Density‐dependent growth and survival in salmonids: Quantifying biological mechanisms and methodological biases

Abstract

Understanding the complex variation in patterns of density‐dependent individual growth and survival across populations is critical to adaptive fisheries management, but the extent to which this variation is caused by biological or methodological differences is unclear. Consequently, we conducted a correlational meta‐analysis of published literature to investigate the relative importance of methodological and biological predictors on the shape and strength of density‐dependent individual growth and survival in salmonids. We obtained 160 effect sizes from 75 studies of 12 species conducted between 1977 and 2019 that differed in experimental approach (sensu Ecological Monographs, 54, 187–211; 65 laboratory experiments, 60 observational field studies, and 35 field experiments). The experimental approach was the strongest factor influencing the strength of density dependence across studies: density‐dependent survival was stronger than growth in field observational studies, whereas laboratory experiments detected stronger density‐dependent growth than survival. The difference between density‐dependent growth and survival was minimal in field experiments, and between lotic and lentic habitats. The shape of density dependence (logarithmic, linear, exponential or density‐independent) could be predicted with 66.7% accuracy based solely on the experimental approach and the density gradient (highest/lowest*100) of the study. Overall, the strength and shape of density dependence were primarily influenced by methodological predictors, while biological factors (predator presence, food abundance, and species) had predictable but modest effects. For both empirical studies and adaptive fisheries management, we recommend using field experiments with a density gradient of at least 470% to detect the proper shape of the density‐dependent response, or accounting for potential biases if observational or laboratory studies are conducted.

Interacting Effects of Density‐Dependent and Density‐Independent Factors on Growth Rates in Southwestern Cutthroat Trout Populations

“…We considered a DIC difference (ΔDIC) greater than 7 (ΔDIC = DIC of model 1 -DIC of model 2) as sufficient evidence for differences in model fit between two models (Spiegelhalter et al 2002;Cain and Zhang 2019). Constructed models included additive and interactive effects between DD, DI, and RGCT life stage (i.e., immature or mature) because all of these mechanisms have been found to affect instantaneous growth dynamics in salmonid populations (Crozier et al 2010;Kovach et al 2016;Grossman and Simon 2020;Matte et al 2020; see Table 2). Preliminary analyses were conducted based on two scenarios:…”

“…The biotic and abiotic characteristics of aquatic environments shape the growth dynamics of aquatic ectotherms, and understanding the link between environmental conditions and instantaneous growth can provide valuable information about what limits the productivity of a fish population (O'Gorman et al 2016;Lindmark et al 2018Lindmark et al , 2019. For example, DD effects on instantaneous growth can indirectly affect population growth through size-dependent mortality rates (Post et al 1999;Matte et al 2020). This knowledge is particularly important for managing aquatic ectotherms as the climate continues to change, since DI and DD effects have been shown to affect the size and productivity of many aquatic ectotherm populations (Crozier and Zabel 2006;Daufresne et al 2009;Crozier et al 2010;Sheridan and Bickford 2011;Myrvold and Kennedy 2015;Nelson et al 2017).…”

“…Competition for limiting foraging resources is a common DD effect that has been demonstrated to influence instantaneous growth rates in salmonids (Newman 1993;Jenkins et al 1999;Grant and Imre 2005;Imre et al 2005;Matte et al 2020), due in part to the territorial behavior that salmonids demonstrate (Grant and Noakes 1987;Nakano 1995;Ward et al 2007). Temperature is a DI effect that can directly control the instantaneous growth rates of salmonids due to thermal regulation of physiological process (consumption rates and metabolism; Brown et al 2004;Armstrong and Schindler 2013).…”

Mechanisms of density dependence in juvenile salmonids: prey depletion, interference competition, or energy expenditure?

“…Food abundance or consumption was also unrelated to fish somatic growth, mortality, or their trade-off. These results are surprising, given the strong effect of fish density on somatic growth and mortality in the study populations (Matte et al 2020b) and across salmonids in general (Grant and Imre 2005, Martinussen et al 2011, Vincenzi et al 2012, Matte et al 2020a). However, increasing fish density led to increasing competitive intensity through augmented aggression and decreasing distance to neighbors.…”

“…Sections were separated by gaps of at least 1 m. All fish were removed from these sections, and YOY trout were stocked to one of four density treatments (0.3, 1, 3 and 7 fish/m 2 ) for a target duration of 21 d (range 9-27 d). These densities encompass those observed across the salmonid literature (Matte et al 2020a), and the natural densities observed in these populations (Matte et al 2020b). Treatments were randomized across sections and years.…”