Burrishoole System Research Articles

Impacts of a record-breaking storm on physical and biogeochemical regimes along a catchment-to-coast continuum.

The impacts of changes in climate are often most readily observed through the effects of extremes in local weather, effects that often propagate through multiple ecosystem levels. Precise effects of any extreme weather event depend not only on the type of event and its timing, but also on the ecosystem affected. Here the cascade of effects following the arrival of an atmospheric river (directed by record-breaking Storm Desmond) across terrestrial, freshwater and coastal zones is quantified, using the Burrishoole system on the Atlantic coast of Ireland as a natural observatory. We used a network of high-frequency in-situ sensors to capture in detail the effects of an unprecedented period of rainfall, high wind speeds and above-average winter air temperatures on catchment and estuarine dynamics. In the main freshwater lake, water clarity decreased and acidity increased during Storm Desmond. Surface heat input, due to a warm and moist above-lake air mass, was rapidly distributed throughout the water column. River discharge into the downstream coastal basin was estimated to be the highest on record (since 1976), increasing the buoyancy flux by an order of magnitude and doubling the water column stratification stability. Entrainment of salt into the outflowing freshwater plume exported resident salt from the inner estuarine basin, resulting in net salt loss. Here, the increased stratification markedly reinforced isolation of the bottom waters, promoting deoxygenation. Measurements of current between the inner estuarine basin and the adjacent coastal waters indicated a 20-fold increase in the volume of seaward flowing low-salinity water, as a result of storm rainfall over the watershed. Storm impacts spanned the full catchment-to-coast continuum and these results provide a glimpse into a potential future for hydrological systems where these severe hydroclimatic events are expected to occur more frequently.

Differential lifetime success and performance of native and non‐native Atlantic salmon examined under communal natural conditions

The lifetime success and performance characteristics of communally reared offspring of wild native Burrishoole (native), ranched native (ranched) and non‐native (non‐native) Atlantic salmon Salmo salar from the adjacent Owenmore River were compared. Non‐native 0+ year parr showed a substantial downstream migration, which was not shown by native and ranched parr. This appears to have been an active migration rather than competitive displacement and may reflect an adaptation to environmental or physiographic conditions within the Owenmore River catchment where the main nursery habitat is downstream of the spawning area. There were no differences between native and ranched in smolt output or adult return. Both of these measures, however, were significantly lower for the non‐native group. A greater proportion of the non‐native Atlantic salmon was taken in the coastal drift nets compared to the return to the Burrishoole system, probably as a result of the greater size of the non‐native fish. The overall lifetime success of the non‐native group, from fertilized egg to returning adult, was some 35% of native and ranched. The ranched group showed a significantly greater male parr maturity, a greater proportion of 1+ year smolts, and differences in sex ratio and timing of freshwater entry of returning adults compared to native, which may have fitness implications under specific conditions.

Temporal and environmental influences on the variation in Atlantic salmon smolt migration in the Burrishoole system 1970–2000

The relationships between a number of environmental variables and the number of Atlantic salmon Salmo salar smolts migrating in the Burrishoole system, western Ireland, were examined over a 30 year period from 1970 to 2000. The number of Atlantic salmon smolts recorded migrating downstream decreased significantly from an annual mean of 11 579 in the 1970s to a mean of 6272 in the 1990s. The primary factor in the decline in Atlantic salmon numbers was a consistent decline in the number of returning adults from the 1970s until the mid 1990s. Timing of the smolt migrations (runs) was consistent throughout the three decades. The mean durations of the smolt runs were 102·1, 92·6 and 103·2 days for the 1970s, 1980s and 1990s respectively. Mean surface water temperatures at key points in the smolt run for the three decades were also similar. Mean water temperature values were 5·4, 5·1 and 5·3° C at the start of the smolt runs and 15·2, 14·9 and 15·3° C at the end of the smolt runs in the 1970s, 1980s and 1990s respectively. Multivariate analysis identified two groups of environmental variables which had a significant influence on the daily smolt catch. One group of variables dominated by photoperiod and temperature operated prior to the smolt run and was considered to regulate the development of smoltification. The second group of variables dominated by total light and water level operated within the smolt run and was considered to control daily smolt migration.

Temporal and environmental influences on the variation in sea trout ( Salmo trutta L.) smolt migration in the Burrishoole system in the west of Ireland from 1971 to 2000

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.

An ecophysiological comparison of wild and hatchery-raised Atlantic salmon (Salmo salar L.) smolts from the Burrishoole system, western Ireland

Total fish trapping facilities that enable monitoring of all migrating fish have been operating in the Burrishoole river system, west of Ireland, since 1970. Ocean ranching of reared salmon smolts has been used since the early 1960s to supplement grilse returns to the rod fishery and as a research tool for assessing marine survival and exploitation. Wild Burrishoole salmon (Salmo salar) smolts have, on average, a fivefold higher return rate to freshwater than hatchery-reared smolts derived from the same stock. To help explain these differences in return rates, histological and physiological data were collected in 2000 from two groups of hatchery-reared salmon smolts just before release and from wild smolts migrating about the same time. The data show striking differences (size, condition factor (CF), blood cortisol and glucose, gill mucous cells) between both smolt groups. Wild smolt were significantly smaller (mean length: 14.8 and 13.9 cm) and thinner (Mean CF: 0.91 on both dates) than hatchery reared smolt (mean length: 20.3 and 19.1 cm; Mean CF: 1.01 and 1.09). Wild smolt had higher basal cortisol levels in April than in May and did not display “typical” quadratic responses to capture stress, while hatchery-reared smolt did display quadratic cortisol stress responses. Similar differences were observed in serum glucose levels. There was considerable variation in gill and skin histology between fish sampled from the same group, even on the same day, although these were generally present at low levels and were not related to catching order or confinement. Hatchery-reared smolt had significantly higher concentrations of mucous cells in both skin and secondary gill lamellae. This preliminary look at Burrishoole wild and reared salmon smolts indicated striking differences between the two groups which may be linked to subsequent marine survival.

Comparison, using minisatellite DNA profiling, of secondary male contribution in the fertilisation of wild and ranched Atlantic salmon (Salmo salar) ova

Secondary males, either subdominant adults or, more usually, parr which mature in freshwater, are known to successfully fertilise Atlantic salmon (Salmo salar) ova. This study has estimated the reproductive success of such secondary males in redds formed by pairs of wild and sea-ranched adults. Eight wild and 11 ranched redds were sampled from the Burrishoole system, western Ireland, in two consecutive years. Representative samples of fertilised ova were screened at three minisatellite loci to reconstruct the genotypes of putative anadromous parents and to estimate the proportion of eggs fertilised by secondary males. Multiple paternal genotypes were detected in 18 of the 19 samples, and of the 1484 progeny examined, 593 could not have been derived from the primary adult male genotype. The level of secondary male contribution detected among wild redds from Burrishoole (mean 28.9%) was similar to that found in previous published studies. The level among ranched redds was significantly higher (mean 48.2%) (P = 0.042). The difference was greater when data from 1995 were considered alone, with the average level of secondary male contribution in ranched redds being almost twice that in wild redds. Minimum numbers of secondary males were not significantly different between wild (mean 2.6) and ranched (mean 2.7) redds.

Genetic impact of escaped farmed Atlantic salmon (Salmo salar L.) on native populations: use of DNA profiling to assess freshwater performance of wild, farmed, and hybrid progeny in a natural river environment

Since Atlantic salmon (Salmo salar L.) used for farming are usually genetically different from local wild populations, breeding of escaped farmed salmon potentially results in genetic changes in wild populations. To determine the likelihood and impact of such genetic change, an experiment was undertaken, in a natural spawning tributary of the Burrishoole system in western Ireland, to compare the performance of wild, farmed, and hybrid Atlantic salmon progeny. Juveniles were assigned to family and group parentage by DNA profiling based on composite genotypes at seven minisatellite loci. Survival of the progeny of farmed salmon to the smolt stage was significantly lower than that of wild salmon, with increased mortality being greatest in the period from the eyed egg to the first summer. However, progeny of farmed salmon grew fastest and competitively displaced the smaller native fish downstream. The offspring of farmed salmon showed a reduced incidence of male parr maturity compared with native fish. The latter also showed a greater tendency to migrate as autumn pre-smolts. Growth and performance of hybrids were generally either intermediate or not significantly different from the wild fish. The demonstration that farmed and hybrid progeny can survive in the wild to the smolt stage, taken together with unpublished data that show that these smolts can survive at sea and home to their river of origin, indicates that escaped farmed salmon can produce long-term genetic changes in natural populations. These changes affect both single-locus and high-heritability quantitative traits, e.g. growth, sea age of maturity. While some of these changes may be advantageous from an angling management perspective, they are likely, in specific circumstances, to reduce population fitness and productivity. Full assessment of these changes will require details of marine survival, homing and reproductive performance of the adults together with information on the F2 generation.

Age and growth of yellow eel, Anguilla anguilla (L.), determined by two different methods

Abstract– Ageing and calculation of growth rate of eels has always been controversial, and remarkably little validation has been reported. This article describes the growth of yellow eels from three lakes in the Burrishoole system, Ireland, as determined by back‐calculation from otoliths prepared by burning and cracking. Eels captured by fyke net ranged in length from 25.0 cm to 98.5 cm and the ages ranged from 6 to 49 years. Back‐calculated growth was slow, with mean annual increments of 2.08 cm, 1.40 cm and 1.97 cm for the three lakes. Yellow eels captured in the Burrishoole system in 1987 and 1988 were marked by Pan Jet inoculated alcian blue dye and released back into the three lakes. Recaptures made over the following three years yielded considerable growth data. Mean annual growth rates recorded were 2.30 cm, 1.07 cm and 0.97 cm. These data compare favorably with the calculated growth rates, thereby validating the otolith analysis for the eels in the Burrishoole system.

Growth rate and age at migration of Anguilla anguilla

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 performance of sea trout, Salmo trutta L., stocks from the Burrishoole system western Ireland, 1970–1994.

Abstract Since 1975, with partial recoveries in 1979 and 1984–1986, the numbers of finnock and adult sea trout, Salmo trutta L., in the Burrishoole system, western Ireland, have declined. The population decreased sharply from 1987, and collapsed in 1989 and 1990. Finnock were virtually absent in 1989 and one‐year maidens equally so in 1990. Sea trout smolt recruitments from 1992 to 1994 were the lowest recorded since 1970. The modal fork length of finnock decreased from 28 cm in 1987 to 26 cm in 1990. Up until 1989, the percentage return of smolts, as finnock, ranged from 11.4 to 32.4%. In 1989 a minimum was reached at 1.5%, with values of 10.0, 3.7, 7.4 and 9.9% from 1990 to 1994. Marine survival of smolts, to total first return to fresh water, historically ranged from 19 to 66%. Survival dropped to 1.8% in 1989. Observed changes in the structure of the sea trout population and the collapse in marine survival suggest that the current west of Ireland sea trout problem is based in the marine habitat.

Metazoan parasite community structure in brown trout from two lakes in western Ireland.

A total of 549 brown trout were caught from two lakes, Lough Feeagh and Bunaveela Lake, in the Burrishoole system, Co. Mayo, Ireland. Seven species of metazoan parasite were detected but only four species occurred in > or = 10% of the sample. The parasite community of the trout was dominated by a single species, Pomphorhynchus laevis, with a prevalence if 58%. Only one allogenic helminth species was recorded from the trout, Diphyllobothrium ditremum, with the community mainly comprising autogenic species. Few differences were found between the two lakes, although the larger lake, Lough Feeagh, did contain some trout which harboured more parasite species and had, on average, fish with higher numbers of individual parasites. Measures of species richness and diversity were very low, indicating an impoverished helminth community with little potential for interspecific interactions. Host age was found to significantly influence these indices of infracommunity structure.

An intensive study of allozyme variation in freshwater resident and anadromous trout, Salmo trutta L., in western Ireland*

Ten polymorphic enzyme coding loci were assayed using starch gel electrophoresis in anadromous (sea trout) and freshwater‐resident trout, Salmo trutta, from two neighbouring river systems draining into Clew Bay in western Ireland. Samples were collected from the Burrishoole and from the neighbouring Newport river system. In the Burrishoole system, two samples were from above waterfalls impassable to migratory trout, two consisted of anadromous smolts or post‐smolts and the remainder were freshwater‐resident trout from freely accessible areas. There were large differences in gene frequency between trout from above impassable barriers and all others sampled, and these two groups also differed substantially from each other. More minor differences were evident between some other samples, particularly between trout from the upper lake and streams entering it and a stream associated with the lower lake. Two anadromous populations therefore occur in the Burrishoole system. In contrast, no differences were evident in two sampling years between anadromous trout and cohabiting freshwater‐resident trout from the upper part of the Burrishoole system. Thus, in the upper part of this system, anadromous trout do not appear to comprise a separate population from non‐migratory trout and interbreeding may occur.

The Release of Reared Salmon Smolts (Salmo salar) into the Burrishoole River System (Western Ireland) and their Contribution to the Rod and Line Fishery

Reared salmon smolts have been released into the Burrishoole river system since 1957. From 1970 it has been possible to record all adult returns. Survival of reared smolts to adult, as measured by numbers of fish returning to fresh water, has averaged 2.1% cf. 8.4% for wild smolts. A number of factors contributing to the comparatively poor returns of reared smolts are outlined. The behaviour of reared adults on return to fresh water is described and it is concluded that once past their homing point, their contribution to the rod and line fishery diminishes. Thus, the exploitation rate of reared grilse below their homing point (L. Furnace) exceeds that above (L. Feeagh). In confirmation, whilst a positive correlation exists between total stock and total catch of reared grilse on L. Furnace, no significant correlation exists between stock and catch on L. Feeagh. In the Burrishoole system, the exploitation rate of wild grilse usually exceeds that of reared and it is only in L. Furnace that reared grilse make a significant contribution to the total rod catch. The mean exploitation rate obtained for Burrishoole reared grilse is low (10.8%) when compared with those recorded for salmon in other systems. The comparable exploitation rate of Burrishoole wild grilse is 14.3%.