Determination of optimal degree days of fasting before slaughter in rainbow trout (Oncorhynchus mykiss)

Abstract

Pre-slaughter fasting is a common practice in aquaculture to ensure emptying of the digestive tract, but little is known about how long fish should be fasted, in terms of degree days (°C d), to clear the gut without affecting their welfare or flesh quality. In this study, 180 rainbow trout (Oncorhynchus mykiss) were fasted for 3, 4, 5, 6, 7 and 9 days (17.2, 22.3, 28.6, 35.3, 41.8 and 55.3 °C d, respectively) and different morphometric, hematological and flesh quality parameters were measured. The slaughter weight, relative growth and hepato-somatic index did not decrease until after 7 days of fasting (41.8 °C d) and stomach content was similar and near zero after 4 days of fasting (22.3 °C d). Conversely, fasting time increased carcass yield.Plasma concentrations of cortisol, glucose and creatine phosphokinase enzyme were lower with increasing days of fasting. However, trout slaughtered after 5 days of fasting (28.6 °C d) had a higher flesh pH at 0 hours post-mortem than 3 and 4 days fasted trout and, consequently, earlier establishment of rigor mortis. Furthermore, liver color could be useful as a stress indicator since its hue decreases after 5 days of fasting (28.6 °C d). In conclusion, a pre-slaughter fasting period from 17.2 ° C d to 22.3 ° C d makes possible to obtain a complete emptying of the digestive system in rainbow trout and simultaneously it can minimize the stress response with consequent better flesh quality. Aquaculture 301, 57-63. Blasco, J., Fernández, J., Gutiérrez, J., 1992. Fasting and refeeding in carp, Cyprinus carpio L.: the mobilization of reserves and plasma metabolite and hormone variations. J. Comp. Physiol. B 162, 539-546. Borderías, A.J., Sánchez-Alonso, I., 2011. First processing steps and the quality of wild and farmed fish. J. Food Sci. 76, R1-R5. Caggiano, M., 2000. Quality in harvesting and post-harvesting procedures-influence on quality. Fish freshness and quality assessment for sea bass and sea bream. Cah. Options Méditerr. 51, 55-61.

Physio-metabolic response of rainbow trout during prolonged food deprivation before slaughter

“…In the present study, a 10-day fast (107 °C d) caused a significant decrease in fish weight and condition factor compared to controls, which coincides with other studies that report a decrease in body weight in rainbow trout after 1 to 6 weeks without food (Sumpter et al 1991;Pottinger et al 2003;Bermejo-Poza et al 2017). The lack of a difference between 0D and 5D trout sustains the hypothesis that fish can quickly adjust their metabolic rate according to food availability (Lines and Spence 2012).…”

“…Both stomach content and digestive somatic index (DSI) decreased in 5D trout (55 °C d), which agrees with Lines and Spence (2012) and McMillan and Houlihan (1992) who estimated an average of 1-5 days of fasting to ensure a completely empty stomach. Also, previous studies by our research group confirm that 19-23 °C d of fasting are enough to empty the stomach in rainbow trout (López-Luna et al 2013;Bermejo-Poza et al 2017). However, 10 days of fasting (107 °C d) were needed to modify gastric pH, increasing it slightly.…”

Fasting combined with long catch duration modifies the physio‐metabolic response and flesh quality of rainbow trout

“…DSI and gastrointestinal content decreased with fasting, and consequently, carcass yield increased. In our study, a fasting time of 135.6°C d was enough to reduce the gastrointestinal content considerably, supporting the theory that gastrointestinal emptying is a highly temperature-dependent process in fish and a fasting period of 1-5 days is required to produce a nearly complete emptying(Bermejo-Poza et al, 2017;Lines & Spence, 2012;López-Luna et al, 2013). HSI also decreased with fasting, possibly due to the use of glycogen reserves, mainly, and other metabolites such as lipids or proteins to maintain homeostasis(Davis & Gaylord, 2011;Torfi Mozanzadeh et al, 2017).…”

“…UF: unfasted; F: fasted; S: short catch duration; L: long catch duration [Colour figure can be viewed at wileyonlinelibrary.com] (see, e.g. Bermejo-Poza et al, 2017, 2018López-Luna et al, 2013).…”

“…Bermejo-Poza et al, 2017;Lines & Spence, 2012;López-Luna et al, 2013). HSI also decreased with fasting, possibly due to the use of glycogen reserves, mainly, and other metabolites such as lipids or proteins to maintain homeostasis(Davis & Gaylord, 2011;Torfi Mozanzadeh et al, 2017).…”