Natural and marine environment
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Browsing Natural and marine environment by Author "Allen, Michelle M."
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Item The changing times of Europe's largest remaining commercially harvested population of eel Anguilla anguilla L(Wiley, 2021-06-04) Aprahamian, Miran W.; Evans, Derek W.; Briand, Cedric; Walker, Alan M.; McElarney, Yvonne; Allen, Michelle M.This study quantifies the processes involved in regulating the European eel population of Lough Neagh, a lake in Northern Ireland. The relationship between glass eel input and silver eel output for the 1923–1997 cohorts was best described by a Beverton–Holt stock recruitment model. Glass eel input time series was not complete and was thus derived from the relationship between catches elsewhere in Europe and Lough Neagh, together with the addition of stocked glass eel. Silver eel output was the sum of silver eel escapement, catch and yellow eel catch converted to silver eel equivalents. Natural mortality increased with glass eel density, ranging from 0.017 to 0.142 year−1. The mean carrying capacity increased from ≈3.25 M silver eels (≈26 kg ha−1) for the 1923–1943 cohorts to ≈5.0 M (≈40 kg ha−1) for the 1948–1971 cohorts before regressing back to ≈3.25 M. The total silver eel output was highest during the late 1970s/early 1980s at 35–45 kg ha−1 year−1 and lowest during the early years of the 20th century and is currently at 10–15 kg ha−1 year−1. The findings are discussed in relation to (a) the ecological changes that have occurred within the lough, associated with eutrophication and the introduction of roach (Rutilus rutilus L.), and (b) the decline of the wider European eel stock across its distribution range. The findings from this study have relevance for the wider management of the European eel stock.Item A comparison of the behaviour and survival of angling vs. trap-sampled S. salar smolts.(Wiley, 2022-06-15) Kennedy, Richard; Rosell, Robert S.; Campbell, Warren; Allen, Michelle M.; Del Villar-Guerra, diegoAn experiment was undertaken, using acoustic telemetry, to compare the survival and migratory timing of Salmo salar L. smolts sampled, under optimal conditions, in a traditional fixed Wolf trap against a sample of rod-caught fish captured using a sensitive angling technique. No significant difference was evident in survival with 83% of both samples detected in the river outflow, 67% of the trap and 76% of the rod samples were detected in coastal waters and finally 43% of the trap and 35% of the rod samples were detected on an offshore array c. 50 km from the river outlet. No significant difference was evident in the time taken for trap- and rod-sampled fish to reach either the river outflow, coastal or offshore waters. Angling, if undertaken sensitively, can provide an effective, resource-efficient and ethically justifiable sampling tool for juvenile salmonid age classes.Item In-river behaviour and freshwater return rates of sea trout, Salmo trutta L., from two coastal river populations(Wiley, 2022-07-22) Kennedy, Richard; Barry, James; Roche, William; Rosell, Robert S.; Allen, Michelle M.The effective management of anadromous Salmo trutta resources is challenging because long-term data on life history, phenology and survival are sparse and most stocks across the range are highly diverse and data-limited. The current study employed acoustic telemetry to tag 448 sea trout across three life stages, to describe the phenology, spawning behaviour and return rates of smolts, finnock (0+ sea age) and adult (≥1+ sea age) sea trout in two Irish river systems during 2018–2021. Tagged smolts (n = 206) exhibited river to sea transition rates of 78%–92% and a number of surviving smolts returned to their natal river as 0+ sea age finnock, exhibiting overall smolt to finnock return rates of between 6% and 17%. Short-term vagrancy occurred among smolts, and 14 individuals were detected in adjacent nonnatal rivers. Finnock tagged during the late summer (n = 205) exhibited a range of behaviours with a minority (<30%) ascending upstream to spawning areas. Tagged adult sea trout (n = 37) ascended upstream to the spawning grounds and between 50% and 80% successfully returned to sea as kelts after spawning. Subsequent return rates of kelts back to the river in the following year ranged from 9% to 40%. The current study indicated that body size was an influential predictor of behaviour and survival across all three life stages. Increased body size was positively associated with marine transition success in smolts, long-term marine survival in kelts and spawning behaviour in finnock. This work further demonstrates the complexity of sea trout life-history dynamics and provides a comparative perspective across different age classes. An understanding of life-history variation, behaviour and survival is fItem Long term abundance patterns of potamodromous brown trout in a large lacustrine catchment in County Fermanagh(Royal Irish Academy, 2021-09-29) Kennedy, Richard; Rosell, Robert S.; McElarney, Yvonne; Gallagher, Kevin; Allen, Michelle M.Long-term survey data detailing brown trout abundance in the Lough Erne catchment in Co. Fermanagh were tabulated from 1968–2016. These data included redd counts and electric fishing surveys across three key spawning tributaries in addition to gill-net surveys of the lake. The abundance of spawning adults fluctuated widely across the time-series and were examined in relation to various pressures, including a major disease epidemic and the invasion of the catchment by zebra mussels. A functional stock-recruitment relationship between adult spawners and young-of-year juveniles in the spawning tributaries was identified and described. Redd counts were significantly lower for the post-zebra mussel time-series (2000–16) than the pre-zebra mussel time-series (1968–99). The post-zebra mussel invasion period was associated with increased water clarity, reduced plankton productivity and changes to the balance of coarse fish species in the lake. The significance of these changes is discussed in relation to the trout stock.Item Some observations on the behaviour of lake-dwelling brown trout in Lower Lough Erne(Royal Irish Academy, 2021-02-23) Kennedy, Richard; Rosell, Robert S.; Allen, Michelle M.Acoustic telemetry was utilised to track a sample of 80 adult wild brown trout across an extensive array of 30 receivers in Lower Lough Erne during 2016 and 2017. The mean detection duration across the array was 142 days, and the majority of tagged fish were detected consistently in the northern basin of the lake. One year after tagging, c.40% of larger fish (>45cm LF) were still actively detected, while only 5-10% of smaller fish (<45cm LF) were detected on the array. In total, nine trout were recaptured by anglers between 2 and 1,152 days post-tagging, with a mean liberty time of 152 days, and a minimum angling exploitation rate of 11.25%. A high proportion of surviving tagged trout (>50%) did not undertake spawning migrations into an influent tributary and remained active within the lake during the reproductive period in November. In total, fourteen tagged trout undertook spawning migrations into a range of tributaries; the mean spawning sojourn was 54 days, and five fish (36%) did not return to the lake post spawning.