Exploring the role of temperature and possible alternative stable states in brook trout (Salvelinus fontinalis) population structure
Most organisms undergo ontogenetic changes, leading to complex life histories. An organism’s prey and habitat preferences change as they age, as may their responses to the same environmental stressors. Therefore, a changing environment may confer a competitive advantage to a particular life stage, leading to population structures dominated by one stage or another. I used survey data of brook trout (Salvelinus fontinalis) in lakes across the Sierra Nevada to investigate if there was evidence for (1) alternative stable states between populations dominated by small or large fish, and (2) trends in population structure across elevation to suggest that temperature plays a key role in determining size structure. I characterized the average fish size, number of size classes, and the evenness of the size class distribution for 42 populations. I found these features by fitting a Bayesian normal mixture model using Markov Chain Monte Carlo simulations. I tested for alternative stable states by looking for bimodality in each feature, and I analyzed each feature’s relationship with elevation using Spearman’s rank correlation. I found that that populations are most often dominated by small individuals, rarely by large individuals, and most often contain either one or two distinct size classes. If large individual body sizes occur as an alternative population state, my results show that such populations occurred rarely, or were not well sampled by these surveys. High elevation was associated with reduced average size and greater numbers of size classes, suggesting that temperature does affect size distributions.