Environmental conditions modify density‐dependent salmonid recruitment: Insights into the 2016 recruitment crash in Wales

Abstract

1. Understanding the effects of density-dependent and density-independent factors on recruitment is often inhibited by difficulties quantifying their relative contributions in highly variable recruitment data. Use of data-driven statistical methods with data that include one or more extreme recruitment events could help overcome these difficulties. 2. Juvenile Atlantic salmon and trout abundances in Wales have declined over the last 2 decades, and 2016 was a notably poor recruitment year in rivers around southern Europe, including England and Wales. The 2016 recruitment crash coincided with extreme winter weather conditions, leading to speculation that unusually warm temperatures and high flows adversely affect salmonid recruitment and caused the 2016 crash, although this remains untested. 3. We developed data-driven statistical models to: (1) describe juvenile salmonid recruitment from density-dependent and density-independent factors; and (2) assess whether the density-independent factors probably contributed to the 2016 salmon recruitment crash. We compiled salmon and trout young-of-year juvenile abundances from electrofishing surveys, egg deposition estimates and river flow and air temperature data from 2001-2017 for seven Welsh rivers, broadly representative of rivers around Wales. We used river flow and air temperature data to derive ecologically and behaviourally meaningful density-independent explanatory variables. 4. Salmonid recruitment in Wales was best described using density-dependent and density-independent factors, especially for salmon: after accounting for a concave relationship with egg deposition, salmon juvenile abundance was reduced under (1) warmer spawning temperatures that might inhibit spawning, and (2) higher flood frequencies during pre-emergence and emergence that might washout eggs or alevins. Results were less clear for trout, perhaps because they are behaviourally more plastic. 5. Our findings provide empirical support for general and predictable effects of temperature and flow during spawning and emergence on salmonid-especially salmon-recruitment in Wales. Furthermore, we suggest that the 2016 salmon recruitment crash was caused-in part-by particularly inclement spawning and

Medium‐term environmental changes influence age‐specific survival estimates in a salmonid population

“…As other salmonid species spawn earlier than grayling, high winter flow events can be detrimental to early life-stages by causing egg wash-out (Warren et al, 2015) and have been shown to contribute to reduced survival of juvenile Atlantic salmon Salmo salar (Gregory et al, 2020). Consequently, it has been suggested that flow management, i.e.…”

“…Although the effect of winter flows on juvenile survival was relatively weak (perhaps because it did not capture variance in flows, discussed below), this finding suggests a need to reconcile habitat management strategies for salmonids with different life histories. As other salmonid species spawn earlier than grayling, high winter flow events can be detrimental to early life‐stages by causing egg wash‐out (Warren et al., 2015) and have been shown to contribute to reduced survival of juvenile Atlantic salmon Salmo salar (Gregory et al., 2020). Consequently, it has been suggested that flow management, i.e.…”

Environmental threats and conservation implications for Atlantic salmon and brown trout during their critical freshwater phases of spawning, egg development and juvenile emergence

“…The combination of all or some of these factors will most likely result in unfavourable conditions for egg development and subsequent recruitment. Gregory et al (2020) found that the 2016 salmon recruitment crash in Wales was most likely caused by the unfavourable combination of warm spawning temperatures, which can inhibit spawning, and higher flood frequencies during egg incubation and emergence, resulting in washouts of eggs and alevins. Such inclement conditions could become more common under future climate change.…”

Warm winters and cool springs negatively influence recruitment of Atlantic salmon (Salmo salar L.) in a southern England chalk stream

“…In 2016, juvenile salmon numbers in England and Wales were among the lowest on record, and it was speculated that these were caused by an unusually warm winter and wet spring (ICES, 2017). A recent study presented evidence suggesting that winter and spring temperatures and discharges were associated with this ‘2016 recruitment crash’ in seven rain‐fed rivers throughout Wales (Gregory et al ., 2020). We observed similarly low juvenile abundance in 2016 on the River Frome, a primarily groundwater‐fed chalk stream in southern England characterized by relatively benign temperature and discharge regimes (Berrie, 1992).…”

“…Following Gregory et al . (2020), a flood was defined as a high pulse when the discharge was between three and five times above the 50% (Q50) river level rate, and the number of floods during pre‐emergence and emergence was calculated from the discharge data. We used River Lab Long Term Monitoring (RLLTM) programme temperature data collected near the East Stoke gauging station (Supporting Information Figure S3) to calculate the mean water temperature during spawning and emergence.…”