DOI: 10.5061/dryad.5tb2rbp85
title: Data from: Contrasting long-term trends in juvenile abundance of a widespread cold-water salmonid along a latitudinal gradient: Effects of climate, stream size and migration strategy

description: A changing climate reshapes the range distribution of many organisms, and
 species with relatively low thermal optima, like many salmonids, are
 increasingly expected to face local population extinctions at lower
 latitudes. Understanding where and how fast these changes are happening is
 of pivotal importance for successful mitigation and conservation efforts.
 We used an extensive electrofishing database to explore temporal trends of
 brown trout juveniles (Salmo trutta L.) in 218 locations from 174 Swedish
 streams, over the last 30 years (1991–2020). We hypothesized that 1)
 declines in abundance have occurred predominately in the warmer, southern
 regions, while increases have occurred in the colder, northern regions, 2)
 larger stream sizes may partly offset negative effects of climate, and 3)
 migrating and resident populations are affected differently by a warming
 climate. We found that abundance of brown trout juveniles generally
 declined in warmer regions, especially in smaller streams (≤ 6 m wide),
 while the abundance increased in colder regions. In larger streams,
 negative effects of higher temperatures were seemingly buffered, as we
 found lower rates of decline or even positive trends. The rate of change
 (i.e. the slopes of the trends in abundance) was more pronounced towards
 the climate extremes and was on average zero in regions with a normal
 annual air temperature (average temperature over 30-year period) around
 5–6 ºC. Warmer climate had stronger effects on migrating compared to
 resident populations, suggesting that climate-induced loss of stream
 connectivity could be an additional factor that hinders recruitment in
 anadromous populations in a changing climate. Considering predictions of
 increasing temperatures and frequency of summer droughts, management of
 cold-water salmonid populations should focus on conserving and restoring
 riparian vegetation, wetlands, climate and thermal refugia, and habitat
 integrity overall. Such measures may, however, not suffice for small
 streams at lower latitudes, unless hydrological connectivity is
 maintained.
description: Data on brown trout abundance
 and stream size (i.e. width and average depth) were extracted from the
 Swedish Electrofishing RegiSter (‘SERS’). Data in SERS are quality assured
 by using a strict sampling protocol (Bergquist et al. 2014), including posterior quality checks.
 Data on brown trout
 juvenile abundance were collected via electrofishing by wading.
 The abundance was
 estimated through successive removals, usually including three
 electrofishing passes over the survey area (Bohlin et al. 1989), or, when this was not possible (i.e.
 &lt;10% of the sampling occasions), from average catch probability
 based on one pass (Bergquist et al.
 2014). Climate variables and water flow estimates were
 provided by the Swedish Meteorological and Hydrological Institute
 (SMHI). Flow estimates were based on the S-HYPE model
 (Bergstrand et al. 2014).
description: Microsoft Excel.