The role of density-dependent individual growth in the persistence of freshwater salmonid populations

Abstract

Theoretical and empirical models of populations dynamics have paid little attention to the implications of density-dependent individual growth on the persistence and regulation of small freshwater salmonid populations. We have therefore designed a study aimed at testing our hypothesis that density-dependent individual growth is a process that enhances population recovery and reduces extinction risk in salmonid populations in a variable environment subject to disturbance events. This hypothesis was tested in two newly introduced marble trout (Salmo marmoratus) populations living in Slovenian streams (Zakojska and Gorska) subject to severe autumn floods. We developed a discrete-time stochastic individual-based model of population dynamics for each population with demographic parameters and compensatory responses tightly calibrated on data from individually tagged marble trout. The occurrence of severe flood events causing population collapses was explicitly accounted for in the model. We used the model in a population viability analysis setting to estimate the quasi-extinction risk and demographic indexes of the two marble trout populations when individual growth was density-dependent. We ran a set of simulations in which the effect of floods on population abundance was explicitly accounted for and another set of simulations in which flood events were not included in the model. These simulation results were compared with those of scenarios in which individual growth was modelled with density-independent Von Bertalanffy growth curves. Our results show how density-dependent individual growth may confer remarkable resilience to marble trout populations in case of major flood events. The resilience to flood events shown by the simulation results can be explained by the increase in size-dependent fecundity as a consequence of the drop in population size after a severe flood, which allows the population to quickly recover to the pre-event conditions. Our results suggest that density-dependent individual growth plays a potentially powerful role in the persistence of freshwater salmonids living in streams subject to recurrent yet unpredictable flood events.

Modeling the influence of flow on invertebrate drift across spatial scales using a 2D hydraulic model and a 1D population model

“…With additional assumptions, they have been used to predict larger spatial patterns of biomass distribution or capacity (Grossman et al, 2002;Hayes et al, 2007;Hughes, 1998), but a remaining challenge is linking spatial patterns of growth and survival to population viability (Anderson et al, 2006b;Armstrong and Nislow, 2012;Frank et al, 2011;Locke et al, 2008). Individual-based modeling (IBM) approaches based on the bioenergetics of specific life stages for individual fish have been the most successful at this integration and have shown great utility in river management contexts (Van Winkle et al, 1998;Vincenzi et al, 2008). Most current IBMs are parameter rich, requiring extensive data and/or natural history knowledge of the particular system for calibration (Breckling et al, 2006).…”

Genetic and life-history consequences of extreme climate events

“…not following extreme events) conditions, but they may be more likely to successfully reproduce when fewer older fish compete for mates. Vincenzi et al [29,39] found in theoretical work that faster growth due to lower density and younger age at reproduction due to size-dependent sexual maturity-or weaker competition for mates-should increase the probability of persistence of marble trout populations affected by extreme events.…”

“…In the case of flash floods and debris flows affecting marble trout, survival of fish is thus expected to be largely determined by chance; for However, the conditions created by such extreme events in the years following their occurrence (e.g. greater per capita resources such as food and space due to transient low population density and mobilization of nutrients, fewer older fish) may increase the prevalence of more opportunistic life histories through selection, phenotypic plasticity or changes in population age and size structure [18,29].…”

“…Sex was assigned using a molecular genetic assay developed for marble trout using the sequence information described in [28]. The movement of marble trout is very limited, and the majority of fish were sampled within the same 50 -100 m stream reach throughout their lifetime [29]. The oldest fish sampled in Lipo and Zak were 8 and 12 years old, respectively.…”

Extinction risk and eco-evolutionary dynamics in a variable environment with increasing frequency of extreme events

“…Variability exists within fish populations in terms of optimal and critical temperatures for life histories such as growth, survival and reproduction [18], and thus traits related to thermal means and extremes can be selected for [19]. On the contrary, severe floods-especially when swift, aseasonal and with longer recurrence interval than species' generation time [20]-are likely to cause high and largely non-selective mortalities, for example by scouring eggs or killing fish by impact with rocks and boulders [21]. These non-selective population and ( potential) genetic bottlenecks [22] caused by a point extreme are likely to contribute to the erosion of adaptive potential of populations (i.e.…”