GABAergic anxiolytic drug in water increases migration behaviour in salmon

Abstract

Migration is an important life-history event in a wide range of taxa, yet many migrations are influenced by anthropogenic change. Although migration dynamics are extensively studied, the potential effects of environmental contaminants on migratory physiology are poorly understood. In this study we show that an anxiolytic drug in water can promote downward migratory behaviour of Atlantic salmon (Salmo salar) in both laboratory setting and in a natural river tributary. Exposing salmon smolt to a dilute concentration of a GABA_A receptor agonist (oxazepam) increased migration intensity compared with untreated smolt. These results implicate that salmon migration may be affected by human-induced changes in water chemical properties, such as acidification and pharmaceutical residues in wastewater effluent, via alterations in the GABA_A receptor function.

Figure 1. Effects of oxazepam on downstream migration in Atlantic salmon smolt.

Downstream migration intensity for Atlantic salmon smolt (mean and 95% confidence intervals) in control and oxazepam-exposed Atlantic salmon smolts in a large-scale migration mesocosm. Migration intensity is inferred from the number of laps registered in the migration pool, and values for each treatment (exposed and control) represents the mean number of laps per smolt individual completed in the migration pool during 70 h, and is based on five independent replicated trials, each containing 20 smolts per treatment.

Figure 2. Effects of oxazepam on downstream migration in Atlantic salmon smolt.

Downstream migration intensity of control and oxazepam-exposed Atlantic salmon smolt in a natural river tributary. Migration intensity was measured as latency to first detection (mean and 95% confidence interval) at the PIT-tag antenna 200 m downstream of the release site.

Figure 3. Downstream migration intensity in Atlantic salmon smolt 0–10 and 60–70 h after exposure to oxazepam.

Downstream migration intensity (mean and 95% confidence intervals) in control (white) and oxazepam-exposed (black) Atlantic salmon smolts during the first 10 h of migration and the last 10 h (between 60 and 70 h) of migration in a large-scale migration mesocosm.

Figure 4. Downstream migration intensity in Atlantic salmon smolt over time and with decreasing concentration of oxazepam in fish muscle tissue:

Downstream migration intensity in control (dashed line) and oxazepam-exposed (solid line) Atlantic salmon smolt as a function of time (lower x axis) and concentration of oxazepam in fish muscle tissue (upper x axis). Values are fitted values from a Loess smoother, and dotted lines are 95% confidence interval bands.

Figure 5. Diel migration activity in Atlantic salmon smolt.

Diel migration activity for control (dashed line) and oxazepam-exposed (solid line) Atlantic salmon smolt. Migration intensity was measured as the mean number of downstream detections per hour per fish individual. The lines are embedded with 95% confidence interval bands, and are the fitted values from a Loess smoother.

Figure 6. Schematic drawing of the migration mesocosm used to monitor downstream migration.

The centre of the rearing tank (diameter 8 m) is enclosed, restricting fish from entering the area. At the narrowest passages between the enclosed area and the tank wall, two PIT-antennas were installed, connected to a reader station. A ‘dent' in the enclosed area created a back eddy, serving as a refuge for the fish from the water current. Schematic drawing by G Hellström.