Data from randomized control trials released hatchery salmon treated with anti-parasitic treatment

Data used in the article "Parasite spillback from fish farms reduce return rate of wild salmon"

Each release group has been used as a randomized control trials (RCT) of hatchery reared salmon smolts where half of the fish has been treated with an antiparasitic drug. Description of this method has been given in various other publications (Vollset et al. 2014, Vollset et al 2016, Skilbrei et al. 2013). The method involves rearing salmon eggs originating from the national Gene Bank to smolt size in hatchery facilities during one year, and then treating the salmon smolts with fish feed pellets coated with emamectin benzoate (SLICE®). These fish are then released into the river or transported in tanks or mobile net pens further out in the fjord before release. The fish are tagged with either coded-wire-tags (CWT; years 2000-2017) or Passive Integrated Transponders (PIT; 2015-2019) so that it is possible to identify them as they are recaptured or registered on an antenna upon their return as adults. In a few trials, another antiparasitic treatment (Substance EX) has been used, but in most cases the EB has been the only available treatment. Releases of hatchery reared salmon in freshwater have not been successful in this system, i.e. very few fish have returned from any group released in the river, lakes or estuary of Vosso. Since the release groups are also a part of a restoration effort of the Vosso salmon, some years fish have only been released in the fjord. There has been some variation in the release sites in the fjords, but for the purpose of this study we group the release groups in either group that has been released in the outer fjord (70-105 km from the river mouth) and the inner fjord (15-70 km from the river mouth), and freshwater (approx -10 to 15 km from the river mouth). The two most prevalent locations are at Manger (WGS84; 60.63918, 4.92149) and Arna (WGS84; 60.50812, 5.37777).

Sea lice surveillance

Sea lice surveillance on sea trout has been conducted at Herdla, the northern peninsula of the island Askøy (WGS84; 60.568972, 4.963010) since 2009. Here, trout have been caught using a trap net that has been developed specifically to capture and treat trout while minimizing sea lice loss during handling (Barlup et al. 2013). From an earlier study by Vollset et al. (2018), it has been shown that the lice numbers on sea trout on this site correlate with the infestation pressure of fish farms in the outer region of the fjord. This area is also one of the largest fish farm zones with coordinated production and fallowing in the outer fjord system where all the released salmon smolts must migrate (see Vollset et al. 2018). This is also the area where surface salinity layers permit salmon lice to overlap with out-migrating salmon smolts (Vollset et al. 2016).

The number of trout caught during the monitoring season has varied with weather conditions, sampling intensity, and number of traps operated. The way that trout are handled is described in more detail in Vollset et al. (2018), but in brief, the trap chambers are checked daily, and individual trout are transferred from the trap using a hand held dip net and are either euthanized and placed in zip-lock bag or transported in a large bucket with aerated water to land. Euthanized samples are kept cold and frozen when at land, and later thawed and counted in the lab, while live samples are counted after being sedated with half dose (0.05 g/L) of MS222 and then assessed for salmon lice in a high-contrast bucket using a headlamp by trained personnel. Since 2015 the sea lice surveillance at Herdla is also operated as a part of the Norwegian national sea lice monitoring program.

We aimed to use a standardized time period from which to assess sea lice numbers on sea trout that can be representative of the lice infestation pressure from when the tagged hatchery salmon smolts are released. When counting sea lice on sea trout, the most observable lice are large chalimus and mobile stages, while recently attached copepods are more likely to be missed. Therefore, we use total lice counts on sea trout from Julian day 135 to 165 as an assessment of the infestation pressure the salmon smolts must experience. This corresponds to approximately 15 May to 15 of June, and is based on a study on progression rate of salmon smolts from hatchery smolt in this area (Vollset et al. 2016). To account for the fact that larger fish will attract more parasites, we use parasites per gram fish per individual and average data to get one index per year. This method is expected to provide a fair index of interannual variation of the infestation pressure.

Table 1 Description of column names in csv file