Study looks at chemical vs electric sedation in reducing pre-harvest stress in fish

It’s been known for centuries that animals consumed by humans undergo a certain amount of stress before they get to the dining table, lowering meat quality and processing efficiencies.

         The use of sedatives before and after harvest to achieve a “rested harvest” has proven to lessen stress effects, but the US Federal Drug Administration (FDA) hasn’t approved any such drugs for use in animals raised for human consumption. A recent study found that using electricity may offer a non-chemical means to sedate farmed fish while in pre-harvest stages.

         The study, by biologists at the Southern Illinois University at Carbondale, compared both the effects of drug (chemosedative) and electricity (electrosedative) on the rested harvest conditions of rainbow trout.

         Although the authors concluded that rested harvest did show reduced pre-harvest stressors in the tested trout — with electricity appearing to improve some aspects of product quality — they encourage other researchers to further look into rested harvest for fish using both electrosedative and chemosedative techniques to develop adequate protocols for fish farmers.

         “One of the most important factors to consider, really from the beginning, is how the technique or technology might feasibly be implemented by a farm,” research team leader Jesse Trushenski, who now works at the Idaho Department Fish and Game as the fish pathologist supervisor, told Aquaculture North America (ANA). “We had quite a few discussions about how the things we were testing at an experimental scale could be adapted and scaled up to match commercial conditions.  There’s a certain element of ‘if you build it, they will come’ in that if the technique/technology is absolutely perfect and offers clear advantages, the industry can always adapt to use it.”

         Currently, the only commercially available FDA-approved fish sedative is a tricaine methanesulfonate product, Tricaine-S, which has to be used 21 days before human consumption. The fish anesthetic is regularly used in hatcheries, labs, and aquariums to immobilize fish for marking or transport and to suppress sensory systems during invasive procedures. The drug is provisionally approved for use in fish in Canada, the United Kingdom, and the United States.

Comparing results

Harvesting and slaughtering methods usually involve some degree of handling and crowding, which tend to increase plasma cortisol, glucose, and lactate levels in fish, and could remain elevated for hours or days. Slaughter methods tend to exacerbate these stresses, the authors said.

         For instance, rainbow trout are typically slaughtered by dewatering (asphyxia; also referred to as “air shock”), and hypoxia is known to induce stress responses in this species, according to the authors of the study.

         Because of stress, hyperactivity, and their consequences, the ease and efficiency of processing (filleting), fillet quality, and shelf life are all negatively affected. Pre-rigor filleting results in the highest filleting yields, but stressful harvest and slaughter conditions can accelerate the timeline of rigor development. Pre-slaughter stress and hyperactivity can also cause external mechanical damage (e.g., scale loss, bruising, lacerations) as well as internal injuries, according to the study.

         Trushenski’s approach was to simulate harvest and slaughter of rainbow trout that typically occur at large commercial farms and processors, and to test possible rested harvest protocols in this context.

         Her team found that direct current (DC) electricity effectively sedates fish and may be an appropriate alternative to chemosedation for rested harvest. Based on the principles of electrophysics, the strength and size of an electric field in water is highly controllable, allowing delivery of electricity to selected areas of a water body or, in the case of aquaculture, to selected holding systems or areas within holding systems.

         Sedation of fish encountering the electrical field is generally quick but is dependent, in part, on the fish’s conductivity, the electrical resistance of its skin and body and its size and surface area. The advantages of electrosedation are that it requires no expensive toxicology testing to prove that sedated fish are safe to eat, has no withdrawal period, poses no concerns regarding chemical disposal, offers faster induction times, and is easy to use. Unlike chemosedation, however, there is the possibility of physical trauma as a result of exposure to DC electricity, depending on the fish, waveform, and environmental conditions, the study found.

         Now Trushenski looks to other researchers to continue the observations she and her team made.

         “Our team probably won’t be doing more of this work, simply because the principal investigator (me) has changed jobs and the students involved are all moving on to other careers,” she explained. “That said, I don’t think the interest in rested harvest has diminished, so I hope other researchers are able to learn from what we have reported and take this research forward.”

— Erich Luening