Seasonal mortality and the effect of body size: a review and an empirical test using individual data on brown trout

Abstract

Summary 1.For organisms inhabiting strongly seasonal environments, over-winter mortality is thought to be severe and size-dependent, with larger individuals presumed to survive at a higher rate than smaller conspecifics. Despite the intuitive appeal and prevalence of these ideas in the literature, few studies have formally tested these hypotheses. 2. We here tested the support for these two hypotheses in stream-dwelling salmonids. In particular, we combined an empirical study in which we tracked the fate of individually-marked brown trout across multiple seasons and multiple years with a literature review in which we compiled the results of all previous pertinent research in stream-dwelling salmonids. 3. We report that over-winter mortality does not consistently exceed mortality during other seasons. This result emerged from both our own research as well as our review of previous research focusing on whether winter survival is lower than survival during other seasons. 4. We also report that bigger is not always better in terms of survival. Indeed, bigger is often worse. Again, this result emerged from both our own empirical work as well as the compilation of previous research focusing on the relationship between size and survival. 5. We suggest that these results are not entirely unexpected because self-sustaining populations are presumably adapted to the predictable seasonal variation in environmental conditions that they experience.

Inadequate food intake at high temperatures is related to depressed mitochondrial respiratory capacity

“…It has been suggested that mitochondrial respiratory capacities will determine the thermal range of ectothermic species (Blier et al, 2014). At an individual level, fish whose mitochondria are least able to perform at higher temperatures are likely to be smaller as a result of stunted growth, they will have lower levels of energy storage and will ultimately have poorer survival and/or reproductive success (Carlson et al, 2008;Elliott and Elliott, 2010;Johnsson et al, 1999). This could also lead to differing habitat selection between conspecifics based on mitochondrial properties.…”

Incorporating temperature-driven seasonal variation in survival, growth, and reproduction into population models for small fish

“…1). All of these relationships are modeled below, with the following exceptions: the influence of growth rate on the duration of the adult stage is considered negligible, since adults of this species live up to 3 yr, and the potential for increased mortality associated with elevated energetic demands of higher growth rates is excluded based on the assumption that self-sustaining populations within their native ranges are physiologically adapted to the seasonal variation in environmental conditions (Carlson et al 2008), so implications of seasonal metabolic changes should be negligible.…”

“…With the exception of overwinter mortality for commercially important stocks (e.g. Carlson et al 2008), annualized survival rates that assume average seasonal conditions prevail in models pertaining to management decisions, particularly for nongame species. General models for estimating rates of natural mortality in fish derive an annual rate based on longevity, mean temperature, size, and growth rate (Pauly 1980, Djabali et al 1993, Hewitt & Hoenig 2005, Gislason et al 2010.…”

“…While seasonal variation in survival has been measured in fish (Carlson et al 2008, Letcher & Horton 2008, incorporating such seasonal dynamics into models has been limited to birds (Etterson et al 2009) and invertebrates (Levin & Huggett 1990, Dobson et al 2011). The present study presents a simple approach for incorporating temperature-dependent variability of demographic rates into matrix models that reflect the influences of seasonality.…”

Distribution and growth of brown trout in pristine headwaters of Central Europe

“…Individual growth influences brown trout, Salmo trutta, populations beginning in the early ontogenetic phases, as demonstrated by the positive correlation between juvenile survival and body size [7]. However, this relationship is highly variable across seasons and populations [8]. In the later phases of the life cycle, individual growth affects the fertility of individuals, as the number and weight of eggs increase with female body size [2], and individuals that grow faster achieve earlier sexual maturation than their conspecifics [9].…”