PERFORMANCE IN SEA OF ATLANTIC SALMON EXPOSED TO CROWDING AND PUMPING DURING PRE-SMOLT PHASE
Introduction
During smolt production, farmed Atlantic salmon are repeatedly handled during crowding, pumping, vaccination, sorting and transport. As a result of this handling the fish may become less robust and more sensitive towards stressors, and less able to perform in sea (Johnston et al. 2003). This issue is presently in focus since the salmon industry faces challenges with high mortality during the sea water phase. Sedation of the fish with Aqui-S during different handling operations is one possibility to decrease the stress and damages that may follow from the repeated handling.
The aim of this study was to investigate how repeated crowding and pumping over a period of five weeks in fresh water affected smolt performance after transfer to sea water. A further aim was to investigate how the use of the sedative Aqui-S during pre-smolt handling affected fish performance during seawater production phase. The project is funded by the Norwegian Seafood Research Fund (FHF) (project #900660).
Materials and Methods
Three thousand pre-smolt were evenly distributed between twelve tanks (flow through FW; 3m3) in March 2014 (mean 53.7±1.0g (±SD)), and allowed to acclimatize for two weeks. The fish were distributed between 4 experimental groups in triplicate: Gr 1 = Control, no crowding/pumping, no use of Aqui-S (AQUI-S, New Zeland Ltd.); Gr 2 = Control, no crowding/pumping, with sedation with Aqui-S in the same intensity as for group 4; Gr 3 = Crowding/pumping fish once every week for five weeks without use of Aqui-S; Gr 4 = Crowding/pumping fish once every week for five weeks with use of Aqui-S. Aqui-S (5 ml/m3) was applied to the tank water during crowding and pumping according to a protocol provided by the distributer ScanVacc. During crowding water level in the tanks were lowered until density of 150 kg/m3 was obtained, and after two hours of crowding the fish were pumped (Heathro pump 6") to an empty tank and back to the original tank. The pumping lasted for 40min. During four of the five treatments five fish per tank were killed by percussive stunning, weighed and measured for length, immediately sampled for blood (glucose, lactate, chloride), and evaluated for scale loss and skin damages (after a scale from 0=nothing; 1=some; 2=severe). Six weeks after experimental start the fish were transported with truck (10h) to the sea water cage farm were they will stay for three months (until July 2014). Groups 1 and 3 that were not given Aqui-S during the experiment were not sedated during transport, while groups 2 and 4 that were sedated during experiment were given Aqui-S during transport. Prior to the transport 12 fish were transferred to 72h sea challenge test (34.5‰), where after they were sampled for blood, and 2nd gill arches were put on liquid N2 and frozen in -80"C for later analyses of Na/K+-ATPase activity to investigate degree of smoltification.
Results
Crowding and pumping caused no mortalities during the freshwater phase. All five weeks of treatments caused elevated levels of glucose and lactate compared to control fish. The levels of lactate were significantly lower in the crowded/pumped fish that were sedated with Aqui-S compared to the crowded/pumped fish that were not sedated. Plasma chloride are currently being analysed and the results will be presented. External welfare score of the fish (Fig. 1a) showed that at the third treatment there were significantly more crowded and pumped fish with scale loss and in some occasions starting wounds (Fig 1b) compared to control fish. The difference in scale loss among crowded/pumped fish and control fish increased in the last sampling. The sedation of fish during crowding/pumping did not affect scale loss.
The truck transport caused elevated levels of glucose among the fish, and the fish treated with Aqui-S had the highest levels of glucose. Levels of lactate did not increase with transport, but Aqui-S caused significantly lower levels of lactate both before (fish sampled from the truck before departure) and after (fish sampled from the truck immediately after arrival) transport. Scale loss increased as a result of transport; however Aqui-S did not influence on scale loss. The fish are presently in three sea cages. Results of performance both during and after three months in sea will be presented.
Discussion and Conclusions
This experiment is a follow up from a previous experiment where long term effects of pumping and crowding were investigated, and where the fish were on-growing in controlled tanks. The fish were significantly stressed from the crowding and pumping in both experiments. However, in the present study sedation of the fish prior and during crowding and pumping caused less stress. Behavioural observations of the sedated fish showed that they swam and behaved similar to the control fish, but they did not burst swim. Plasma chloride will show the progress of smoltification, and may differ between the groups. Also Na/K-ATPase activity in the sea challenge test will detect differences in smoltification. An earlier experiment showed that crowded and pumped fish were less smoltified than control fish. Scale loss was significantly more severe in crowded and pumped fish compared to control fish and the difference increased with time during the freshwater phase. Also the transport caused severe scale loss. The three month long sea cage phase will reveal whether the scale loss and beginning wounds observed during freshwater phase have long term effects on survival and performance. In the previous experiment it was seen that the control fish experienced high mortality as a consequence of transfer to sea water, and it was concluded that some handling of fish my adapt them to the coming handling associated with the transport. The present experiment will tell if this is also the case when transporting the fish to an on-growing commercial farm situation and if the use of sedative during handling in freshwater can have positive effects on survival rate and growth of Atlantic salmon in sea.
References
Johnston IA., Manthri S., Alderson R., Smart A., Campbell P., Nickell D., Robertson B., Paxton C.G.M., Burt M.L (2003). Freshwater environment affects growth rate and muscle fibre recruitment in seawater stages of Atlantic salmon (Salmo salar L). The Journal of Experimental Biology 206, 1337 - 1351.
