Aquaculture Europe 2023

September 18 - 21, 2023


Add To Calendar 20/09/2023 12:00:0020/09/2023 12:15:00Europe/ViennaAquaculture Europe 2023ASSESSING RAINBOW TROUT WELFARE AT SLAUGHTER: AN INTEGRATIVE APPROACH USING BEHAVIOURAL, PHYSIOLOGICAL, PROTEOMIC AND QUALITY INDICATORS WITH A NOVEL TEMPERATURE STUNNING METHODStrauss 3The European Aquaculture Societywebmaster@aquaeas.orgfalseDD/MM/YYYYaaVZHLXMfzTRLzDrHmAi181982


Saraiva J.L.1,2* , Faccenda F.3, Cabrera-Álvarez M.J.1,2, Povinelli M.3, Hubbard P.C.2 , Cerqueira M.2, Farinha A.2 , Secci G.4 , Tignani M.V. 4 ,  & Parisi G.4


1 Fish Etho Group Association, Olhão, Portugal

2 Centre of Marine Sciences, Faro, Portugal

3 Fondazione Edmund Mach, S. Michele all’Adige, Italy

4 Dipartimento di Scienze e Tecnologie Agrarie , Alimentari , Ambientali e Forestali , Università degli Studi di Firenze




Fish are now widely recognised to be sentient animals .  Good welfare practices are  therefore  not only ethical, but they also  are demonstrated to improve fish quality and may even drive consumer choice. A critical point in the life of a captive fish is the final stages of the ir production. Not only this moment has the risk of dramatically affecting the welfare of the individuals, but it can also cause serious economic impacts to the farm, since the slaughter process can affect meat quality and carcass appearance. The most common method to slaughter fish (including trout) is by asphyxia either in ice-water or in the open air. However, this method induces prolonged and intense suffering, which is a poor practice in ethical, commercial, and legal terms. To achieve a humane slaughter practice, a stunning method needs to be implemented before slaughter, and it must render the fish immediately unconscious until death. In this regard, electrical stunning prior to slaughter has been proposed as a humane stunning method for trout , although there are anecdotal reports of trout  carcass  and fillet damage, with consequent decrease in value. In this experiment we tested a newly devised temperature stunner, in which the fish were immersed in water that achieves water temperatures of -8ºC still in liquid state. The objective was to evaluate and compare the effectiveness and welfare effects of four types of stunning methods in rainbow trout O. mykiss: cold shock by fast-chilling (FC) as a novel method, asphyxia (ASP) as the current method, electrical stunning (ES) as a humane method, and anaesthesia with MS-222 (AN) as a positive control. We used a multi-level approach to address the welfare of 176 juvenile trout, combining behaviour ( individual  swimming activity, equilibrium, opercular movement and eye-roll), physiology (heart rate and amplitude of electrocardiogram signal ) and stress biomarkers (plasma cortisol and osmolality) , and proteomics. We then proceeded to analyse the effects on shelf-life  fillet and quality of fish subjected to each of these methods, using a wide range of indicators (namely rigor mortis , water content, fillet colour, pH and ATP degradation) . This is, to our knowledge, the most detailed assessment of  rainbow  trout welfare at slaughter.

Behavioural indicators showed that  ES  presented quick and effective induction of unconsciousness (i.e., absence of all  consciousness indicators in 20s ), while AN  was  slower yet 100% effective. Fish subjected to FC  showed signs of extremely poor welfare (e.g., gill haemorrhage ,  brain damage, eye freezing and thawing, heavy mucus release)  and this method  was not only ineffective (only 13% lost consciousness) but also inconsistent (they recovered consciousness quickly).  ASP also induced extremely poor welfare ,  with  fish  dying slowly (up to 20 min or more) and maintaining consciousness until death in 44% of cases.

Physiology indicators showed that  ES  had higher osmolality than AN , while ASP higher osmolality than AN and FC . Fish subjected to  ASP  had  higher cortisol  levels than AN ,  while  ES and FC showed  large variation in this hormone level . Regarding electrocardiogram signal,  ES  presented stronger heartbeat than ASP , while  AN  showed stronger heartbeat than ASP and FC. No differences were found in heart rate.

 Regarding proteomics, FC and ASP  had the highest impact on the brain proteome with bimodal effects. On one hand, FC registered a significant number of reactions, biological and metabolic processes. On the other hand, ASP results suggest suppression of the coping mechanisms and therefore an allostatic cost that may infer higher suffering. Interestingly, AN also induced a significant impact on the brain proteome as indicated by significant involvement of programmed cell death processes, which is probably linked to the opioid nature of MS-222.

 Regarding quality indicators,  ASP and FC showed  faster rigor  onset  than AN and ES until 6h post-mortem .  Energy depletion  was  lower in AN and ES than  in ASP and FC ,  while  AN  showed  higher pH than all the other treatments, and  ASP  presented the  lowest pH. Fish subjected to  ASP showed the highest  fillet  weight loss after rigor , ES and AN  the lowest.

 Our results demonstrate that, other than anaesthesia (which is  generally  not permitted in fish farmed for food), electrical stunning consistently  showed good results as a humane stunning method. This  was  presented at all levels of welfare analysis, as well as in fillet quality terms. The fast-chilling method, on the other hand,  presented  very poor results both in welfare and in quality and  did  not seem to be  a  viable  humane alternative to asphyxia.

 Additionally, the proteome analysis opens new windows into the selection of fine-scale biomarkers of welfare and, more specifically, provides very interesting insights into the brain mechanisms of rainbow trout at slaughter.