3.3 | Colorado Pikeminnow diet composition
To estimate Colorado Pikeminnow contemporary and historical diet composition, we used a Bayesian stable isotope mixing model in the R package MixSIAR (Stock et al ., 2018). We used this model to estimate the posterior probability distributions of the proportion each fish and invertebrate prey contributed to Colorado Pikeminnow’s isotopic signatures. The model accounts for uncertainty resulting from multiple prey items, variation in signatures among prey and the predator, and isotopic fractionation (trophic discrimination factors) between predator and prey. However, for the mixing model to be informative, isotope signatures among prey items need to be statistically dissimilar (Stock et al ., 2018). Therefore, we first estimated isotopic differences in prey items using two Bayesian ANOVAs (one for δ13C and one for δ15N) with vague priors and assessed significant differences using posterior credible intervals with the R packagebrms (Bürkner, 2017). All prey items except Flannelmouth Sucker and Speckled Dace were dissimilar by at least one isotope for both time periods but we kept all species separate with the understanding that Flannelmouth Sucker and Speckled Dace posterior probability distributions would be similar and less uniquely informative. We excluded Channel Catfish as potential prey because a laboratory feeding study suggested it is unlikely to greatly contribute to Colorado Pikeminnow’s diet (Gilbert et al ., 2018).
Because we lacked historical invertebrate samples, we used contemporary invertebrate samples to estimate historical invertebrate isotopic values. To do so, we assumed a mean isotopic shift in invertebrates from the contemporary to historical period but retained the variability observed in contemporary samples. Following this assumption, we estimated historical invertebrate δ13C and δ15N values by calculating the mean difference of δ13C and δ15N between periods for known invertivorous fishes (Flannelmouth Sucker, Speckled Dace, and Fathead Minnow) and then subtracted this from each contemporary invertebrate isotope value (mean value shift of δ13C = +0.32 and δ15N = +3.14).
We used one mixing model for each time period to assess changes to the resource use of Colorado Pikeminnow. To complete input data for each mixing model, we used separate trophic discrimination factors (mean and standard deviation) for potential fish prey (∆13C = 0.82 ± 0.10 and ∆15N = 2.76 ± 0.35) and invertebrates (∆13C = 0.83 ± 0.29 and ∆15N = 2.93 ± 0.23) as estimated by Franssen et al . (2017). Priors for the proportion of prey in the diet for each model included 0.05 for invertebrates with the remaining proportion distributed evenly among fish prey species. We chose these prior probabilities because the most comprehensive Colorado Pikeminnow diet study suggested the species converts entirely to piscivory by 150 mm TL (Vanicek and Kramer, 1969), although a more recent stable isotope analysis suggested invertebrates contributed to the diet of fish as large as 300 mm TL (Franssen et al ., 2019). We computed each mixing model with three chains of 3,000,000 iterations, discarded the first 200,000, and thinned chains by retaining every 100th draw (Stock et al ., 2018). We again determined model convergence (all R-hat <1.01) and report predicted posterior distributions of proportional contributions of each prey item to the diet of Colorado Pikeminnow between time periods. As with all prior analysis, we assessed changes between periods by subtracting posterior probability distributions of interests to determine the probability the difference was greater than zero.