Experimental evidence of gradual size‐dependent shifts in body size and growth of fish in response to warming

Abstract

A challenge facing ecologists trying to predict responses to climate change is the few recent analogous conditions to use for comparison. For example, negative relationships between ectotherm body size and temperature are common both across natural thermal gradients and in small‐scale experiments. However, it is unknown if short‐term body size responses are representative of long‐term responses. Moreover, to understand population responses to warming, we must recognize that individual responses to temperature may vary over ontogeny. To enable predictions of how climate warming may affect natural populations, we therefore ask how body size and growth may shift in response to increased temperature over life history, and whether short‐ and long‐term growth responses differ. We addressed these questions using a unique setup with multidecadal artificial heating of an enclosed coastal bay in the Baltic Sea and an adjacent reference area (both with unexploited populations), using before‐after control‐impact paired time‐series analyses. We assembled individual growth trajectories of ~13,000 unique individuals of Eurasian perch and found that body growth increased substantially after warming, but the extent depended on body size: Only among small‐bodied perch did growth increase with temperature. Moreover, the strength of this response gradually increased over the 24 year warming period. Our study offers a unique example of how warming can affect fish populations over multiple generations, resulting in gradual changes in body growth, varying as organisms develop. Although increased juvenile growth rates are in line with predictions of the temperature–size rule, the fact that a larger body size at age was maintained over life history contrasts to that same rule. Because the artificially heated area is a contemporary system mimicking a warmer sea, our findings can aid predictions of fish responses to further warming, taking into account that growth responses may vary both over an individual's life history and over time.

Growth impacts in a changing ocean: insights from two coral reef fishes in an extreme environment

“…Such reduction in body size is consistent with the temperature-size-rule (TSR) (i.e. body size decrease at higher temperature) (Atkinson 1994;Kingsolver and Huey 2008;Ohlberger 2013;Horne et al 2017;Huss et al 2019) and is particularly evident in aquatic environments (Forster et al 2012;Horne et al 2015), where fish and other aquatic organisms' average body size has already declined by 5-20% over the last two decades (Baudron et al 2014;Audzijonyte et al 2016;van Rijn et al 2017). Experimental temperature-size responses, models and meta-analyses suggests that body size will further reduce by 3-5% per degree of warming in aquatic arthropods, while in fishes it may decline by 14-24% by 2050 under a higher green-house gases emission scenario (A2 scenario, IPCC 2007;Cheung et al 2013;Pauly and Cheung 2018a).…”

Individual variation and interactions explain food web responses to global warming

“…Such population-level metrics vary not only with individual growth but also with mortality, and can therefore not be used to infer warming effects on body growth. The variation in body growth responses to high temperatures over ontogeny observed in fishes in natural systems [4] as well as experimentally in unfished populations over long time scales ( [7], figure 2b) is associated with larger size at young ages ( [4,7]; figure 2c), but no [4] or smaller [7] increases in size at older age (figure 2c). Responses to temperature differences occurring across generations may differ from plastic responses of individuals arising from changes in temperature-and sizedependent physiological and ecological processes, owing to evolutionary adaptation.…”

“…Responses to temperature differences occurring across generations may differ from plastic responses of individuals arising from changes in temperature-and sizedependent physiological and ecological processes, owing to evolutionary adaptation. For example, Huss et al [7] found that warming also resulted in gradually faster growth of small individuals over more than 10 generations (figure 2b), suggesting that evolutionary adaptations to warming may need to be accounted for. Alternatively, the longer growth season or effects on resource availability caused by warming may enable small individuals to grow faster as well as large individuals to maintain growth, and hence increase in size at age owing to fast growth when young.…”

Multigenerational exposure to elevated temperatures leads to a reduction in standard metabolic rate in the wild

“…Moreover, using populations exposed to contrasting thermal habitats for many generations allowed us to examine long‐term responses to elevated temperatures (Huss, Lindmark, Jacobson, van Dorst, & Gårdmark, ). This avoids the limitations of short‐term laboratory experiments that expose animals to high temperature for only one or a few generations.…”