Table 1_Experience of spiny dogfish Squalus acanthias interactions with aquaculture installations in Norway.xlsx

The spiny dogfish (Squalus acanthias) is a common shark species found along the Norwegian coast. This coast is also utilized for salmon farming, and farmers report incidents of spiny dogfish biting through nets and often entering the fish cages. This causes breaches that allow farmed salmon to escape leading to both financial loss and an ecological risk to endangered wild salmon populations. Unfortunately, the extent and impact of such incidents has not been studied. Here, we conducted survey-based research among the fish farms, unravelling geographical distribution, cause, frequency and impact of spiny dogfish incidents in Norway. We quantified the experiences of fish farmers regarding spiny dogfish incidents and found that most incidents were localized in southern and western Norway during autumn and winter periods. Most spiny dogfish attacks are in groups, primarily targeting the base of fish cages and often attacking the same cage multiple times. These attacks are mostly associated with the presence of dead fish in the cages; the timely removal of carcasses largely mitigates these incidents. However, considering the impact of escapees on wild fish populations, combined with periods of elevated fish mortality within the cages when it is difficult to rapidly remove dead fish, multiple mitigation approaches are necessary. Moreover, spiny dogfish is listed as a vulnerable species and an effective measure in keeping them away from sea cages will reduce their mortality associated with fish farms. Hence, we also discuss suitable shark-deterrents as mitigating measures without harming either the spiny dogfish or the farmed fish.