Impact of salmon farming on Atlantic cod spatio-temporal reproductive dynamics

Abstract

Salmon farming in marine net pens is a major activity in many temperate regions. This industry may affect coastal ecosystems in several ways, such as with waste pollution and parasite spillover. Less is known about the extent to which salmon farming disrupts the use of inshore spawning grounds by wild fish, such as the Atlantic cod Gadus morhua. Acoustic telemetry was therefore used to explore cod space use during the spawning season in a coastal region in mid-Norway with multiple salmon farms. Acoustic receivers were placed in clusters at 5 known cod spawning grounds and 6 nearby salmon farms. Data from 481 adult cod caught at the spawning grounds during 2017-2019 and equipped with acoustic telemetry transmitters were analysed. Overall, fewer fish were detected at farms than spawning grounds, even when accounting for distance from release point. Individual cod residency (days detected / duration of spawning period) was generally higher at the spawning grounds close to farms but low at the farms themselves, with little apparent spawning at the farm localities. In contrast, spawning was clearly occurring at the nearby spawning grounds, with cod spending weeks (n = 316) or months (n = 158) there during the spawning period. Males had longer residence times at spawning grounds than females, likely linked to the cod mating system. Overall, we found little support for the assertion that salmon farms disrupt inshore spawning dynamics of cod using nearby spawning grounds presently, either by attracting spawners to farms or causing fish to leave these grounds.

Movement diversity and partial sympatry of coastal and Northeast Arctic cod ecotypes at high latitudes

“…Morita et al, 2012). Previous studies on the habitat use of spawning cod have documented several sex-specific behaviours, both related to residency, with males spending more time at spawning sites than females (Skjaeraasen et al, 2021), and to aggregation, with females aggregating over smaller areas than males during daytime (Dean et al, 2014). We observed no effect of sex on cod behaviour and while it is possible that this is caused by random noise in our spawning-season data, we argue that it is more likely that this aspect is inhibited by the spatial resolution of the analyses and further scrutiny of the within fjord movements would be necessary to determine if the behaviour of males and females differ during the spawning period.…”

Attraction of cod Gadus morhua from coastal spawning grounds to salmon farms

“…Details of telemetry receiver grid deployment, detection range testing, the capture of fish, the fish tagging procedure, and the transmitter specifications are given in (Skjaeraasen et al 2021) and therefore not reiterated in detail here. In brief, Innovasea VR2W receivers were deployed at 33 fixed stations in a semiexposed coastal region in mid-Norway in November 2016.…”

“…In brief, Innovasea VR2W receivers were deployed at 33 fixed stations in a semiexposed coastal region in mid-Norway in November 2016. These receivers detected and stored acoustic signals emitted from acoustic transmitters, with an estimated detection efficiency of ~50% at 500 m from the transmitter (see Skjaeraasen et al 2021). Receivers were moored in catch clusters (CC) at 5 known cod Gadus morhua spawning grounds (Skjaeraasen et al 2021): Glasøysvaet (CC1), Lauvøysvaet (CC2), Araneset (CC3), Åkvika (CC4) and Dromnessundet (CC5).…”

“…These receivers detected and stored acoustic signals emitted from acoustic transmitters, with an estimated detection efficiency of ~50% at 500 m from the transmitter (see Skjaeraasen et al 2021). Receivers were moored in catch clusters (CC) at 5 known cod Gadus morhua spawning grounds (Skjaeraasen et al 2021): Glasøysvaet (CC1), Lauvøysvaet (CC2), Araneset (CC3), Åkvika (CC4) and Dromnessundet (CC5). Single receivers were also placed on 6 salmon Salmo salar farms neighboring these spawning grounds (Fig.…”

“…However, the spatial movement of the commercially valuable coastal Atlantic cod, also commonly observed in association with fish farms, has been less studied. In response, Skjaeraasen et al (2021) studied the possible impact of sal mon farms on the spawning dynamics of cod on monitored spawning grounds.…”