Does cortisol manipulation influence outmigration behaviour, survival and growth of sea trout? A field test of carryover effects in wild fish

Abstract

For anadromous brown trout Salmo trutta, the transition from life in freshwater to the marine environment is an inherently challenging and dangerous period characterized by high levels of mortality. As such, smoltification is a relevant life-history phase to examine how physiological state, in particular glucocorticoids, influences fitness-oriented endpoints such as migration timing and survival. We experimentally assessed the effect of cortisol by combining passive integrated transponder (PIT) telemetry with a physiologically relevant exogenous cortisol manipulation (i.e. intracoelomic injection) in juvenile sea trout in the Gudsø Stream, Denmark. Individual survival, migration behaviour (timing and speed), and growth were assessed for 4 treatment categories: control (CO), sham (SH), and low-(LW; 25 mg kg −1) and high-dose (HI; 100 mg kg −1) cortisol. There was no difference in the timing of migration among treatments, but trout in the HI treatment had lower survival rates to the lower station (41.6%) when compared to the CO (53.9%) and SH (52.3%) groups. After migration, the system was electroshocked again to contrast growth of trout that remained in the system. HI, LW and SH individuals recaptured in the stream had lower growth rates for length than the CO treatments; HI and LW also had significantly lower growth rates for mass than CO trout. Future monitoring of this population may demonstrate the long-term repercussions of chronic stress as trout return from the ocean. This study provides contributions to our understanding of the relationship between organismal condition and fitness while elucidating the potential for carryover effects, i.e. lasting effects that influence future success.

Stress and food deprivation: linking physiological state to migration success in a teleost fish

“…We did so to test whether the semi-chronic cortisol manipulation had a similar influence to the more 'natural' food deprivation stressor. In terms of growth, brown trout in the cortisol treatment had significantly lower instantaneous growth rates for both length and mass relative to fish in the control and FD treatments, which is consistent with previous exogenous manipulations of cortisol in this species (Midwood et al, 2014 and other teleost fishes (e.g. rainbow trout, Oncorhynchus mykiss; Gregory and Wood, 1999).…”

“…In the short distance between S1 and S2, the stream flows through a millpond, with a small fish ladder at its outflow just upstream of S2. This millpond has previously been identified as a bottleneck that limits downstream migration for brown trout in the system (Midwood et al, 2014; passage at S2 is therefore expected to be naturally lower than that at S1.…”

“…longer than acute but not particularly prolonged such that it would be chronic). Sham treatments were not included in the present study, but previous work on brown trout has suggested that, relative to a control, treatment with cocoa butter alone reduced growth rate (length), but did not affect survival (Midwood et al, 2014). Circulating plasma cortisol levels in sham-treated brown trout were also not found to differ significantly from a control (K. Birnie-Gauvin, K. S. Peiman, M.H.L., K.A., K. M. Gilmour and S.J.C., unpublished data).…”

“…In each section, between 46 and 53 brown trout greater than 12 cm in total length were collected, giving a total of 202 individuals. The total length (cm) and wet mass (g) of each individual was recorded and a uniquely coded 23 mm PIT tag (RI-TRP-RRHP, 134 kHz, 0.6 g mass in air, Texas Instruments, Plano, TX, USA) was inserted into the body cavity following methods outlined in Midwood et al (2014). A similar approach in the con-generic Atlantic salmon (Salmo salar) was found to have high rates of tag retention and survival (Larsen et al, 2013).…”

Short-term and long-term effects of transient exogenous cortisol manipulation on oxidative stress in juvenile brown trout

“…The validation study used the same population and the same products (cortisol and vehicle) as we used here, and similar methods for elevating cortisol have been used in several other studies of the same trout population (Midwood et al, 2014(Midwood et al, , 2016. We therefore did not measure individual cortisol levels in the current study as the objective was to investigate average treatment effects.…”