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Identifying mechanisms underlying individual body size increases in a changing, highly seasonal environment: The growing trout of West brook

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Journal of Animal Ecology
Authors:
Benjamin Havens Letcher at United States Geological Survey
Benjamin Havens Letcher
Keith H Nislow at University of Massachusetts Amherst
Keith H Nislow
Matthew J O'Donnell at United States Geological Survey
Matthew J O'Donnell
Andrew Whiteley at University of Montana
Andrew Whiteley
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Abstract and Figures

As air temperature increases, it has been suggested that smaller individual body size may be a general response to climate warming. However, for ectotherms inhabiting cold, highly seasonal environments, warming temperatures may increase the scope for growth and result in larger body size. In a long‐term study of individual brook trout Salvelinus fontinalis and brown trout Salmo trutta inhabiting a small stream network, individual lengths increased over the course of 15 years. As size‐selective gains and losses to the population acted to reduce body sizes and mean body size at first tagging in the autumn (<60 mm) were not observed to change substantially over time, the increase in body size was best explained by higher individual growth rates. For brook trout, increasing water temperatures during the spring (when both trout species accomplish most of their total annual growth) was the primary driver of growth rate for juvenile fish and the environmental factor which best explained increases in individual body size over time. For brown trout, by contrast, reduction in and subsequent elimination of juvenile Atlantic salmon Salmo salar midway through the study period explained most of the increases in juvenile growth and body size. In addition to these major trends, a considerable amount of interannual variation in trout growth and body size was explained by other abiotic (stream flow) and biotic (population density) factors with the direction and magnitude of these effects differing by season, age‐class and species. For example, stream flow was the dominant growth rate driver for adult fish with strong positive effects in the summer and autumn, but flow variation could not explain increases in body size as we observed no trend in flow. Overall, our work supports the general contention that for high‐latitude ectotherms, increasing spring temperatures associated with a warming climate can result in increased growth and individual body size (up to a point), but context‐dependent change in other factors can substantially contribute to both interannual variation and longer‐term effects.
Map of the study area in Massachusetts, USA (above) and a representation of trout life cycles and our sampling strategy (below). In the map, the mainstem (West brook) and tributary watersheds are colour‐shaded and the tagging study area is shown with black and white dashes. Impassible (square) and passable (circle) waterfalls are indicated on the stream network (blue lines). The map was made using ArcMap v10.8.1 and datasets included the National Hydrography Dataset (U.S. Geological Survey, 2020b), the National Elevation Dataset (U.S. Geological Survey, 2020a) and the National Hydrography Dataset Plus High Resolution Dataset for flow lines (U.S. Geological Survey, 2018). In the life cycle graph, the spiral represents aging across seasons (vertical and horizontal lines) from spawning through the spring of age‐2+ (trout can live to age‐4+, but we limit to age‐2+ for simplicity). Spawning occurs between autumn and winter and embryos incubate in streambed gravels over the winter. Fry emerge from the gravel in early spring and are large enough to tag in the autumn of their first year (age‐0+). Tagging samples are represented by open circles and the closed blue circles represent ages at which adults could contribute to spawning (here, age‐1+ through age‐4+). The thicker line on the spiral represents the range of ages used for ‘growth year 0/1’ described in the text.
Map of the study area in Massachusetts, USA (above) and a representation of trout life cycles and our sampling strategy (below). In the map, the mainstem (West brook) and tributary watersheds are colour‐shaded and the tagging study area is shown with black and white dashes. Impassible (square) and passable (circle) waterfalls are indicated on the stream network (blue lines). The map was made using ArcMap v10.8.1 and datasets included the National Hydrography Dataset (U.S. Geological Survey, 2020b), the National Elevation Dataset (U.S. Geological Survey, 2020a) and the National Hydrography Dataset Plus High Resolution Dataset for flow lines (U.S. Geological Survey, 2018). In the life cycle graph, the spiral represents aging across seasons (vertical and horizontal lines) from spawning through the spring of age‐2+ (trout can live to age‐4+, but we limit to age‐2+ for simplicity). Spawning occurs between autumn and winter and embryos incubate in streambed gravels over the winter. Fry emerge from the gravel in early spring and are large enough to tag in the autumn of their first year (age‐0+). Tagging samples are represented by open circles and the closed blue circles represent ages at which adults could contribute to spawning (here, age‐1+ through age‐4+). The thicker line on the spiral represents the range of ages used for ‘growth year 0/1’ described in the text.
… 
Average (±SD) stream flow (m³ s⁻¹) for each season over the course of the study in the West brook, Massachusetts, USA.
Average (±SD) stream flow (m³ s⁻¹) for each season over the course of the study in the West brook, Massachusetts, USA.
… 
Average (±SD) stream temperature (°C) for each season (the range of dark grey to light grey corresponds to spring, summer, autumn, winter) over the course of the study in the West brook, Massachusetts, USA. Lines represent linear regressions and numbers near the beginning of each set of points are slopes (* signifies p‐values <0.05).
Average (±SD) stream temperature (°C) for each season (the range of dark grey to light grey corresponds to spring, summer, autumn, winter) over the course of the study in the West brook, Massachusetts, USA. Lines represent linear regressions and numbers near the beginning of each set of points are slopes (* signifies p‐values <0.05).
… 
Estimated abundances over time for brook trout (black), brown trout (grey), Atlantic salmon (light grey) and the sum of all three species (dashed line). Solid lines represent linear regressions of abundance over time for each species. Atlantic salmon fry were no longer stocked after 2004.
Estimated abundances over time for brook trout (black), brown trout (grey), Atlantic salmon (light grey) and the sum of all three species (dashed line). Solid lines represent linear regressions of abundance over time for each species. Atlantic salmon fry were no longer stocked after 2004.
… 
Mean body length (mm) across years for each growth year (columns), season (rows) and species (brook trout = black, brown trout = grey). Numbers in the panels represent slopes of linear regressions of length over time (brook trout, top; brown trout, bottom) and the number of stars next to slopes indicated significance at p < 0.05 (*) or p < 0.001 (**). Age‐0 means in the autumn include untagged fish <60 mm to represent the full size range of observed fish.
+5
Mean body length (mm) across years for each growth year (columns), season (rows) and species (brook trout = black, brown trout = grey). Numbers in the panels represent slopes of linear regressions of length over time (brook trout, top; brown trout, bottom) and the number of stars next to slopes indicated significance at p < 0.05 (*) or p < 0.001 (**). Age‐0 means in the autumn include untagged fish <60 mm to represent the full size range of observed fish.
… 
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78 
|
  J Anim Ecol. 2023;92:78–96.wileyonlinelibrary.com/journal/jane
Received: 26 May 2022 
|
 Accepted: 24 September 2022
DOI : 10.1111/136 5-265 6.13 83 3
RESEARCH ARTICLE
Identifying mechanisms underlying individual body size
increases in a changing, highly seasonal environment: The
growing trout of West brook
Benjamin H. Letcher1 | Keith H. Nislow2 | Matthew J. O'Donnell1 | Andrew R. Whiteley3 |
Jason A. Coombs4 | Todd L. Dubreuil1 | Daniel B. Turek5
1U. S. Geological Survey, Eastern
Ecological Science Center, Silvio O. Conte
Research Laboratory, Turners Falls,
Massachusetts, USA
2US Forest Service, Nor thern Research
Station, Amherst, Massachusetts, USA
3Wildlife Biolog y Program, Department
of Ecosystem and Conservation
Sciences, Franke College of Forestry and
Conser vation, University of Montana,
Missoula, Montana, USA
4Department of Environmental
Conser vation, University of
Massachusetts, Amherst, Massachusetts,
USA
5Department of Mathematics and
Statistics, Williams College, Williamstown,
Massachusetts, USA
Correspondence
Benjamin H. Letcher
Email: bletcher@usgs.gov
Funding information
U.S. Fish and Wildlife Service; U.S. Forest
Service
Handling Editor: Marlène Gamelon
Abstract
1. As air temperature increases, it has been suggested that smaller individual body
size may be a general response to climate warming. However, for ectotherms
inhabiting cold, highly seasonal environments, warming temperatures may in-
crease the scope for growth and result in larger body size.
2. In a long- term study of individual brook trout Salvelinus fontinalis and brow n tr o u t
Salmo trutta inhabiting a small stream network, individual lengths increased over
the course of 15 years. As size- selective gains and losses to the population acted
to reduce body sizes and mean body size at first tagging in the autumn (<60 mm)
were not observed to change substantially over time, the increase in body size
was best explained by higher individual growth rates.
3. For brook trout, increasing water temperatures during the spring (when both
trout species accomplish most of their total annual growth) was the primary
driver of growth rate for juvenile fish and the environmental factor which best
explained increases in individual body size over time.
4. For brown trout, by contrast, reduction in and subsequent elimination of ju-
venile Atlantic salmon Salmo salar midway through the study period explained
most of the increases in juvenile growth and body size.
5. In addition to these major trends, a considerable amount of interannual variation
in trout growth and body size was explained by other abiotic (stream flow) and
biotic (population density) factors with the direction and magnitude of these ef-
fects differing by season, age- class and species. For example, stream flow was
the dominant growth rate driver for adult fish with strong positive effects in the
summer and autumn, but flow variation could not explain increases in body size
as we observed no trend in flow.
6. Overall, our work supports the general contention that for high- latitude ec-
totherms, increasing spring temperatures associated with a warming climate
can result in increased growth and individual body size (up to a point), but
© 2022 The Authors. Journal of Animal Ecology © 2022 British Ecological Society. This article has been contributed to by U.S. Government employees and
their work is in the public domain in the USA.
[Correc tion added on 17 November 2022, after first Online publication: In affiliation 5, the city name “Williamsburg” has been changed to “Williamstown”.]
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... Further, the linear relationship between logged values of k and mass depends on individual fish body shape, which is known to change with size in brook trout (Goerig et al., 2020) and the location of temperature recording in the coelomic cavity (Negus & Bergstedt, 2012). What remains unknown is the effect that individual fish body size, outside of sizes examined in previous studies, has on k of smaller bodied brook trout typical of headwater streams (Kanno et al., 2012;Letcher, Nislow, O'Donnell, Whiteley, Coombs, Dubreuil, & Turek, 2022). Thermal transfer rate impacts the lag in temperature that implanted temperature recording tags detect as fish experience large temperature changes over shorter periods of time. ...
... Atlantic salmon which were then subjected to a 2°C h −1 temperature increase; these authors found no effect of size on internal temperature. The size range of fish in the current study ranged from 127 mm (FL) to 228 mm (FL) and represents the typical size of adult brook trout that may be available for tagging in small streams (Kanno et al., 2012;Letcher, Nislow, O'Donnell, Whiteley, Coombs, Dubreuil, & Turek, 2022) and are large enough to carry the 1.3 g tag burden (Winter, 1983). ...
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