Fasting up to 34 °C days in rainbow trout, Oncorhynchus mykiss, has little effect on flesh quality
Introduction
Fish are often fasted before slaughter in aquaculture production systems, but concerns have been raised regarding the maximum duration of fasting in different species commonly produced in marine (Mørkøre et al., 2008) and continental aquaculture (Barcellos et al., 2010), including trout (FAWC, 1996).
Fasting prior to slaughter evacuates the gut and reduces oxygen demand and waste production (Robb, 2008). However, fasting may also increase stress levels, thus influencing post mortem biochemical processes, mostly anaerobic glycolysis and ATP degradation rate. If the stress is high enough, it affects the onset and release of rigor mortis, which determines the duration of fish freshness (Poli et al., 2005). It has also been shown that the rate of spoilage depends on several factors, such as the fish species and the amount of food found in the intestines (Macagnano et al., 2005). By reducing the amount of faeces in the intestines, spoilage is delayed, and digestive enzyme activity is reduced after rigor mortis has occurred. Thus, short term fasting has an effect on product shelf-life (Huidobro and Tejada, 2004).
Very little is known about the optimum period of fasting prior to slaughter in fish and it is not clear how long rainbow trout can be starved before flesh quality is compromised. Despite the lack of data, both the Farm Animal Welfare Council and the Humane Slaughter Association recommend that the maximum limit for fasting should be 48 h before slaughter (Farmed Animal Welfare Council FAWC, 1996, Humane Slaughter Association, HSA, 2005). Nevertheless, it is not uncommon for rainbow trout in the wild to survive long periods of food deprivation (e.g. several months over winter, Ashley, 2007). Prolonged starvation is shown to influence quality of fillets (Einen and Thomassen, 1998) but the effects of short-term fasting are not clear. Muscle pH, water holding capacity and rigor mortis have been reported to be influenced by starvation prior to slaughter in fish (Álvarez et al., 2008, Mørkøre et al., 2008) but no information is available on how they change in relation to time in rainbow trout subject to short-term fasting. However, Ferreira-Pinto et al. (2007) point out that feeding interruption may be applied beyond 48 h in order to improve fish quality.
Less is known about the best time of day to slaughter trout in terms of the final product quality. However, the choice of an optimal moment to slaughter is of a great importance, as it may be dictated by diurnal rhythms in physiological stress parameters, including cortisol (Polakof et al., 2007) and glucose (Holloway et al., 1994). In fish, the effects of fasting on daily changes of several plasma metabolites and hormones like glucose, lactate, cortisol, GH or thyroxin have been characterised in a number of species demonstrating in several cases that some of those changes are dependent on feeding (Figueroa et al., 2000, De Pedro et al., 2005). According to the literature reviewed above, no clear conclusions can be made so far about the best time to slaughter trout and the effect on flesh quality is not known. This optimal time to slaughter can determine when the effects of transport, pre-slaughter handling and slaughter may be less stressful. A lower stress response during those procedures should also result in fewer problems in relation to flesh quality. In addition, few studies have considered a wide variety of indicators and their diurnal rhythms, comparing fasted fish with a significant number of control (i.e., fed) fish. That is important to be able to suggest a time frame in which commercial slaughter is most feasible.
In this study we aimed to analyse the impact of short term fasting (up to 3 days) and time of day of slaughter on several flesh quality parameters in rainbow trout.
Section snippets
Fish material and experimental design
The study was carried out at the aquaculture facilities of the School of Forestry Engineering (Madrid, Spain). The fish farm is located on a small slope divided into terraces where raceways are set. The terrace arrangement allows the downward water flow to be distributed among the different raceways by means of channels. For the experiments, two parallel raceways (volume 5.16 m3) were filled with freshwater from an underground well, supplying a constant water flow. We used 180 rainbow trout (
Physicochemical flesh quality
The significance of the main effects as well of their interactions is presented in Table 1. Significant interactions between the hour of slaughter, duration of fasting and hours post mortem were found for each flesh indicator and thus displayed in Fig. 1, Fig. 2, Fig. 3.
Overall, muscle pH showed a marked tendency to decrease up to 24 h post mortem and then stabilise, with very small variations. The minimum pH was reached just at that point or later on.
Initial muscle pH at 08H00 did not differ
Physicochemical flesh quality
Overall, flesh quality was not compromised after the three-day fasting period prior to slaughter, since it may be too short a period of time to elicit important changes in flesh. Although the hour of slaughter showed significant interactions with the duration of fasting and sampling time, it did not have a major impact on flesh quality, although fish slaughtered at 20H00 showed a higher variability on muscle pH among days of fasting.
In general terms, the pH of the flesh always fell after
Acknowledgements
This project was financed in part by Project AGL2010-19479 (Spanish Ministry of Science and Innovation) on the analysis of the effect of fasting and slaughter hour on stress and product quality in the rainbow trout.
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