• Prystay, Tanya S.
  • Elvidge, Chris K.
  • Twardek, William M.
  • Logan, James M.
  • Reid, Connor H.
  • Clarke, Shannon H.
  • Foster, Jordann G.
  • Cooke, Emma L. L.
  • Cooke, Steven J.


Fish are commonly sedated to render them immobile and thus easier to handle for research, veterinary, and aquaculture practices. Since sedation itself imposes a significant challenge on the targeted fish, the selection of sedation methods that minimize physiological and behavioral disturbance and recovery time is essential. Two popular sedation methods include the chemical tricaine methanesulfonate (MS‐222) and electrosedation. Although many studies have already investigated the physiological consequences of these methods, there is limited research examining the latent behavioral effects on fish. Using Largemouth Bass Micropterus salmoides as a model species, we compared the postsedation behaviors of fish that were sedated with either MS‐222 or electrosedation to those of a control group exposed to the same handling protocol. Immediately after sedation, fish exposed to either treatment demonstrated lower reflex scores than the control group. Time to resume regular ventilation did not differ between chemically sedated and electrosedated fish; however, electrosedated fish regained equilibrium faster (mean ± SE = 154 ± 20 s) than fish that were exposed to MS‐222 (264 ± 30 s). Locomotor activity and swimming performance were assessed at 5‐, 30‐, or 60‐min intervals, beginning after individuals had recovered from sedation sufficiently to regain equilibrium. For all postsedation intervals, locomotor activity was two times greater in the electrosedated group than in the control and MS‐222 groups. Other behavioral measures (refuge emergence time, activity level, and flight initiation distance) and swimming performance did not differ at 5, 30, or 60 min postrecovery for any of the treatment groups. Our results indicate that while both chemical and electrical sedation methods result in impairment (i.e., sedation) immediately after treatment, these behavioral effects do not persist beyond 5 min postrecovery, and the two methods have similar impacts on Largemouth Bass. However, we caution that these results cannot be extrapolated to other fish species without further study.


Fish Collection

Largemouth Bass were collected from Lake Opinicon (44.5590°N, 76.3280°W), Ontario, Canada, during June 2015. Fish were collected via angling by using a variety of lure types, and landing time was limited to less than 20 s to reduce stress associated with capture and anaerobic exercise (Cooke et al. 2003). Test fish (all between 300 and 400 mm TL) were held for 24 h in offshore floating net‐pens (1.2 × 1.2 × 1.2 m) to allow for acclimation to captivity (Wilson et al. 2015). Surface water temperature within the net‐pens was measured three times daily with an alcohol thermometer and ranged from 15°C to 23°C.


Sample sizes and size ranges of the Largemouth Bass in each treatment group are summarized in Table 1. In all treatments, we recorded the latency between the attainment of stage IV sedation (or the end of the 5‐min control treatment) and the resumption of regular ventilation activity (at least once every 5 s) and equilibrium, which we refer to as the “postsedation recovery time.” Fish in the MS‐222 treatment were placed individually into coolers (66 × 34 × 31 cm) filled with lake water and dissolved MS‐222 at 100 mg/L and were held in the coolers until they reached stage IV sedation. The fish were then transferred to a recovery cooler that received a constant flow of freshwater. Fish in the electrosedation treatment were placed into a Smith‐Root Portable Electrosedation System (PES) unit (Smith‐Root, Inc., Vancouver, Washington) that was installed in an insulated container (107 × 48 × 47 cm), with the electrodes placed 69 cm apart. The PES unit was operated with a constant setting (3‐s operation with standard pulsed DC at 100 Hz, 90 V, and a 25% duty cycle; after Rous et al. 2015). Fish were placed perpendicular to the electrodes during sedation (Rous et al. 2015). Control fish were divided into two equal groups and placed in a cooler filled with lake water to the same depths as the sedative treatments for 5‐min periods. Initial examination revealed no behavioral differences attributable to cooler dimensions, so the two control groups were pooled into a single group.

Reflex impairment index

After the fish regained equilibrium, we applied a five‐stage reflex impairment test (reflex action mortality predictors [RAMP]; Raby et al. 2012). Reflexes require the coordination of neurological and physiological functions (Davis 2010), making them a relevant metric for this study. We assigned RAMP scores based on five independent reflex assessments that were scored on a binary scale (0 = impaired; 1 = unimpaired), resulting in an overall range of 0–5. The five reflexes consisted of (1) regaining orientation within 3 s after being flipped upside down; (2) avoidance behavior, evidenced as an attempt to burst swim away during pinching of the caudal fin; (3) ocular control, demonstrated by a fish's ability to roll its eye to maintain level pitch when turned on its side; (4) body flex, shown by a fish's attempt to struggle free while being held out of water by two hands positioned on the middle of its body; and (5) a positive head complex response, evidenced by a regular ventilation pattern of opening and closing the lower jaw within 5 s while the fish was held out of water.