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.

Keywords: Baltic Sea; body size; climate change; coastal ecosystem; fish; life history; population; temperature; temperature-size rule.

Figure 1

Study area. The location (left) and map (right) of the artificially heated enclosed coastal ecosystem, the Biotest Lake (heated from 1980 onward), and its reference area. Full arrows indicate the warm water inlet and outlet (transported in tubes indicated by hatched arrow)

Figure 2

Optimum temperature for growth and daily water temperature. (a) The observed mean (±1SD, shaded areas) daily water temperatures in the heated Biotest Lake (red) and its reference area (blue) during the ice‐free season during the time period 1989–2003. (b) The optimum temperature for perch net energy gain (energy available for body growth, see Figure S1) as a function of body length with the mean length and optimum temperatures of 1 and 3 year old perch indicated with dashed lines (70 and 150 mm, respectively)

Figure 3

Warming effects on fish growth trajectories. Growth trajectories of different cohorts (each point representing mean length‐at‐age from back‐calculated individual growth trajectories and each line one cohort) of perch in (a) the artificially heated enclosed coastal ecosystem, the Biotest Lake (1969–2004), and (b) its adjacent reference area (1962–2004). The first point in each (cohort) line represent 1 year old individuals, the second 2 year olds, etc. 1, 3 and 5 year old individuals are highlighted in white. The light red area indicates the period during which the Biotest Lake received warm water

Figure 4

Warming effects on fish size‐at‐age. Body lengths, based on back‐calculated length‐at‐age, of (a) 1 year old perch and (b) 3 year old perch in the artificially heated enclosed coastal ecosystem, the Biotest Lake (red symbols), and its reference area (blue symbols), and (c) the resulting difference in mean body length between areas for 1 year old perch (black symbols) and 3 year old perch (white symbols). Solid regression lines represent significant (p < 0.05) relationships. The light red area in (c) indicates the period during which the Biotest Lake received warm water

Figure 5

Warming effects on length‐specific body growth of fish. Length‐specific annual growth rates (G _L, based on back‐calculated length‐at‐age) of (a) newborn and (b) 140–160 mm 3 year old perch in the artificially heated enclosed coastal ecosystem, the Biotest Lake (red symbols), and its reference area (blue symbols), and (c) the resulting mean difference in body growth between areas for newborn perch (black symbols, left y‐axis) and 3 year old perch (white symbols, right y‐axis). Solid regression lines represent significant (p < 0.05) relationships. The light red area in (c) indicates the period during which the Biotest Lake received warm water