Concurrent habitat and life history influences on effective/census population size ratios in stream‐dwelling trout

Abstract

Lower effective sizes (Ne) than census sizes (N) are routinely documented in natural populations, but knowledge of how multiple factors interact to lower Ne/N ratios is often limited. We show how combined habitat and life-history influences drive a 2.4- to 6.1-fold difference in Ne/N ratios between two pristine brook trout (Salvelinus fontinalis) populations occupying streams separated by only 750 m. Local habitat features, particularly drainage area and stream depth, govern trout biomass produced in each stream. They also generate higher trout densities in the shallower stream by favoring smaller body size and earlier age-at-maturity. The combination of higher densities and reduced breeding site availability in the shallower stream likely leads to more competition among breeding trout, which results in greater variance in individual reproductive success and a greater reduction in Ne relative to N. A similar disparity between juvenile or adult densities and breeding habitat availability is reported for other species and hence may also result in divergent Ne/N ratios elsewhere. These divergent Ne/N ratios between adjacent populations are also an instructive reminder for species conservation programs that genetic and demographic parameters may differ dramatically within species.

Effective number of breeders, effective population size and their relationship with census size in an iteroparous species,Salvelinus fontinalis

“…These results suggest that published estimates of effective population size obtained with random samples of individuals of mixed ages for iteroparous species with overlapping generations and even those based on unadjustedN b can be biased, and should thus be considered with caution. As seen in other studies [15][16][17], we also found thatN eðadj2Þ /N c ratios were variable across populations in the same geographical area, ranging from a high of 0.29 (CY) to a low of around 0.01 (e.g. CV, TB).…”

“…Resident salmonid populations inhabiting small streams generally exhibit relatively short generation times, facilitating the study of the relationship between effective and census population sizes. Recent studies have produced variable estimates of N b /N c for stream brook trout populations, with large differences among studies as well as among populations within studies in these ratios [15][16][17]. The differences have been linked to differences in habitat variability and habitat quality, which in turn result in differences in census population size and potentially also in life-history traits such as age and size at maturation, and age-specific survival or reproductive lifespan [15,17].…”

Population variation in density‐dependent growth, mortality and their trade‐off in a stream fish

“…positive logarithmic; Bellows, ), but did not support our initial prediction that mortality would increase exponentially after habitat saturation. Thus, it is unlikely that space was a limiting factor in our experimental sites (Grant & Kramer, ), perhaps because food abundance is low in Cape Race (Belmar‐Lucero et al, ; Hutchings, ). In their review of density‐dependent growth across salmonid species, the high frequency of logarithmic density‐dependent growth led Grant and Imre () to suggest that stream‐dwelling populations are regulated by density‐dependent growth at low densities and density‐dependent mortality at higher densities.…”

“…Consequently, boulder coverage may contribute to weakening density‐dependent growth in populations that exhibit a strong boulder gradient, such as FW (Figure ). Conversely, starvation and stress are assumed to be the main causes of mortality in our streams due to the low food abundance and negligible predation risk (Belmar‐Lucero et al, ; Hutchings, ). Thus, increasing temperature and flow may increase mortality rates by increasing energetic costs (Smith & Li, ; Utz & Hartman, ), while acidic pH is a well‐known driver of mortality in brook trout of this system (Yates, ).…”

“…Each section was randomly assigned one of four YOY stocking densities (0.3, 1, 3 and 7 fish/m 2 ) for a total of 36 sections per year replicated over three consecutive summers (2016–2018). These densities were selected to be representative of those experienced in the study streams – FW and WN can exhibit densities as low as 0.3 fish/m 2 (Hutchings, ; unpublished ), while densities in BC can reach upwards of 5 fish/m 2 locally (Belmar‐Lucero et al, ; unpublished ). We aimed for an experimental duration of 21 days, which is sufficient to measure changes in salmonid growth and mortality (Le Cren, ; Elliott, ).…”