended gaps in data collection. We designed and tested a novel pass‐through half duplex (HDX) antenna system that allows the antenna loop to (1) break away from one river bank under predetermined strain and (2) split into two separate cables, thereby shedding entangled debris that could otherwise damage or dislodge the antenna system. After break away events, our system can be rapidly reconnected and redeployed without the need for personnel to enter the water, reducing maintenance time and costs while minimizing gaps in data. In locations where pass‐through antennae are prone to flood damage, this system offers distinct advantages over traditional designs.

European eel (Anguilla anguilla) recruitment has been declining at least since the early 1980s at the scale of its distribution area. Since the population is panmictic, its stock assessment should be carried out on a range-wide basis. However, assessing the overall stock during the continental phase remains difficult given its widespread distribution among heterogeneous and separate river catchments. Hence, it is currently considered by the International Council for the Exploration of the Sea (ICES) more feasible to use glass eel recruitment data to assess the status of the overall population. In this study, we used Glass Eel Recruitment Estimation Model (GEREM) to estimate annual recruitment (i) at the river catchment level, a scale for which data are available, (ii) at an intermediate scale (6 European regions), and (iii) at a larger scale (Europe). This study provides an estimate of the glass eel recruitment trend through a single index, which gathers all recruitment time-series available at the European scale. Results confirmed an overall recruitment decline to dramatically low levels in 2009 (3.5% of the 1960–1979 recruitment average) and highlighted a more pronounced decline in the North Sea area compared to elsewhere in Europe.

This paper reviews the life history of brown trout and factors influencing decisions to migrate. Decisions that maximize fitness appear dependent on size at age. In partly anadromous populations, individuals that attain maturity at the parr stage typically become freshwater resident. For individual fish, the life history is not genetically fixed and can be modified by the previous growth history and energetic state in early life. This phenotypic plasticity may be influenced by epigenetic modifications of the genome. Thus, factors influencing survival and growth determine life‐history decisions. These are intra‐ and interspecific competition, feeding and shelter opportunities in freshwater and salt water, temperature in alternative habitats and flow conditions in running water. Male trout exhibit alternative mating strategies and can spawn as a subordinate sneaker or a dominant competitor. Females do not exhibit alternative mating behaviour. The relationship between growth, size and reproductive success differs between sexes in that females exhibit a higher tendency to migrate than males. Southern populations are sensitive to global warming. In addition, fisheries, aquaculture with increased spreading of salmon lice, introduction of new species, weirs and river regulation, poor water quality and coastal developments all threaten trout populations. The paper summarizes life‐history data from six populations across Europe and ends by presenting new research questions and directions for future research. freshwater resident, habitat, migration, partial anadromy, Salmonidae, sex ratio

Climate projections indicate that the frequency and severity of extreme precipitation events will increase over the next century. Although a large number of lakes across the globe are systematically monitored, it is rare to have a wide range of ecological indices measured at high enough frequency and over a sufficient time scale to allow characterisation of the response of a lake ecosystem to such events. We present data from Lough Feeagh, a relatively large oligotrophic lake in Ireland, which was at the epicentre of a once in 250-year precipitation event in July 2009 when 50 mm of rain fell in less than 2 hours. The effects of the resulting flood on the water column stability, chemistry, biology, and metabolism were examined, and data from multiple years before and after the event were used to ascertain the significance and longevity of any observed changes. The flood caused the water column to destabilise prematurely and depressed primary production. Bacterial biomass was high in the month after the flood, and zooplankton assemblages in late 2009 were significantly different from those of other years. Changes in all these variables combined to produce lower rates of gross primary production and higher rates of respiration than those measured between 2010 and 2014, resulting in more pronounced negative net ecosystem production than in the other years. Despite all these changes, the normal seasonal cycles resumed in 2010, and it appears that this rare but significant event did not have a long-term impact on the ecosystem functioning of the lake.

The European eel (Anguilla anguilla) population has been in decline at least since the 1960s and reliable regional information, particularly on the spawner production and escapement (i.e. the silver eel life stage), is a requirement of the EU stock recovery regulation. Two comparable time series exist in Burrishoole (Ireland) and Imsa (Norway), with monitoring of total silver eel production since the early 1970s. Numbers of emigrating silver eels fell significantly (p < 0.0001) in the 1980s (breakpoints: Burrishoole 1982; Imsa 1988), in both catchments from >4000 eels per annum to ∼2000 eels per annum. The proportion of male eels dropped and the average size of female eels increased. Biomass of silver eels escaping has remained similar in Burrishoole (1.1/1.2 kg/ha), but not in Imsa (2.1/0.9 kg/ha) between the early period and the 2000s. Factors that govern the onset of eel maturation (silvering) and the annual production of silver eels are little understood. In this paper, the influence of time-lagged environmental variables on silver eel production is examined. Annual variation in the time series was partly (r2 Burrishoole = 0.43, Imsa = 0.46) explained by variation in water temperature and water level. Annual number of migrating eels in both catchments was positively related to summer temperature and summer water flow, negatively related to summer temperatures in the previous year, and in the Burrishoole, also negatively related to high water levels in September/October. The models did not transfer well between catchments, indicating likely catchment specific environmental factors impacting on eel production. The reduction in eel numbers observed in both catchments, accompanied by the change in sex ratio and mean weight of females that contribute to maintain biomass production, calls into question the advisability of basing a spawner escapement recovery target on biomass alone, while numbers and proportions of males decline. Locked until 9.5.2019 due to copyright restrictions. This is a pre-copyedited, author-produced version of an article accepted for publication in [ICES Journal of Marine Science] following peer review. The version of record is available online at: https://doi.org/10.1093/icesjms/fsy053

A simple approach is proposed to fit a body growth model for the European eel Anguilla anguilla to data-poor case studies. The model is a modified von Bertalanffy curve allowing for delayed sex determination and sexual dimorphism. The proposed procedure provides preliminary estimates of model parameters on the basis of average age and body length of silver eels.

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)

A flexible panel consisting of 38 informative microsatellite markers for Salmo trutta is described. These markers were selected from a pool of over 150 candidate loci that can be readily amplified in four multiplex PCR groups but other permutations are also possible. The basic properties of each markers were assessed in six population samples from both the Burrishoole catchment, in the west of Ireland, and Lough Neagh, in Northern Ireland. A method to assess the relative utility of individual markers for the detection of population genetic structuring is also described. Given its flexibility, technical reliability and high degree of informativeness, the use of this panel of markers is advocated as a standard for S. trutta genetic studies.

Estimates of fish abundance from electrofishing surveys depend on accurate estimation of capture probability. We examine in this paper how estimates of capture probability and abundance of Atlantic salmon from multi-pass removal sampling can be improved by comparing the results of an experimental programme of closed electrofishing sites within selected rivers in west-central Norway, and those obtained from open electrofishing sites established for monitoring long-term juvenile Atlantic salmon population abundance within the Burrishoole catchment, western Ireland. We first establish that the Carle & Strub method provides a more robust estimate of population abundance than the Zippin and Seber methods. We then show how prior information on capture probability may be used to improve the accuracy of the abundance estimate in open sites. We also show that the use of prior information with single-pass electrofishing may improve the accuracy of the abundance estimate so that it is comparable with that of multi-pass electrofishing in terms of stock prediction while requiring less sampling effort.

This is a author-produced PDF of a 'Just in' article published in Canadian Journal of Fisheries and Aquatic Sciences following peer review. The definitive publisher-authenticated version 'The response of North Atlantic diadromous fish to multiple stressors including land use change: a multidecadal study Elvira de Eyto, Catherine Dalton, Mary M Dillane, Eleanor Jennings, Philip McGinnity, Barry O'Dwyer, Russell Poole, Ger G Rogan, David Taylor Canadian Journal of Fisheries and Aquatic Sciences, Published on the web 19 May 2016, 10.1139/cjfas-2015-0450' is available from DOI 10.1139/cjfas-2015-0450', Reduction of freshwater habitat quality due to land use change can have significant impacts on diadromous fish. Partitioning this impact from other potential drivers, such as changing marine conditions and climate, is hampered by a lack of long term datasets. Here, four decades of data were used to assess the impact of land use change on Salmo salar L. and anadromous Salmo trutta L. in the Burrishoole catchment, Ireland, one of the few index sites for diadromous fish in the North Atlantic. Land use change was found to have no significant impact on the freshwater survival of either salmon or trout. However, climate impacted significantly on the survival of salmon and trout in freshwater, with poor survival in years with wetter warmer winters, coinciding with positive North Atlantic Oscillation values. Additionally, cold springs were associated with higher survival in trout. The addition of hatchery salmon into the salmon spawning cohort coincided with low freshwater survival. Our results highlight the necessity for a broad ecosystem approach in any conservation effort of these species.

A cooperative effort gathered a large European length-at-age data set (N = 45,759, Lat. 36S-61N Long. 10W-27E) for Anguilla anguilla, covering one century. To assess the effect of global warming during the last century and habitat effects on growth, a model was fitted on the data representing the conditions met at the distribution area scale. Two GLMs were designed to predict eel log(GR): one model was fitted to the whole data and the other was fitted to the female data subset. A model selection procedure was applied to select the best predictors among sex, age class, five temperature parameters and six habitat parameters (depth, salinity and four variables related to the position in the catchment). The yearly sum of temperatures above 13 � C (TempSUP13), the relative distance within the catchment, sex, age class, salinity class and depth class were finally selected. The best model predicted eel log(GR) with a 64.46% accuracy for the whole data and 66.91% for the female eel data. Growth rate (GR) was greater in habitats close to the sea and in deep habitats. TempSUP13 variable had one of the greatest predictive powers in the model, showing that global warming had affected eel growth during the last century.

Low dissolved oxygen concentrations are of increasing concern in aquatic ecosystems, particularly at the interface between freshwater and marine environments. Oxygen depletion occurs naturally in many perennially stratified systems and it remains to be seen how climate change will affect these habitats. This is due, in part, to a lack of high-resolution, long-term data describing interannual variability in dissolved oxygen concentrations within stratified basins. Physicochemical parameters for Lough Furnace, an ecologically important tidal lagoon, were assessed using daily measurements (2009–14) from an undulating CTD (conductivity, temperature and depth) profiler and observations of tidal exchange flow. Continuous vertical saline stratification existed, with anoxia (

The relationships between a number of environmental variables and the daily sea trout (Salmo trutta L.) smolt output in the Burrishoole system were examined between 1971 and 2000. The average number of smolts migrating each year in the Burrishoole system decreased significantly from 3814 in the 1970s and 3899 in the 1980s to 1520 in the 1990s. A historic low of 769 smolts was recorded in the year 2000. The annual smolt runs were divided into a number of points, the duration in days to the 5, 50 and 95% points of the run. All of the points took substantially longer to reach in the 1980s and 1990s than in the 1970s, and they occurred earlier in the year. Sea trout smolt runs have shifted from discrete well-defined periods in the 1970s to more spread out less distinct runs in the 1990s. Two sets of environmental variables were identified as having an important role in smolt migration, regulating factors and controlling factors. Regulating factors operate before the smolt run and affect the physiological process of smoltification. The primary regulating factors were photoperiod and temperature. Controlling factors operate within the smolt run, affect the physical process of migration. The primary controlling factor was absolute water level, followed by change in water level, temperature and photoperiod. It is likely that the significant decline in the number of smolts migrating from the Burrishoole system over the period also had some effect on the dynamics of the smolt run.

The length, age and growth rate were investigated for downstream migrating male and female eels in the unexploited Burrishoole system, western Ireland. Significant differences were found in the age and length at migration with the larger, older female eels also showing faster annual growth as early as the first year in fresh water. Female eels normally migrated at lengths from 40.5 cm, exceptionally to 92.9 cm, and male eels at lengths between 28.9 and 46′0 cm. Back-calculation showed an irregular pattern of fast and slow annual growth. Mean annual growth increments were almost always greater for females than for males.

The eel migrations in Burrishoole, Ireland, have been studied from 1959 to 1988. Numbers of eels in the catches have decreased. The mean length of male eels has increased by 16.5% and females by 27.6%. Total weight of catch has not been affected by the drop in numbers. A change in sex ratio from 94.5 % males in 1962 to 37.5% in 1988 has also occurred. A decrease in elver recruitment into freshwater and a change in the environmental status of the system are discussed as being possible reasons for these changes.