Bell et al. 2021

Summary:

This paper investigated the effects of climate change (i.e., rising stream temperatures and changing hydrological conditions) on the distribution of five trout species in the northern Rocky Mountains of Montana. The authors used over 21,000 fish surveys to parameterize a model to explore past and future distributions of native cutthroat (Oncorhynchus clarkii lewisi) and bull trout (Salvelinus confluentus) and invasive brook (Salvelinus fontinalis), brown (Salmo trutta), and rainbow trout (Oncorhynchus mykiss). They found that by 2080, the occupancy of both native cutthroat and bull trout is expected to decrease by 16 and 39%, respectively. However, the decline in cutthroat occupancy was primarily the result of interactions with invasive species, whereas the decline in bull trout occupancy was the result of climate-induced changes. 

Key take-homes:

• Declines in native bull trout were primarily due to declines in summer flow and increases in summer stream temperatures, with little influence from invasive trout species ​within stream ecosystems.

• Declines in native cutthroat trout were primarily due to the presence of invasive species, particularly rainbow trout. Without invasive species present, cutthroat trout occupancy was predicted to increase by 26% by 2080 despite rapid changes in stream temperature and flow. 

• Invasive brown and rainbow trout are able to tolerate more extreme projected changes in climate than the native trout species. As a result, brown and rainbow trout are expected to occupy a greater amount of habitat than if climate change did not occur.

Management Implications:

• Both native cutthroat and bull trout had low rates of colonization, suggesting that native species are unlikely to recolonize streams once they are gone from that location.

• The intensity of climate change matters. If climate change proceeds faster than expected, more native trout are expected to be lost and invasive species will be favored. 

• The distinct threats to native cutthroat and bull trout populations highlight the need for tailored conservation, management, and adaptation strategies. Conservation of cutthroat trout should be aimed at reducing invasive species while conservation of bull trout should be aimed at protecting and restoring cold-water habitats.