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Kootenay Lake is a large, oligotrophic waterbody in southern British Columbia renowned for recreational fisheries for piscivorous rainbow trout (Oncorhynchus mykiss) and bull trout (Salvelinus confluentus). Long-term datasets showed an increase in large-bodied (>2 kg) piscivore abundance followed by a collapse of the kokanee (Oncorhynchus nerka) prey population in 2013 and subsequent decline of large-bodied piscivores. An unprecedented post-collapse state formed in 2015–2018, characterized by low kokanee spawner abundance and biomass and high catch rates for small-bodied (<2 kg), slow-growing piscivores. Bioenergetics model estimates of average historical (1961–2008) piscivore consumption was 29.3% of the average historical (1993–2008) kokanee prey supply (biomass and production), but increased to 78.7% in 2011, immediately preceding kokanee collapse. From 2015–2018, kokanee did not recover due to persistently poor juvenile survival; estimated piscivore consumption relative to prey supply remained high (73.0%), suggesting that kokanee were trapped in a predator pit. Although the ultimate and interacting causes of the predator build up remain uncertain, overcoming current depensatory dynamics may be aided by kokanee stocking or increasing harvest on still-abundant, unsatiated piscivores. [ABSTRACT FROM AUTHOR]

In this experiment, competition was observed among native bull trout ( Salvelinus confluentus (Suckley, 1859)) and non-native brook trout ( Salvelinus fontinalis (Mitchill, 1814)) in artificial streams. In intraspecific competitions, brook trout engaged in territorial interference foraging strategies, and their foraging success was correlated with occupancy of the lead position in the stream. No correlation was apparent for bull trout, most of which engaged in nonterritorial scramble foraging tactics. In interspecific competitions, four stream environments were constructed in which fish density, habitat complexity, and current velocity were altered. Bull trout outcompeted brook trout for food in simple pool habitat devoid of cover when competition was head-to-head (density = 3 fish·m−2) between the species. When competitor number was doubled in this habitat, the two became equal competitors. At this higher density, bull trout again outcompeted brook trout for food when the habitat was changed to a complex riffle with substrate cover. Brook trout were more aggressive towards bull trout than vice versa, and interspecific aggression was decreased by low density, cover, and high stream velocity. Territorial brook trout aggressively interfere with their competitor for access to resources, but the success and intensity of this tactic against bull trout may be mitigated by environmental factors. [ABSTRACT FROM AUTHOR]

An information-theoretic approach was used to determine the association between brook trout ( Salvelinus fontinalis) invasiveness and a suite of abiotic and biotic factors, measured at 80 sites from 51 streams in the Canadian Rockies. The streams selected had confirmed brook trout invasions and were identified as current or historical nursery habitat for native bull trout ( Salvelinus confluentus). The biomass at most sites was strongly or completely dominated (95%-100%) by one species or the other, and sites were classified as having high brook trout invasiveness (>60% of the biomass of the community relative to remnant bull trout) or low invasiveness (<40%). Among abiotic factors, high brook trout invasiveness was positively associated with stream temperature and undercut bank habitat, but negatively associated with large in-stream substrate (cobbles and boulders). Among biotic factors, brook trout invasiveness was negatively associated with co-occurring rainbow trout ( Oncorhynchus mykiss) or brown trout ( Salmo trutta), two other introduced species. Brook trout appear to dominate communities over native bull trout where thermal or habitat niche opportunities are provided for them, although other non-native species may restrict their invasion into bull trout streams. [ABSTRACT FROM AUTHOR]