An official website of the United States government
Climate-driven decreases in body size have been documented for a variety of taxa and proposed as a universal response to climate change. However, empirical support among taxa, including fishes, has been mixed, with some fishes growing larger at higher temperatures, and causal mechanisms for faster or slower growth under debate. We simulated effects of climate warming on bluegill (Lepomis macrochirus) growth and consumption and used linear regression and boosted regression trees (BRTs) to model length-at-age for bluegill from Michigan lakes from 1945 to 2019. Bioenergetics models showed bluegill growth and consumption both increase under climate warming. In contrast, linear regression revealed that bluegill ages 1–4 decreased (–0.20 to –0.55 mm/year) in mean length-at-age and that ages 5–8 increased or did not statistically change. BRTs demonstrated that growth had a unimodal relationship with surface water temperature and degree days, peaking at intermediate values. This mismatch between simulations and empirical data may be from increased recruitment leading to increased food limitation at higher temperatures. Future research should empirically test this hypothesis and assess the consequences for ecosystem functions and services.
more »
« less
This content will become publicly available on November 5, 2026
Long‐Term and Regional‐Scale Data Reveal Divergent Trends of Different Climate Variables on Fish Body Size Over 75 Years
Across many ecto‐ and endothermic organisms, climate change has induced a general shift towards smaller body sizes. Several existing hypotheses (e.g., Temperature Size Rule—TSR, metabolic theory) contribute to our understanding of climate‐driven changes in body size. However, empirical support for climate‐induced reductions in body size is mixed with some species growing larger under warmer temperatures, and underlying mechanisms are under debate. To address these inconsistencies, we used Bayesian hierarchical modeling to determine if mean length‐at‐age (proxy for growth) changed from 1945 to 2020 for age classes of 13 freshwater fish species. Then, we used boosted regression trees (BRTs) to disentangle the impacts of climate change on growth from other environmental factors. Hierarchical modeling revealed that 37% of age classes were decreasing in mean length through time (69% were qualitatively decreasing). BRTs demonstrated that growing degree days and mean annual surface water temperature had varying effects on growth. For cold‐and cool‐water adapted fishes, length‐at‐age usually increased as a function of degree days but decreased as a function of surface temperature. Warm‐water adapted fishes, however, typically decreased in response to both degree days and surface temperature. The direction of change in length‐at‐age as a function of surface temperature corresponded to the direction of change over time for 62% (8/13) of species. Overall, we found widespread decreases in length, including age classes from all thermal guilds and juveniles (contrary to TSR assumptions). Mixed results in prior literature may result from choosing different variables to represent climate warming and/or not considering age‐specific length responses. When specific climate variables and age are considered, climate change effects on body size may be more predictable at large temporal and spatial scales than previously thought. Continued decreases in length for the youngest and oldest fishes could lead to biodiversity loss and diminished ecosystem functions and services.
more »
« less
- Award ID(s):
- 2228256
- PAR ID:
- 10654467
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Global Change Biology
- Volume:
- 31
- Issue:
- 11
- ISSN:
- 1354-1013
- Subject(s) / Keyword(s):
- Freshwater, Lakes, Climate, Fishes, Historical Data
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Rising temperatures are associated with reduced body size in many marine species, but the biological cause and generality of the phenomenon is debated. We derive a predictive model for body size responses to temperature and oxygen (O 2 ) changes based on thermal and geometric constraints on organismal O 2 supply and demand across the size spectrum. The model reproduces three key aspects of the observed patterns of intergenerational size reductions measured in laboratory warming experiments of diverse aquatic ectotherms (i.e., the “temperature-size rule” [TSR]). First, the interspecific mean and variability of the TSR is predicted from species’ temperature sensitivities of hypoxia tolerance, whose nonlinearity with temperature also explains the second TSR pattern—its amplification as temperatures rise. Third, as body size increases across the tree of life, the impact of growth on O 2 demand declines while its benefit to O 2 supply rises, decreasing the size dependence of hypoxia tolerance and requiring larger animals to contract by a larger fraction to compensate for a thermally driven rise in metabolism. Together our results support O 2 limitation as the mechanism underlying the TSR, and they provide a physiological basis for projecting ectotherm body size responses to climate change from microbes to macrofauna. For small species unable to rapidly migrate or evolve greater hypoxia tolerance, ocean warming and O 2 loss in this century are projected to induce >20% reductions in body mass. Size reductions at higher trophic levels could be even stronger and more variable, compounding the direct impact of human harvesting on size-structured ocean food webs.more » « less
-
Abstract Rising ocean temperatures pose significant threats to marine ectotherms. Sensitivity to temperature change varies across life stages, with embryos often being less tolerant to thermal perturbation than adults. Antarctic notothenioid fishes evolved to occupy a narrow, cold thermal regime (−2 to +2°C) as the high-latitude Southern Ocean (SO) cooled to its present icy temperatures, and they are particularly vulnerable to small temperature changes, which makes them ideal sentinel species for assessing climate change impacts. Here, we detail how predicted warming of the SO may affect embryonic development in the Antarctic bullhead notothen,Notothenia coriiceps. Experimental embryos were incubated at +4°C, a temperature projected for the high-latitude SO within the next 100–200 years under high emission climate models, whereas control embryos were incubated at present-day ambient temperature, ∼0°C. Elevated temperature caused a high incidence of embryonic morphological abnormalities, including body axis kinking/curvature and reduced body size. Experimental embryos also developed more rapidly, such that they hatched 68 days earlier than controls (87 vs. 155 days post-fertilization). Accelerated development disrupted the evolved timing of seasonal hatching, shifting larval emergence into the polar winter when food availability is scarce. Transcriptomic analyses revealed molecular signatures of hypoxia and disrupted protein-folding in near-hatching embryos, indicative of severe cellular stress. Predictive modeling suggested that temperature-induced developmental disruptions would narrow seasonal reproductive windows, thereby threatening population viability under future climate scenarios. Together, our findings underscore the vulnerability of Antarctic fish embryos to higher water temperature and highlight the urgent need to understand the consequences of disruption of this important trophic component on ecosystem stability in the SO. Significance StatementAntarctic fishes evolved cold-adapted phenotypes suited to the stable thermal conditions of the Southern Ocean, yet are threatened by rising temperatures. The impact of rising temperatures on early life stages in Antarctic fishes is not well understood; our findings show that projected warming may induce premature hatching, developmental abnormalities, and molecular stress responses in embryos, potentially reducing recruitment and leading to population instability and trophic-level ecosystem disruptions. These results underscore the urgency of assessing climate-driven vulnerabilities across life stages of Antarctic marine organisms to refine population projections and enhance conservation strategies amid ongoing environmental change.more » « less
-
The seasonal onset of reproduction is constrained in many systems by a need to first accumulate energetic reserves. Consequently, the observation that larger individuals reproduce earlier may be due to a negative relationship between size and mass‐specific basal metabolic rate that is shared across diverse taxa. However, an untested prediction of this hypothesis is that individuals should be metabolically efficient enough to escape energetic constraints above a certain size threshold. Seasonally reproducing species, such as temperate fishes, that must recover winter energy losses before reproduction and exhibit indeterminate growth are ideal models to test this prediction. We harness decade‐long behavioral data on parental male smallmouth bass,Micropterus dolomieu, to investigate contributions of energetic allometry to differences in reproductive timing. At the population level, peak seasonal reproductive timing (i.e. the median date on which eggs were found in nests each year) was negatively related to degree days – a measure of thermal energy experienced – before reproduction. At the individual level, degree days accumulated by males before reproduction was related to male size and condition in every year, but the impact of temperature on reproductive timing by the largest males was relaxed in most years. Additionally, we used our data to replicate the analyses of two previous studies ofM. dolomieupopulations and found virtually identical negative associations between male body size and degree days accumulated before reproduction. Our results suggest that in smallmouth bass the onset of seasonal reproduction is constrained by basal metabolic rate – as indicated by total length – and that large individuals can escape size‐associated energetic constraints. We reveal a more complicated relationship between size and reproductive timing than earlier studies, which may be relevant for many species. Knowledge of this relationship is critical to understanding how a changing climate will influence population dynamics of economically, ecologically and recreationally important species likeM. dolomieu.more » « less
-
Decreases in body sizes of animals related to recent climate warming can affect population persistence and stability. However, direct observations of average sizes over time and their interrelationships with underlying density-dependent and density-independent processes remain poorly understood owing to the lack of appropriate long-term datasets. We measured body size of two species common to headwater streams in coastal and Cascades ecoregions of the Pacific Northwest of North America over multiple decades, comparing old-growth and managed forests. We found consistent decreases in median length of Coastal Cutthroat TroutOncorhynchus clarkii clarkii,but a coexisting species, the Coastal Giant SalamanderDicamptodon tenebrosus, appears to be more resilient to size changes over time. Based on observed trends, adult trout have decreased in length by 6–13% over the last 30 years. Length decreased more in larger compared to smaller animals, suggesting that these effects reflect changes in growth trajectories. Results from a model-selection approach that included hydroclimatic and biological information as covariates in one of our study ecoregions demonstrated that stream temperature alone did not explain observed length reductions. Rather, a combination of density-dependent (animal abundances) and local density-independent factors (temperature, habitat, and streamflow) explained observed patterns of size. Continued decreases in size could lead to trophic cascades, biodiversity loss, or in extreme cases, species extirpation. However, the intricate links between density-independent and density-dependent factors in controlling population-level processes in streams need further attention.more » « less
