Non‐native species and lake warming negatively affect Arctic charSalvelinus alpinusabundance; deep thermal refugia facilitate co‐existence

Abstract

This study finds that non-native species and warming temperatures have significant negative effects on Arctic char Salvelinus alpinus abundance in Irish lakes. Eutrophication was not important at the range of total phosphorus tested (0.005-0.023 mg l −1 ). Model results predict that S. alpinus occur across the temperature range sampled (8.2-19.7 C) when non-natives are absent, but S. alpinus catch is predicted to be close to zero irrespective of temperature when non-native catch is high. This result indicates that to persist, S. alpinus may require a habitat where non-natives are at low abundance or absent. Salvelinus alpinus segregated from other species along the thermal axis, inhabiting significantly colder water and actively avoided non-native species, which appeared to limit their distribution. The thermal niche realized by S. alpinus in non-native dominated lakes was thus compressed relative to native dominated lakes and S. alpinus population density was significantly lower. These findings were consistent even when the only non-native present was Perca fluviatilis. Temperature appeared to limit the distribution of non-native species, such that the presence of deep thermal refugia is currently facilitating S. alpinus co-existence with non-natives in associated lakes. Diet analysis identified P. fluviatilis as potential predators and competitors. This study provides strong evidence that non-native species are a key driver of low S. alpinus abundance in Irish lakes. Temperature increases associated with climate change are identified as a secondary concern, as they could erode S. alpinus' thermal niche and lead to their extirpation. The lower thermal buffering capacity of shallow lakes identifies these as higher risk systems. Salvelinus alpinus conservation in Ireland should focus on preventing future illegal non-native species introductions because unlike other stressors (e.g., eutrophication etc.), species introductions are rarely reversible.

Potential climate change impacts on Arctic char Salvelinus alpinus L. in Ireland

“…Morrissey‐McCaffrey, Kelly, Shephard and Kelly‐Quinn () found Arctic char in Irish lakes were widely distributed in all habitats. Habitat use by Arctic char was more restricted in terms of depth and habitat in the lakes where mixed fish communities occurred.…”

“…Morrissey‐McCaffrey et al. () and Kelly et al. (, ) found strong evidence that mixed fish species introductions are the major driver of Arctic char decline through competitive exclusion of Arctic char from the most productive areas of these lakes.…”

“…Morrissey-McCaffrey, Kelly, Shephard and Kelly-Quinn (2018) found Arctic char in Irish lakes were widely distributed in all habitats. were separated according to temperature and habitat where mixed fish communities were present.…”

“…Smith and Kwak (2015) fish tagging results revealed fish mortality, displacement and increased activity that occurred due to disturbance from a flood event as a result of a storm. -McCaffrey et al (2018) and Kelly et al (2014Kelly et al ( , 2017 found strong evidence that mixed fish species introductions are the major driver of Arctic char decline through competitive exclusion of Arctic char from the most productive areas of these lakes. Irish Arctic char are currently classified as "vulnerable" (King et al, 2011) and a number of non-climate drivers are contributing to the pressures on these populations.…”

Climate warming is predicted to enhance the negative effects of harvesting on high‐latitude lake fish

“…This expectation assumes that the outcomes of interactions with other species do not change. For instance, studies on Arctic charr closer to the southern border of its distributional range indicate that an experienced temperature rise alone did not affect the production of such an Arctic charr population, but when non‐native fish (percids) were abundant, Arctic charr population's production decreased with increasing temperature (Morrissey‐McCaffrey, Shephard, Kelly, & Kelly‐Quinn, ). Our projections of changes in growth and stock biomass are robust to small (±5%) changes in growth parameters ( L ∞ and K max ) and qualitatively consistent across the investigated range of natural‐mortality parameters (Appendix ).…”