Negative impacts of the sea lice prophylactic emamectin benzoate on the survival of hatchery released salmon smolts in rivers

Abstract

Emamectin benzoate (EB) is a prophylactic pharmaceutical used to protect Atlantic salmon (Salmo salar) smolts migrating out of rivers and into the ocean against sea lice parasites.Randomized control trials comparing the marine survival of smolts treated with EB to a control group is used to calculate the fraction of marine mortality attributable to sea lice parasitism. However, it is assumed that there is no baseline difference in survival induced by the application of EB treatment. We used a combined laboratory and field study approach to investigate the potential impacts of EB treatment on behaviour and survival of hatchery-reared Atlantic salmon in western Norway. In aquaria experiments, EB-treated salmon smolts did not differ significantly in exploratory behaviour. Fish from treated groups responded similarly to simulated predator attack with spontaneous escape and elevated gill beat rate. Three rivers in the Osterfjord system of western Norway were selected for field experiments, Dale, Modalen, and Vosso. Dale River smolts were treated with intraperitoneal EB injections and had lower probability of detection in a wolf trap downstream of the release site than control smolts.Salmon smolts raised in the Vosso River hatchery were treated with EB delivered in their food and were detected on PIT antennas at the rivermouth at lower rates than control fish, but only when released at downstream sites. Calculation of risk ratios suggested that the bias in mortality caused by treatment with EB decreased the estimated survival of treated fish from an expected 18% to 46%, reducing the observable negative impact of sea lice on Atlantic salmon smolts.The results suggest that estimates of the fraction of mortality attributable to sea lice may be underestimated due to lower baseline survival of treated fish caused by treatment and bring urgent attention towards a potential systematic underestimation of the impacts of sea lice on wild salmon.

Direct evidence of increased natural mortality of a wild fish caused by parasite spillback from domestic conspecifics

“…Despite this new designation, owing to the industrial production of fish being farmed in open sea cages, sea lice are not an isolated stressor. Atlantic salmon in Norway are facing urgent threats from multiple angles [9,21]. Multiple stressors in ecological systems are challenging to unravel because interactions can be synergistic, confounding management efforts.…”

“…Evidently, the anti-lice treatment only partially shielded the out-migrating salmon from sea lice during periods of high infestation pressure. This is in line with other recent studies suggesting that (i) salmon lice in western Norway are resistant to emamectin benzoate [31], (ii) the effect of treatment in RCTs is only significant and positive when infestation pressure is high [19], (iii) treatment may not be effective across all individuals in the group [20], and (iv) treatment may also incur negative effects on marine survival cancelling out the positive effect of the antiparasitic treatment on the return rate of salmon [21].…”

“…Some have raised concerns about how efficient these antiparasitics are at shielding out-migrating salmon smolts [14,19]. For example, studies have shown that there is variation in the amount of drug taken up through oral administration within groups of treated fish [20], and that the treatment itself has been shown to reduce the survival of salmon smolts as they migrate to sea [21]. But perhaps of most concern is that the treatment (e.g.…”

Negative association of sea lice from fish farms on recreational fishing catches of Atlantic salmon

“…Jackson et al, 2013;Skilbrei et al, 2013;Vollset et al, 2016). However, recent studies indicate potential methodological biases (Lennox et al, 2020;Vollset et al, 2023), raising concerns about the accuracy of previous findings and the validity of subsequent conclusions. Obtaining quantitative estimates of overall stock abundance typically demands year-round trapping facilities, which are limited (Thorstad et al, 2014).…”

“…Other researchers have disputed this conclusion and find, using similar data, that the density of farm lice substantially affects wild salmon returns (Gargan et al, 2012;Krkošek et al, 2013;Otero et al, 2011;Shephard & Gargan, 2017. Moreover, recent studies suggest that the impact of sea lice may have been underestimated in RCTs, as indicated by the reduction in the survival of salmon smolt associated with the treatment (Lennox et al, 2020), and by findings of reduced treatment efficacy attributed to increased resistance among sea lice (Vollset et al, 2023).…”