Abstract
In organisms with flexible and indeterminate growth, demographic density dependence can affect both individual fitness and population dynamics. Recent work in stream salmonids suggests that individual growth rates can be depressed even at low population densities, which warrants examination of populations previously assumed to be below carrying capacity. We investigated the effects of population density on individual growth rates in a threatened population of steelhead (Oncorhynchus mykiss) in a tributary to the Clearwater River in Idaho, USA. We followed a mark‐recapture design where we visited 16 study sites on average five times per year from 2010 to 2012. The 95% confidence interval for average growth rates (% change in body mass per day) for subyearlings were [0.82, 1.0]) and [0.16, 0.30] for yearling steelhead. Variance decomposition showed that the variation in growth rates could be attributed equally to individual‐ and visit‐level factors in subyearlings, whereas almost two‐thirds of the variance in yearling growth rates could be attributed to individual‐level factors. Growth rates in the subyearling age class were negatively related to the densities of yearling steelhead, but not vice versa. Yearling growth rates showed no evidence of density dependence. Finally, density in interaction with water temperature did not affect growth rates of either age class. Our results demonstrate that density dependence can pose constraints on individual growth rates at low population densities (<1 fish m−2) in stream salmonids, and underscore the importance of considering age classes separately when studying density dependence in age‐structured populations.
Highlights
Wild populations are regulated if they persist for many generations, are bounded at some level above zero and below infinity, and show a return tendency towards some long-term mean (Turchin 1995, Hixon et al 2002)
We study the effects of population density on individual growth rates in juvenile age classes of steelhead (Oncorhynchus mykiss) in a population listed as threatened under the Endangered Species Act (NMFS 2010)
There is evidence in the salmonid literature of intercohort competition in age-structured populations (Nordwall et al 2001, Kaspersson and Hojesjo 2009). To examine these potentially differential effects of age class density on individual growth rates, we explicitly modeled the effects of each age class and the total density
Summary
Wild populations are regulated if they persist for many generations, are bounded at some level above zero and below infinity, and show a return tendency towards some long-term mean (Turchin 1995, Hixon et al 2002). The mechanism responsible for population regulation is demographic density dependence, because abiotic factors alone cannot produce a return tendency (Haldane 1953, Hixon et al 2002). Density dependence occurs when present or past population size affects the population’s per capita growth rate (Sinclair and Pech 1996, Turchin 1999). Whereas mortality directly affects the numbers of individuals in an age class, birth rates can be MYRVOLD AND KENNEDY affected via subsequent changes to the individuals’ fitness, which in turn can affect reproductive success and future population size (Rose et al 2001)
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