Fish can be affected by physical and psychological stressors during harvest and prior to slaughter. Rested harvest (sedation prior to slaughter) can be used to prevent the negative effects that stress can have on meat quality, taste and animal welfare.
Chemosedatives can be used for this process but they are subject to US Food and Drug Administration (FDA) regulation and there are currently none approved for rested harvest application in the US. Electrosedation protocols have been developed for various species, but this technology has not been evaluated previously in the context of rested harvest.
As part of the study, triplicate groups of rainbow trout (12 fish/group, ~500 g/fish) were exposed to three minutes of crowding and chasing directly after capture (control) or following a 30 second treatment with eugenol (10 mg/L) or one of five electrosedation protocols.
After the challenge, fish were sampled to determine blood chemistry profiles (0, 30, and 60 min post-challenge, three fish/time point) or slaughtered by dewatering to determine time to mortality and rigor, processing efficiency, and fillet quality (three fish/time point).
Fish treated with eugenol exhibited a significantly longer time to mortality (21.5 min), compared to control fish (13.6 min) and electrosedated fish (13.0-15.5 min), said Dr Trushenski.
"The time to rigor did not vary significantly among treatments (27.5-38.3 min postmortality), but one-third of the eugenol-treated fish did not go into rigor within the evaluation period (60 min), suggesting we may have underestimated time to rigor in this treatment," said Dr Trushenski.
Compared to unchallenged reference fish, plasma cortisol and lactate increased in all treatments following the pre-slaughter stress challenge; however, the response of fish treated with eugenol or certain electrosedation protocols appeared to be less robust and more transient.
There was no statistically significant effect of rested harvest on fillet appearance or pH, though some haemorrhaging was observed among fish exposed to the chemo- and electrosedation protocols.
Overall, the results suggest that rested harvest may be beneficial for rainbow trout slaughter and processing, but further research is needed to refine the protocols and characterise their effects.
