Introduction
Climate change has threatened coastal ecosystems through elevated sea temperatures while contending with increased nutrients through expanding development. Both warmer waters and higher nutrient loads have resulted in intensification of coastal oxygen deficits (Breitburg et al. 2018). Adequate dissolved oxygen (DO) concentrations through finfish farms are paramount as low oxygen can directly impact the health and growth of fish. In the short-term, low oxygen can impact feed intake and feed conversion rates, and if prolonged, can result in fish stress, vulnerability to diseases, and mortality (Oldham et al. 2019). Open net-pen farms that contain high stocking densities or exhibit low oxygen during warmer months, may use oxygenation devices to increase DO (Berillis et al. 2016). However, the transport and distribution of oxygen can be costly in ocean environments thus it is only occasionally used and the viability of these systems have yet to be adequately examined. This study explores the effectiveness of oxygenation in an Atlantic salmon net-pen farm to inform farm managers on its use as climate change threatens the industry.
Materials and methods
This study was conducted at a Cooke Aquaculture Atlantic salmon farm consisting of 6 circular cages (48 m in diameter, 10 m deep), located within a bay in southwest Nova Scotia. The study period was between June 1 and November 30 2019, with the deployment of oxygenation within each of the cages between August 4 and November 6. The oxygen system was the NetOx Net (OxyVision AS, Norway), which releases micro-bubbles of oxygen as a curtain at 8m depth. The cage analyzed in this study contained 2 DO and temperature sensors (AquaMeasure, InnovaSea Systems, Inc., Bedford, Nova Scotia), one at 2m and one at 7m. Additionally, there was a reference sensor located ~75 m outside the cage. Satellite sea surface temperature (SST; 4km AVHRR Pathfinder Version 5.3) produced by NOAA National Centers for Environmental Information, was also used as a far field reference for cage temperature. Physical (e.g. tide, wind stress) and biological factors (e.g. theoretical fish consumption), which typically affect oxygen dynamics within a farm, were examined to determine how oxygenation may interact with those processes.
Results
Through June 1 – August 26, there was a strong stratification between 2m and 7m for both temperature and oxygen. The activation of oxygenation (Aug. 4) coincided with a sharp drop in temperature at both depths. There was a significant difference in temperature when comparing 3 weeks before/after the onset of oxygenation, with average values dropping from 16.7 to 15.4℃ and from 14.3 to 13.2℃ for 2m (p<0.001) and 7m (p<0.001), respectively (Fig. 1A). However, this drop was not observed in the satellite SST. On August 26th, autumn mixing began and stratification broke down resulting in a negligible difference between depths. While the temperatures declined at the onset of oxygenation, there was a concurrent increase in oxygen from an average value of 7.3 to 7.8 mg L-1 and from 8.3 to 8.9 mg L-1 at 2m and 7m, respectively. This resulted in an overall significant increase in oxygen on average 3 weeks after oxygenation compared to 3 weeks before (p<0.001; Fig. 1B).
Discussion
The results of this study suggest that although SST was similar before and after oxygenation, the system may have caused an upwelling of cooler waters which decreased temperatures throughout the cage. Similar findings were described in studies by Endo et al. (2018) and Bergheim et al. (2006). This decrease in temperature increased solubility while decreasing fish metabolism, furthering an increase in oxygen concentrations. Overall, the results imply that the oxygenation system increased upwelling and DO at the cage level until autumn mixing became the dominant source of re-aeration. This suggests that the system was effective promoting an increase in DO, but it does not need to run for the entire duration of the warm season, it may only be useful until de-stratification, which would save the farm associated economic costs.
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