Natural Limitations and Bull Trout

Native trout are naturally limited by other environmental variables besides water temperature. These limitations occur at varying spatial scales and include both biotic (e.g., productivity, fish community, etc.) and abiotic features (amount of woody debris, substrate composition, water velocity, groundwater input, natural sediment loads, natural flow regimes, etc.). Further, persistence in suitable, but isolated habitats may be limited if connectivity with neighbouring populations is naturally restricted and supplementation or re-colonization following local population decline or extirpation is not possible. These naturally limiting factors mean that a specific native trout population may never and would not be expected to achieve the reference system capacity of 100%, which is set using the most abundant, pristine native trout populations in the province. Understanding the influence of natural limitations on native trout populations today is difficult due to the confounding spatial and temporal effects of human disturbance and harvest. However, the relative status of native trout prior to extensive human disturbance and harvest (i.e., early 1900s) was assessed by the Historic Adult Density (HAD) FSA score, which will be used as a surrogate measure of natural limitations within each watershed in the species historic range. The HAD score is ranked from 1 (very low native trout density) to 5 (very high native trout density), but for this purpose was converted to a percentage-based natural limitations value. It was assumed there is a linear relationship between system capacity and natural limitations (Figure 1).