Shrinking sizes of trout and salamanders are unexplained by climate warming alone

Abstract

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 Trout Oncorhynchus clarkii clarkii, but a coexisting species, the Coastal Giant Salamander Dicamptodon 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.

Figure 1

Map of our study sites including (a) Flynn Creek and Needle Branch, Coast Range, and (c) Mack Creek (old-growth and second-growth sections), Cascade Range. All sites are located in Oregon, USA. Individual body-size measurements of Coastal Cutthroat Trout and Coastal Giant Salamander were collected from our study sites over the last 60 years. [Figure developed Kelly Christiansen (USDA Forest Service, PNW Research Station); Created in ArcGIS PSMFC GIS, Airbus, USGS, NGA, NASA, CGIAR, NCEAS, NLS, OS, NMA, Geodatastyrelsen, GSA, GSI and the GIS User Community].

Figure 2

Median fork length (mm) of adult Coastal Cutthroat Trout across sites over time. Bias corrected prewhitened (BCP) Sen’s slope values (mm decade⁻¹) represent the rate of change in median size over time (i.e., rate of shrinking size if negative). Shaded area marks 25th–75th percentile band. For detailed statistics, see Tables S1–S4 in Supporting Information.

Figure 3

Median snout-vent length (mm) of Coastal Giant Salamander in Mack Creek over time. Bias corrected prewhitened (BCP) Sen’s slope values (mm decade⁻¹) represent the rate of change in median size over time (i.e., rate of shrinking size if negative). Shaded area indicates 25th–75th percentile band. For detailed statistics, see Tables S1–S4 in Supporting Information.

Figure 4

Bias corrected prewhitened (BCP) Sen’s slope values (mm decade⁻¹) representing the rate of change in size (i.e., rate of shrinking size if negative) of adult Coastal Cutthroat Trout and Coastal Giant Salamander over time across length percentiles and sites. Filled symbols denote statistically significant trends (Mann–Kendall test p < 0.05). For detailed statistics, see Tables S1–S6 in Supporting Information.

Figure 5

Density-dependent (i.e., abundance of freshwater vertebrates) and density-independent factors (i.e., habitat size, temperature, and discharge) in Mack Creek over time. (a) Covariate factors measured during annual sampling events at the old-growth and second-growth sections of Mack Creek. (b) Hydroclimatic covariate factors were estimated based on daily time series obtained from a long-term stream gage station located in Mack Creek. See detailed description of these covariate factors in Table 1. *Statistically significant negative trends over time. For detailed trend analyses and statistics, see Table S9 in Supporting Information.

Figure 6

Effect size (standardized values ± 95% CI) of covariate factors from the best-supported model (Tables 3, 4; Tables S10, S11) explaining median size of adult Coastal Cutthroat Trout and Coastal Giant Salamanders in Mack Creek. The best-supported model included both density-dependent (abundance of animals) and density-independent (stream discharge, temperature, habitat size) factors. These biotic and abiotic factors are fully described in Table 1.