Role of maturation and mortality in portfolio effects and climate resilience

Abstract

The portfolio effect plays a critical role in population productivity and stability. Age structure of spawning salmon represents an example of portfolio effects such that the risks of experiencing unfavorable conditions are spread across time. However, the distribution of maturation ages for Pacific salmon (Oncorhynchus spp.) is increasingly concentrated into fewer and younger ages, which may impact population resilience to climate change. We explored the population dynamics of Sacramento River fall-run Chinook salmon (O. tshawytscha) under different age structure scenarios using a life-cycle model and compared two mechanisms that can underlie these changes – mortality and maturation. In addition, we tested whether age structure promotes resilience to drought. We found that high age structure diversity increased the stability of population size and harvest compared with low diversity. However, mean population size responded differently depending on the underlying mechanism. Reduced mortality of adult fish ages 4-5 increased escapement whereas delayed maturation decreased escapement. Overall, high age structure diversity was able to buffer against the adverse effects of droughts by reducing the variability of population size and harvest compared with low diversity. Results suggest that age structure promotes stability of salmon in an increasingly variable climate.

Local buffer mechanisms for population persistence

“…For instance, at the metapopulation level, among-individual variation in the proportion of time spent in freshwater and the ocean (a lifehistory trait) led to asynchronous population dynamics in sockeye salmon (Oncorhynchus nerka) [33]. This effect also holds at the population level, as recently shown for chinook salmon (Oncorhynchus tshawytscha) [34]. Disentangling the effects of spatial heterogeneity and among-individual trait variation is challenging [35], yet crucial for understanding their interaction with other buffer mechanisms and for predicting the fate of populations.…”

Temporal allele frequency changes in large‐effect loci reveal potential fishing impacts on salmon life‐history diversity

“…Diversity in life‐history traits, such as age at maturity or migration timing, may buffer populations against environmental changes, and can reduce variability among years in the numbers of fish that return to spawn and can be sustainably harvested from salmonid populations (e.g. Carvalho et al., 2023; Connors et al., 2022; Cordoleani et al., 2021; Gharrett et al., 2013; Hoelzel et al., 2019; Schindler et al., 2010). Fishing has been shown to cause evolutionary changes in the mean age and size at maturity of salmon populations (Czorlich et al., 2022).…”

Growth and longevity of the endangered freshwater pearl mussel (Margaritifera margaritifera): Implications for conservation and management

“…The comparatively high life expectancy of FPM may enable high age structure diversity within populations, and therefore promote population stability and resilience to adverse environmental conditions (Carvalho et al, 2023;Haag & Rypel, 2011;Roff, 1992).…”

“…FPM's long reproductive life span may represent an example of portfolio effect where the risk of experiencing unfavourable environmental conditions and reproductive failure is spread over time and multiple age classes (Carvalho et al, 2023;Schindler et al, 2010).…”

“…This may enhance the reproductive output of the population and maintain the population between favourable recruitment events (Berkeley et al, 2004;Carvalho et al, 2023). The risk spreading, however, may not be effective if high juvenile mortality is prolonged over decadesa situation that currently describes most FPM populations (Geist, 2010).…”