Interspecific competition reduces the performance of Atlantic salmon, Salmo salar: implications for restoration programs

Competition from non-native salmonids is potentially impairing efforts to restore Atlantic salmon (Salmo salar) to parts of their historical range. In three separate meta-analyses, we collected 104 effect sizes from 25 published papers to quantify the effect of both native and non-native salmonids on the performance (i.e. behaviour, habitat use, growth and survival) of Atlantic salmon. The presence of other species had negative effects on the performance of Atlantic salmon; in particular, non-native species and brown trout (Salmo trutta), whether native or non-native, had the most negative effects. Contrary to our predictions, the negative effects of other species were not exacerbated in laboratory compared to field studies, and did not increase with total salmonid abundance, or the relative body size of the competitors. However, most studies in our analysis were conducted under laboratory conditions and at densities much higher than found in nature. Thus, a realistic assessment of the potential success of restoration programs when interspecific competitors are present should include more studies conducted under natural conditions.

Temperature dynamics in Klamath River off-channel floodplain restoration sites depend on location within the watershed

The mid-Klamath River is heavily impacted by altered streamflow and warm water temperatures, which contribute to the decline of native salmonids. In an effort to restore critical salmonid habitat, the Karuk Tribe, National Forest Service, and Mid-Klamath Watershed Council have created a variety of off-channel floodplain ponds that provide habitat for juvenile Coho Salmon (Oncorhynchus kisutch) and other juvenile salmonids such as Steelhead (O. mykiss). One purpose of these ponds is to provide cool water refuges for juvenile salmonids during high summer water temperatures. However, no studies have quantified how these ponds vary in temperature regimes across the river floodplain. In July 2020, we placed 30 temperature sensors in 9 off-channel ponds and 2 creeks (Seiad Creek and Horse Creek) in the mid-Klamath floodplain. We used a multivariate auto-regressive state space (MARSS) models to determine the number and spatial arrangement of distinct thermal regimes in floodplain ponds and tributaries. We found that pond temperatures have lower daily maximums and fluctuate less than tributary temperatures. We also found that Seiad Creek, Seiad Creek ponds, Horse Creek, and Horse Creek ponds all have different patterns of temperature change throughout the summer. Historical data (2010-2019) for Alexander and Stender Ponds showed that over time, daily fluctuations in pond water temperature became less drastic. This pattern was also observed by MKWC in their reports on Alexander and Stender Ponds (MKWC 2020; Wickman et al. 2020). More stable water temperatures in the ponds contrast to creek temperatures, which continue to fluctuate widely on a daily basis during summer. Fish monitoring data from MKWC show that coho growth rates are higher in these two ponds, which suggests that coho experience a metabolic benefit from more stable water temperatures (MKWC 2020; Wickman et al. 2020). Overall, our analysis provides deeper insight into the thermal benefits of floodplain habitats and off-channel ponds on the mid-Klamath River, and informs the future collection of fish data that will reveal more precise information about how floodplains benefit salmonids.

Conservation of endangered galaxiid fishes in the Falkland Islands requires urgent action on invasive brown trout

Non-native salmonids are protected in the Southern hemisphere where they sustain aquaculture and valuable sport fisheries, but also impact on native galaxiid fishes, which poses a conservation conundrum. Legal protection and human-assisted secondary releases may have helped salmonids to spread, but this has seldom been tested. We reconstructed the introduction of brown trout (Salmo trutta) to the Falkland Islands using historical records and modelled its dispersal. Our results indicate that establishment success was ~88%, and that dispersal was facilitated by proximity to introduction sites and density of stream-road crossings, suggesting it was human assisted. Brown trout has already invaded 54% of Falkland rivers, which are 2.9-4.5 times less likely to contain native galaxiids than uninvaded streams. Without strong containment we predict brown trout will invade nearly all suitable freshwater habitats in the Falklands within the next ~70 years, which might put native freshwater fishes at a high risk of extinction

The interactive effects of stream temperature, stream size, and non-native species on Yellowstone cutthroat trout

Climate change and non-native species are considered two of the biggest threats to native salmonids in North America. We evaluated how non-native salmonids and stream temperature and discharge were associated with Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) distribution, abundance, and body size, to gain a more complete understanding of the existing threats to native populations. Allopatric Yellowstone cutthroat trout were distributed across a wide range of average August temperatures (3.2 to 17.7ºC), but occurrence significantly declined at colder temperatures (<10 ºC) with increasing numbers of non-natives. At warmer temperatures occurrence remained high, despite sympatry with non-natives. Yellowstone cutthroat trout relative abundance was significantly reduced with increasing abundance of non-natives, with the greatest impacts at colder temperatures. Body sizes of large Yellowstone cutthroat trout (90th percentile) significantly increased with warming temperatures and larger stream size, highlighting the importance of access to these more productive stream segments. Considering multiple population-level attributes demonstrates the complexities of how native salmonids (such as Yellowstone cutthroat trout) are likely to be affected by shifting climates.

Environmental DNA revealed the fish community of Hokkaido Island, Japan, after invasion by rainbow trout

In freshwater ecosystems, invasive salmonid fishes can have a significant impact on native fish species. Detecting the invasion and its negative effects is critical for the conservation of native fish communities. We examined the species composition and seasonal changes in the freshwater fish community, including salmonids, on the Kamikawa Plain, Hokkaido Island, Japan, using environmental DNA (eDNA) metabarcoding. We detected 23 fish species in 176 samples collected from 16 sites over 12 months (October 2018 – August 2019). Between 11 and 20 species were detected at each site, including five native salmonids (Oncorhynchus masou, Oncorhynchus keta, Parahucho perryi, Salvelinus leucomaenis leucomaenis and Salvelinus malma krascheninnikova). The invasive alien rainbow trout Oncorhynchus mykiss was detected at all 16 sites and it was the most commonly detected salmonid. Although we found no obvious competitive exclusion of native salmonids by rainbow trout in the study area, the invasive species occurred more often and at more sites than any of the natives. We also determined the occurrence and seasonal changes in the fish community, classified as native salmonids, invasive rainbow trout, Cypriniformes and other benthic fishes. There were fewer species overall in winter, but the sites with higher species richness in winter were on the lower reaches of the river. In addition, we detected domestic invaders, such as the topmouth gudgeon, Pseudorasbora parva, although they were less prevalent than rainbow trout. These results show the effectiveness of eDNA metabarcoding, which can be used for surveying species richness at an ecosystem scale. In particular, the detection of the early stages of establishment and spread of invasive species can be achieved by eDNA monitoring.

Stable isotope niche convergence in coexisting native and non-native salmonids across age classes

Niche divergence resulting from coevolution is commonly believed to favour coexistence among competing species; however, recent investigations have demonstrated that an unexpected niche convergence can occur when native and non-native species coexist. Yet, our understanding of the ontogenetic characteristics of this niche convergence remains limited. In the present study, we quantified the stable isotope niche of native brown trout (Salmo trutta) in allopatry and sympatry with non-native brook trout (Salvelinus fontinalis) across four age classes. Our results demonstrated that brown trout displayed a stable isotope niche closer to brook trout in sympatry than in allopatry, which was likely driven by an increased consumption of terrestrial invertebrates by sympatric brown trout. Stable isotope niche overlap was the strongest for young-of-the-year individuals and the intensity of overlap between sympatric native brown trout and non-native brook trout decreased during ontogeny. These findings indicate that niche convergence between the species occur at the earliest age class of the native species and are maintained across ontogeny.