Data from: Influence of autumn body size on following spring migration probability and timing in juvenile brown trout (Salmo trutta)
Data files
Mar 24, 2026 version files 224.49 KB
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Data_CJFAS.xlsx
183.84 KB
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Migration_probability_analysis.R
11.10 KB
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Migration_timing_analysis.R
7.83 KB
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README.md
6.81 KB
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Recapture_analysis.R
8.70 KB
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Supplementary_figures.R
6.20 KB
Abstract
Body size is frequently invoked to explain within-population variation in migratory behaviour because of its far-reaching implications for physiological and ecological processes. Here, we investigated how autumn body size influenced spring river-to-sea migration probability and timing in juvenile brown trout (Salmo trutta) in the river Nybroån, Sweden. To achieve this, we used a combination of individual-based tracking and trapping during two consecutive years. We report a non-linear relationship between autumn body size and probability of spring migration, with migration likelihood increasing steeply up to a body length of ~110 mm before declining with further increasing body size. Among migrants, larger individuals migrated earlier than smaller conspecifics. Our results highlight that autumn body size may be used as a reliable predictor of migratory behaviours in juvenile brown trout and underscore the utility of using recruitment surveys to forecast life-history transitions and suggest that probabilistic reaction norms based on body size can support monitoring and conservation strategies in migratory fish populations.
Description of the data and file structure
This repository contains data and statistical methods, written in R and organised as RStudio scripts for Forsberg et al. (2025). "Influence of autumn body size on following spring migration probability and timing in juvenile brown trout (Salmo trutta)"
Description of the data and file structure
Data Collection Methods:
Data for this study consists of measurements of autumn body length, as well as next-spring migration propensities and timing of individual brown trout (Salmo trutta) migrating from the river Nybroån in southernmost Sweden, and to the Baltic Sea. Data was collected over two consecutive years and across 12 sites within the river using passive biotelemetry with stationary and continuously operating antenna arrays. Data also includes additional information of untagged fish migrating through the system that was used as a control to asses the generalisability of the main text results.
Files and variables
File: Data_CJFAS.xlsx
Description: Excel file containing raw data used for analysis and generation of main text figures as well data used for generation of supplementary figures.
Sheet: Migration_data
Description: Data used to test for associations between autumn body size and individual migration propensity and timing.
Variables
Information on Data Column Labels:
- TAG_ID: Passive integrated transponder ID of tagged fish.
- Tagging_date: The specific date a fish individual was PIT-tagged.
- Tagging_year: The specific year a fish individual was PIT-tagged.
- Site: Categorical variable differentiating fish site of origin.
- Total_length_mm: The total body length (mm) of fish individuals at tagging.
- Spring_migrant(yes/no): Fish that were detected as downstream migrants in the spring following capture and tagging (1) or not detected as downstream migrants in the spring following capture (0).
- Migration_date: The specific date a fish individual was last detected in the system. n/a denotes individual fish that did not migrate (no data available).
- Migration_Year: The specific year a fish individual was detected as a spring migrant. n/a denotes individual fish that did not migrate (no data available).
- Location: Specific location used to determine migration timing (Migration_Date) of a fish individual detected/registered (A1,A2,A3,A4 or Trap) migrating during spring. n/a denotes individual fish that did not migrate (no data available).
Sheet: Recap_data
Description: Data used to test for associations between autumn body size and size at migration as well as data for individuals recaptured alive in the stream approximately one year after tagging.
Variables
Information on Data Column Labels:
- TAG_ID: Passive integrated transponder ID of tagged fish.
- Total_length_recapture_mm: The total body length (mm) of fish individuals at recapture.
- Total_length_tagging_mm: The total body length (mm) of fish individuals at tagging.
- Tagging_date: The specific date a fish individual was PIT-tagged.
- Migration_Year: The specific year a fish individual was recaptured.
- Phenotype: Categorical classification of whether the recaptured individuals was a migrant or non-migrant.
Sheet: Untagged_smolts
Description: Individual body length data of untagged brown trout smolts captured in the Köpingebro fish trap during the study period. Data was used to compare body length distributions between tagged and untagged migrants.
Variables
Information on Data Column Labels:
- Total_length_mm: The total body length (mm) of fish individuals at capture.
- Phenotype: Categorical classification. All listed as smolts (i.e. migrants).
- Date_captured: The specific date a fish individual was captured.
- Migration_Year: The specific year a fish individual was recaptured.
Sheet: Data_fig_S1
Description: Individual body length data for brown trout captured during autumn electrofishing in 2023 and 2024. Data was used to illustrate autumn body length distributions of all captured individuals and those sufficiently large to be tagged with a passive integrated transponder (PIT) tag.
Variables
Information on Data Column Labels:
- Total_length_mm: The total body length (mm) of fish individuals at capture.
- Group: Categorical classification of distinguishing captured individuals from those captured and tagged.
- Year: The specific year a fish individual was captured.
Sheet: Data_fig_S3
Description: Migration data of tagged fish detected or recaptured as migrants during the main spring migration period (March-June) across the study period together with environmental data for discharge and water temperature.
Variables
Information on Data Column Labels:
- Date: Specific date during the migration period.
- Number_of_migrants: The number of unique IDs detected migrating.
- Year: The specific year of migration.
- DOY: Day number of year.
- N_smolt_annual: Total number of unique IDs detected that year.
- N_smolt_prop: Proportion of unique IDs detected migrating that day (Number_of_migrants/N_smolt_annual).
- Discharge: Water discharge in cubic meters per second.
- Temperature: Water temperature in degrees Celsius.
Sheet: Data_fig_S4
Description: Data of cumulative migration patterns during spring for tagged individuals detected migrating and untagged individuals captured in the trap or detected in the automatic fish counter.
Variables
Information on Data Column Labels:
- Migration_year: The specific year of migration.
- Date: Specific date during the migration period.
- DOY: Day number of year.
- Group: Categorical classification distinguishing tagged from untagged individuals.
- Cumulative_prop: Date specific cumulative proportion of migration.
File: Migration_probability_analysis.R
Description: Code used to perform statistical analyses of spring migration probability as well as main (Figure 2) and supplementary (Figure S2) text figures.
File: Migration_timing_analysis.R
Description: Code used to perform statistical analyses of spring migration timing as well as main (Figure 3) and supplementary (Figure S5 and S6) text figures.
File: Recapture_analysis.R
Description: Code used to perform statistical analyses of autumn body length and body length at migration relationship as well as main (Figure 4) and supplementary (Figure S7 and S8) text figures.
File: Supplementary_figures.R
Description: Code used to generate supplementary figures S1, S3, and S4.