THE EFFECTS OF OXYGEN SUPPLEMENTATION ON FARMED ATLANTIC SALMON Salmo salar BEHAVIOR USING ACOUSTIC TELEMETRY

Introduction

The solubility of oxygen in seawater is dependent on pressure, salinity, and temperature, where warmer waters decrease the solubility of oxygen. Temperature and dissolved oxygen (DO) togethe r  play a crucial role in the growth and health of  farmed fish  (Elliott & Elliott, 2010). When DO falls below the optimal range (~6.0 mgL-1 at 15°C) fish  stress  and mortality  levels increase. In the past few years, examples of mortality event have been documented worldwide,  associated with hypoxic water events interacting  with st ress from fac tors such as disease (Galea et al ., 2018;  Evans, 2020). In order to counteract the negative effects of low DO on fish health , farms  have initiated oxygen supplementation including  pumping  of oxygen  nanobubbles  via  diffusers placed inside  sea cages . The respons e of fish to changes  in oxygen as well as the presence of bubblers was investigated on a commercial Atlantic salmon (Salmo salar )  farm using acoustic tags to monitor 3D fish position .

Materials and Methods

 The  farm  study site is located in St. Margarets Bay, Nova Scotia, Canada with 6 net pens measuring 48m diameter with maximum depth of 11 m and arranged in one column running southwest to northeast. Fifteen adult salmon (60-72 cm and 4.5 kg) were  surgically  implanted with VEMCO acoustic tags (model V9P-180).  An acoustic array was installed consisting of 8 HR2 receivers  at  2 and 7m depth, used to triangulate horizontal position of each fish. Tags were programmed to transmit resulting data on 3D position every 3 seconds.

 An oxygen supplementation trial was  conducted  in autumn 2019, during which compressed oxygen was continuously supplied to all cages on site. The study cage was supplied with 4 ring-shaped diffusers placed at 7m depth.  No control cage was available due to the risk of fish mortality during low DO months. Temperature and DO data were collected using AquaMeasure wireless sensors  in cage center at 2m and 7m depth.

Three fish variables were calculated using positioning data: velocity [ms-1 ], distance from cage center [m], and turning angle [°], and depth [m]. The study was split into two periods 1) during the oxygen trial (2 Oct 2019 – 7 Nov 2019) and 2) after the trial (8-25 Nov 2019).

Results

 Temperature ran ged from ~8-14°C during the study ,  with  similar  values between 2 and 7m depths during the entire time series . There was a  gradual  temperature decrease through the autumn, then declining more steeply at the  water column overturn in early November. Oxygen was always slightly higher at 2m than 7m, fluctuating over a range of 7-8.3 mg l-1 until the end of October when it rapidly declined to a low of 6.0 mg l-1. From that minimum, oxygen showed a continual increase throughout November to  9 mg l-1 during the fall overturn

Swimming velocity of the tagged population averaged 0.72 ± 0.39 ms-1 with higher velocities during the day than at night. Fish depth showed a daily cycle with deeper values during day than at night. The tagged population averaged 14.5 ± 5.1 m from cage center swimming in a counterclockwise direction with an average turning angle of 53.2 ± 44.1°.

During the oxygen trial, all tagged individuals displayed a slower swimming speed,  negatively related to water temperature (Fig. 1), contrary to literature. Furthermore, 77% of tagged individuals showed a change towards cage edge during the trial compared to after, with 85% of tagged fish displaying straighter turning angles during the trial. During the trial, 85% of the tagged fish swam significantly shallower than after the trial.

 Climate change is triggering increased sea surface temperatures and decreased dissolved oxygen, leading to more sites requiring supplemental oxygen.  Our work suggests behavioural changes associated with  oxygen supplementation . However the implications  of  these changes for feeding, growth, and fish health are not yet clarified . Continuously developing  farm  sensing technology associated with intervention such as oxygenation  provides  further  scope  for  management options leading to improved fish welfare and more sustainable aquaculture.

References

Elliott, J. M., & Elliott, J. A. (2010). Temperature requirements of Atlantic salmon, brown trout and Arctic charr: predicting the impacts of climate change. Journal of Fish Biology.

 Evans, O. (2020).  Authorities investigating Mowi 20,000 salmon mortality . Retrieved from Salmon Business: https://salmonbusiness.com/authorties-investigating-mowi-20000-salmon-mortality/ . Accessed 21 March 2021.

 Galea, S., Street, E., & Dunlevie , J. (29 May 2018). Macquarie Harbour salmon: 1.35 million fish deaths prompt call to ‘empty’ waterway of farms . Retreived from ABC News Australia: https://www.abc.net.au/news/2018-05-29/salmon-deaths-in-macquarie-harbour-top-one-million-epa-says/9810720 . Accessed on 1 April 2021.