Introduction
The transport from fish farms to processing- and harvesting facilities causes salmon stress by putting them through a series of exhaustion and recuperation phases involving crowding, transport in boat, pumping to waiting cage and additional pumping before being killed and bled. Furthermore, the salmon production industry report too high and varying core temperature of fish after processing because of inefficient cooling of the fish (Tobiassen, 2013). Stress and increased temperature are important factors influencing blood coagulation mechanisms and normally causes decline in blood coagulation time (Taveres-Dias and Olivera, 2009). Reduced coagulation time may result in insufficient bleeding and lower quality and shelf life of fish products (Olsen, 2011; Olsen, 2014; Roth, 2009; Roth, 2005). However, few studies have been done on the effect of shorter blood clotting time on the quality of salmon products.
The aim of this study was to see if stress and temperature influences the coagulation time of blood for farmed salmon and see if this affects the quality of the finished product (smoked fillets).
Materials and Methods
Three experiments were conducted on farmed salmon (body length 63±3 cm, weight 3.3 ±0.9 kg) during the period of October to December 2014. In the first experiment, the effect of temperature on blood coagulation time was investigated. Three parallels of blood samples from unstressed salmon (N = 6) distributed among four different water baths temperatures 0.5°C, 6.4°C (same temperature as holding tank), 10°C and 16°C. The blood was kept at constant temperature respectively and blood clotting at the different temperatures was recorded at first signs of coagulation.
In the second experiment, the fish (N = 20) were placed in a large open system swim tunnel (1400 L fish chamber, water temperature 6.4 °C). During the swimming trial, water velocity was gradually increased from 0.5 to 2 BL/s (1.2 m/s) in 10 minutes. The first 11 fish became exhausted and were taken out and killed as they stopped swimming, the remaining 9 were chased around with a dipnet until they ceased swimming. All fish were killed immediately after exhaustion.
In the third experiment, the fish were exposed to crowding for 1.5 hrs (N=10) and 3 hrs (N=10) at the same density as in holding seine for pumping to processing facilities. A control group (N=10) was taken out before crowding. Crowding was done by gradually reducing the volume of the net pen with a sweep net until the wanted density was achieved.
In all experiments, fish were killed by blow to the head and blood was collected from the caudal vein. Blood clotting was evaluated following the method previously described by Ruis and Bayen (1997). All the fish were bled for 30 minutes in seawater (~8 °C), and stored on ice before instrumental measurement of residual blood in the muscle. Instrumental calculation of the residual blood along the spinal cord was carried out with the help of hyperspectral imaging of smoked fillets (Heia, 2012 ). Images were evaluated by a panel of three persons and scored 0-3, 0 means no blood and 3 means large continuous detections of blood along the centerline.
Results and Discussion
The time it took for blood to coagulate was significantly (p < 0.001, r2 = 0.72) reduced from 33±6 min (median ± mad) at 0.5˚C to 9±3 min at 16˚C. Swimming has a non-significant effect on blood coagulation time from 15±5 min in the control group to 10±3 in the swum group. Additional chasing reduced blood-clotting time to 9±5 min. Crowding for 1 and 3 hour significantly (p < 0.01) shortened blood coagulation time from 21±7 min in the control group to 11±4 min and 10 ±3 min respectively. The control group from the crowding experiment had a longer coagulation time than the control group from the swimming experiment despite same water temperature. This may be an indication that the fish were stressed in the holding tanks.
Crowding fish for three hours significantly (p < 0.05) increased the residual blood score compared to the control and in the one hour crowding group. Both swimming groups also increased the residual blood score significantly (p < 0.05). There was a significant (p <0.01) negative correlation between blood clotting time and residual blood score with longest coagulation time having lowest blood score. The probable explanation is that blood coagulates and impairs the effect of bleeding and hence increased the amount of residual blood.
We conclude that high temperature and stress reduce the effect of bleeding and thereby the quality of smoked salmon fillets. Stress and temperature should be kept low to secure a good quality salmon product.
References
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