The pre-slaughter period is one of the most critical for fish and can affect welfare and influence the quality of the final product during storage. The procedures that are carried out in this period are: fasting, handling and crowding , defishing or pumping, capture and finally death. All of these steps, regardless of how well done, can cause adverse effects on your fish. Stress during the antemortem period can interact with and influence the physiology of the animal , worsening their performance. Additionally, in the postmortem period , biochemical processes generated by the antemortem stress act on the muscle affecting the quality and durability of the final product. There are often unavoidable situations where multiple stress factors interact and affect the final condition. Aquaculture production involves the management of relatively small animals (less than 1 kg) with many thousands of individuals in small spaces (between 37 and 120 million farmed fish were killed for human consumption in 2010). Therefore, from an individual point of view, animal welfare is an important issue in aquaculture. In this sense, it is crucial to obtain information on how the handling of fish during pre-slaughter and slaughter periods can affect their welfare and the quality of the final product. This knowledge will help to propose changes in current handling methods during the final phase of the production process, being one of the critical points that most affect the welfare of the fish.
Crowding is the first stage in harvest and transportation operations. Wall (2001) comments that crowding is one of the main causes of discomfort during harvest , being shortage of oxygen the most common problem associated with crowding . However, even maintaining high oxygen levels, crowding changes many other aspects of fish physiology, the effects of which can be observed up to several days later (Ortuño et al., 2001). Different species respond to this intervention in different ways. Under commercial conditions, the density of fish used for seabass crowding is approximately 250 kg/m3 . In sea bass, there are few studies on the effects of crowding and increased physical activity during pre-slaughter operations.
Technology now make possible to monitor the behaviour of small groups of individual fish as bioindicators of wellbeing . Bio-loggers may detect unusual patterns in fish heart rate, which could serve as an early indicator of whether fish health or welfare is becoming compromised. The use of heart rate bio- loggers to monitor fish heart activity, has shown to provide unique insights into the physiological and behavioural state of fish over time (Hvas et al., 2020).
The objective of the present study was to assess the effects of confine ment in the heart activity of seabass during the period of crowding using individually implanted bio-loggers, which can detect seabass stress levels during crowding events. Commercial size seabass were held in a flow-through system with full strength seawater for a whole year. Dissolved oxygen was maintained close to saturation. Fish were held in a rectangular tank of 10000 L capacity. Stoc king density was 11 kg/m3.
Ten days prior to the experiment , nine commercial- size seabass were implanted with DST milli-HRT loggers (StarOddi® , size: 13.0 × 39.5 mm, weight: 11.8 g) wich monitor heart rate (fH), electrocardiogram (ECG) and internal temperature (range 5 °C to +45 °C). These loggers are also equipped with real-time clock with accuracy of ±1 min/ month.
These loggers recorded heart and acceleration hourly for that particular fish for the 36 h prior to crowding and then every 2 minutes for the 180 minutes which was the crowding period. After this period, fish were sacrificed and the loggers recovered.
Dur ing crowding, the level of water was lowered (mimicking bringing the fish to the surface) and fish were cornered at one end of the tank with a net which was place across the tank preventing them from moving. During the crowding exposure period fish were held at an estimated density of 118 kg/m3.
This experiment was performed in the summer with a water temperature of 28ºC and in the winter with a water temperature of 9.5ºC.
Fish used in this experiment had an average standard length of 32. 48 ± 3. 25 cm in the summer and 27.66±2.1 cm in the winter and an average wet weight of 555. 20 ± 141.97 g in the summer and 389 ± 68.14 g in the winter.
Base line values of heart rate were 57 ± 2 beats per minute (bpm) in the winter and 69 ± 2 bpm in the summer, showing a strong relationship with water temperature . Averaged heart rates and accelerations showed slightly variations along the 180 minutes that lasted crowding, but no significant effects were observed by crowding procedure during winter . In the summer, however, the heart rate of fish increased significantly at the beginning of the crowding period and slowly decreased over time, with great variation among individuals.
T herefore, the effects of crowding on heart rate seem to depend on the seasonal period when it is performed at the farms.
References
Brijs, J. et al. Remote physiological monitoring provides unique insights on the cardiovascular performance and stress responses of freely swimming rainbow trout in aquaculture. Sci Rep 9, 9090 (2019).
Hvas M, et al. Heart rate bio-loggers as welfare indicators in Atlantic salmon (Salmo salar) aquaculture. Aquaculture Vol 529 : 735630 (2020).
Ortuño J et al. Effects of short-term crowding stress on the gilthead seabream (Sparus aurata L.) innate immune response. Fish Shellfish Immunology, 11:187–197 (2001)
Wall, A.J. 2001. Ethical considerations in the handling and slaughter of farmed fish, in: Kestin, S.C. & Warriss, P.D. (Eds.), Farmed Fish Quality. Fishing News Books, Oxford, UK, pp. 108–115.