DOI: 10.5061/dryad.qz612jmhs
title: Exceptional longevity in northern peripheral populations of Wels catfish (Silurus glanis)

description: Studies of life-history variation across a species range are crucial for
 ecological understanding and successful conservation. Here, we examined
 the growth and age of Wels catfish (Silurus glanis) in Sweden, which
 represent the northernmost populations in Europe. A total of 1183
 individuals were captured, marked and released between 2006 and 2020.
 Mark-recapture data from 162 individuals (size range: 13–195 cm) were used
 to estimate von Bertalanffy growth curve parameters which revealed very
 slow growth rates compared to catfish within the core distribution area
 (central Europe). The fitted von Bertalanffy growth curve predicted a 150
 cm catfish to be around 40 years old, while the largest recaptured
 individual (length 195 cm) was estimated to be 70 (95% CI 50–112) years
 old. This was substantially older than the previously documented maximum
 age of a catfish. The weight at length relationships in these northern
 peripheral populations were similar to those documented for catfish in
 central Europe indicating that resources did not constrain growth. This
 indicates that the slow growth and exceptional high age in the northern
 catfish populations are the result of lower temperatures and/or local
 adaptations.
description: Sampling of catfish was conducted from 2014 to 2020 in Lake
 Möckeln (56° 39′ 48.9′′N 14° 8′ 57.7′′ E) and between 2006 and 2020 in
 River Emån (57° 7′ 46.7′′ N 16° 30′ 10.8′′ E) (Fig. 1). With the aim to
 capture the complete size structure of the focal populations we sampled
 individuals in both systems (Lake Möckeln and River Emån) between May and
 September using two methods, longlining for targeting adults (&gt; 70
 cm) and fyke-nets for juveniles and subadults. Longlines consisted of a
 floating mainline (100–400 m depending on the sampled area) to which
 monofilament (1.2 mm) leaders (1–1.5 m, looped to Scotty release clips)
 were fitted with single treble hooks and baited with native cyprinids were
 attached every 10–20 m. Fyke-nets varied in design/size between systems,
 in Lake Möckeln we employed a large two-armed fyke-net (20–30 m arms, 1.5
 m diameter entrance) whereas in River Emån we used a smaller single-arm
 fyke net (8 m arm, 0.5 m diameter entrance). Both fyke-nets and longlines
 were initially set randomly throughout the systems but over time placed in
 hot-spot locations to obtain higher n-values. All gear were checked
 daily. All fish were measured (total length) and
 weighted immediately after capture and date of capture was recorded. Using
 a measuring tape, small individuals (&lt; 40 cm) was measured to the
 nearest mm whereas the length of larger fish was determined to the closest
 cm. The weight of small individuals was determined to nearest gram
 (Berkley 50 lbs/22 kg), larger individuals to the nearest 0.1 kg
 (Steinberg Systems SBS-KW-300/100-O). All individuals were marked with a
 PIT-tag (Passive Integrated Transponder from Biomark, 23 mm HDX), injected
 in the pelvic girdle or the abdominal cavity. Recaptured individuals, as
 identified by the PIT-id, was measured and weighted to be able to compare
 growth since the initial day of tagging before released. The sample
 procedure of a catfish took &lt; 5 min. The
 mark-recapture data was used to construct a von Bertalanffy growth curve
 (VBGC) of age at length by adopting the method by Fabens 1965 (sensu Hamel
 2014). A reformulated VBGC-equation (ΔL = (L∞ − Lm)(1 − e(−K × ΔT)) was
 fitted to the mark-recapture data of growth between mark and recapture
 (ΔL), number of years between mark and recapture (ΔT, fractional values)
 and length at tagging (Lm). The parameters K and L∞ for the VBGC-equation
 were estimated iteratively by non-linear regression using the nls-function
 in the package stats (version 4.1.2, part of base R) in R (version 4.1.2;
 R Core Team). Only individuals recaptured 10 months or more after tagging
 were included (N = 162) to ensure that all individuals had experienced at
 least one growth season. The last recapture with the longest time span
 between mark and recapture was used for individuals with repeated
 recaptures such that every individual contributed with one observation of
 growth. Length at hatching (0.7 cm) was used as L0 in the VBGC-equation. A
 reformulated VBGC-equation (Age = log<sub>e</sub> ((Length −
 L∞)/(L0 − L∞))/− K) with the L0 and the estimated parameters K and L∞ was
 used to predict age at length. The length (L) at weight (W) relationship
 was analysed by fitting a classic length–weight model with a power
 function (W = a × Lb). The parameters a and b were estimated by fitting a
 linear regression of log<sub>e</sub>(W) as a function of
 log<sub>e</sub>(L).