Recent studies have shown that anguillid eel populations in habitats spanning the marine-freshwater ecotone can display extreme plasticity in the range of catadromy expressed by individual fishes. Carbon and nitrogen stable isotope analysis was used to differentiate between European eels (Anguilla anguilla) collected along a short (2 km) salinity gradient ranging from <1 per thousand to approximately 30 per thousand in Lough Ahalia, a tidal Atlantic lake system. Significant differences were recorded in mean delta13C, delta15N and C:N values from eels collected from fresh, brackish and marine-dominated basins. A discriminant analysis using these predictor variables correctly classified ca. 85% of eels to salinity zone, allowing eels to be classified as freshwater (FW), brackish (BW) or marine (MW) residents. The results of the discriminant analysis also suggested that a significant proportion of eels moved between habitats (especially between FW and BW). Comparisons of several key population parameters showed significant variation between eels resident in different salinity zones. Mean condition and estimated age was significantly lower in MW eels, whilst observed length at age (a correlate of growth) was significantly higher in MW eels, intermediate in BW and lowest in FW eels. This study has demonstrated that the ecology of eels found along a short salinity gradient can be extremely plastic and that stable isotope analysis has considerable utility in demonstrating intra-population variation in diadromous fishes.

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Measurements were made on 274 spawn clumps and 271 adult frogs in three consecutive years at breeding sites in the west of Ireland. Mean clutch size was 1008 ± 20 ova, mean egg diameter was 2·32 ± 0·01 mm, and mean clutch weight was 11·62 ± 0·37 g. At any particular body length, larger clutch sizes were associated with smaller egg sizes and vice versa. Clutch size, egg size and clutch weight all increased allometrically with body size. The bigger females produced more and smaller eggs and a heavier clutch, relative to their body size, than smaller ones. Egg size and clutch weight also increased allometrically with female age. Age‐specific clutch weight increased during the first two years of maturity and declined slightly thereafter. Age‐specific egg size increased with age throughout life. At any age, bigger females produced more but not larger ova, and at any length, older females contained larger but fewer eggs. Both bigger and/or older females exhibited greater variability in the reproductive parameters than smaller and/or younger ones. Laboratory mating experiments showed that egg fertility was positively correlated with absolute male length and with the relative male/female length in amplexus. The fertilizing ability of males, however, declined with mating frequency, irrespective of male length, clutch size or the time interval between successive matings. In the wild, egg fertility ranged from 24–100%, although over 90% fertility was common.

A total of 1560 breeding frogs were collected in three consecutive years at sites in the west of Ireland. Alt were weighed and measured and the ages of 702 individuals were determined by skeletochronology. The length‐weight relationship differed between gravid females, spent females and males, with the gravid females showing the largest weight gain for any increase in length. The majority of males bred for the first time at two years, but most females delayed maturity until they were a year older. While the two‐year olds were of similar size, the females subsequently grew at a faster rate and achieved larger maximal sizes than the males. However, both sexes exhibited a decelerating growth rate with age. There was no apparent difference between the sexes in post‐maturation survival, which was estimated to be about 50% annually. The oldest frogs were seven years old.

The Shannon, Ireland's largest river, is used for hydroelectricity generation since 1929. Subsequently, the Electricity Supply Board assumed responsibility for management of its eel stocks, due to the impact of the hydro-dam on recruitment to the commercial fishery. In order to negate a decline in juvenile recruitment resulting from the installation of hydroelectric facilities, management was focused on stocking lakes with elvers and fingerling eels. These were trapped at the hydropower facilities and in estuarine tributaries during their up-stream migrations. Due to the decline of natural recruitment in more recent times, attempts have also been made to develop an estuarine glass eel fishery. Stock levels are then monitored through annual surveys of the population trends of juvenile (glass eel, elver), growing phase (yellow eel) and downstream migrating pre-spawners (silver eels). Survey results and fishery management programmes are reviewed in this article. In addition to the long-term effects the hydroelectric facilities have had on the stock levels, there is also an annual effect on the migratory patterns of downstream migratory silver eels. In the lower reaches of the river system flow rates are regulated by the hydroelectric stations. We review previous work that had highlighted the importance of flow in determining the timing of the silver eels migrations, and assess the relationship between flow and migration in more detail through the use of hydroacoustic and telemetric studies. Current research on seaward migrating silver eel populations, suggests that spawner escapement rates can most effectively be increased by trapping migrating eels at fishing weirs located up-stream of the power station and transporting them towards the estuary.

Age was determined in three anuran species inhabiting a temperate oceanic climate (Rana temporaria, Bufo calamita and Bufo bufo) by means of the histological structure of the phalanges. Narrow, haematoxylinophilic lines were visible in these, and evidence from the examination of subadults and frog (R. temporaria) recaptures showed the lines to be annual. The validity of using the lines as age indicators was tested most extensively on natural frog populations and the effects of endosteal resorption were not found to be significant. The evidence for this conclusion was provided directly by recapture data and indirectly by an analysis of histological measurements made on the phalanges.

The parasites of 121 eels from three contrasting sites in the Corrib catchment area, western Ireland, were investigated. Thirteen species, Ergusilus gibbus, Diplosromun gasrerosrei. Diplosrornrtnr spurhaceum. Sphaerosroma branrae. Borhrocephalus claviceps. Proreocephalus mrrc~rocepholus. Cavrullunus lacusiris, Cucullanus rruriae, Parayuimperia renerrima, Ruphidascaris acus. Acunrhocephalus clavulu, Aconlhocephulus lucii and Pomphorhvnchus luevis, were recorded. Two species, P. macrocephalus and P . renerrima, have not previously been reported from Ireland. Microhabitat preferences of the parasites were noted. Variations in the occurrence and intensities of the parasites observed were analyzed in relation to sampling period. host habitat and characteristics of the eel populations studied. A variety of factors were shown to be of importance, including composition of the fish communities and distributional patterns of intermediate hosts and piscivorous birds. Differences were noted in the parasitocoenoses of eels in still and running water sites. The occurrence and intensities of infection of several parasite species were shown to be related to age and size of host: the occurrence of B. cluviceps. C . fruiiae, P. ienerrima and R. acus was shown to be related to either age or size ofeels, which is accounted for by the fact that eels become increasingly piscivorous with age and increasing size. Little evidence of interspecific interactions was noted.

MacNamara, R., and McCarthy, T. K. 2012. Size-related variation in fecundity of European eel (Anguilla anguilla). – ICES Journal of Marine Science, 69: . Declining European eel (Anguilla anguilla) recruitment has focused attention on conservation of potential spawners leaving continental waters. Fecundity of wild, seaward-migrating silver-phase eels was shown to be size-related and higher than previously reported from artificial maturation experiments. Reliable information on fecundity is essential for stock modelling and future development of eel management policies.