Where does the pollution come from?
The increase in heavy metals over a relatively short period of time indicates that crabs are exposed to ongoing and increasing pollution. It has been difficult to explain this, as emissions of various toxic elements have generally been reduced in Norway. The crabs in the study by Ervik et al. were caught at various locations in one of Norway's main aquaculture areas (12, 13). There are ten fish farms in this area, the oldest of which has been in operation since 1996.
In 2018, a total of 1.35 million tonnes of farmed salmon and trout (wet weight) were sold in Norway, which required 1.8 million tonnes of fish feed (dry weight) (14). The fish feed contains both inorganic and organic pollutants. The environmental toxins that are not deposited in the farmed fish are left at the bottom of the fjord under open farming cages, in the form of uneaten feed and fish faeces.
Table 1 shows measurements of arsenic, lead, cadmium and mercury in fish feed, farmed salmon and trout, taken by the NFSA and the Norwegian Institute of Marine Research (Seafood Data) in 2018 (15, 16). Emissions from fish farming are calculated as content in fish feed minus content in produced salmon and trout. After comparing this with data from the Norwegian Environment Agency on total emissions in Norway (7), we calculated the share that comes from aquaculture. The aquaculture industry is not listed as a source of emissions of these substances on the Norwegian Environment Agency's website. The environment status page on the website specifically states that emissions of cadmium from fish feed are not included in the calculation of total emissions (7). If we include the emissions from aquaculture, total emissions of cadmium are an incredible 29 per cent higher (Table 1).
Table 1
Emissions of arsenic and some heavy metals in Norway in 2018
| | Arsenic (As) | Cadmium (Cd) | Mercury (Hg) | Lead (Pb) |
|---|
| Emissions from aquaculture | | | | |
| Mean concentration in fish feed in 2018 (mg/kg) (15) | 2.4 | 0.16 | 0.03 | 0.04 |
| Content in 1.8 million tonnes (consumption 2018) (kg) (15) | 4 320 | 288 | 54 | 72 |
| Mean concentration in farmed salmon and trout, 2018 (16) | 0.89 mg/kg | < LOQ1 | 0.02 mg/kg | < LOQ1 |
| Content in 1.35 million tonnes of salmon and trout (production figures 2018) (kg) | 1 202 | - | 27 | - |
| Emissions: content in fish feed minus content in produced salmon and trout (kg) | 3 118 | 288 | 27 | 72 |
| Total emissions in Norway | | | | |
| Emissions in Norway in accordance with the Norwegian Environment Agency in 2019 (7) | 23 tonnes | 1 000 kg | 500 kg | 87 tonnes |
| Addition from aquaculture in 2018 (%) | + 14 | + 29 | + 6 | + < 0.1 |
The emissions from uneaten feed and fish faeces are spread or accumulated depending on local currents and will impact on the facilities to varying degrees. In 2013, the Norwegian Environment Agency examined cadmium and copper levels in crabs caught at a distance of 80–1000 metres from three salmon farms in Sogn og Fjordane, Møre og Romsdal and Nord-Trøndelag respectively (17). Aquaculture uses around 1700 tonnes of copper annually to impregnate farming nets (figures from 2019), and around 80–90 % of this leaks into the sea and accumulates on the seabed (18).
The study concluded that there were no clear indications that fish farms are a source of cadmium in the marine environment. However, at two of the farms, the mean level of cadmium and copper was 1.6–2.4 times higher in brown crab meat caught 80 metres from the farm compared to 1000 metres. At the third facility, the level of cadmium and copper was the same regardless of the distance to the facility. At all locations, the levels of cadmium and copper were co-variants (Spearman correlation coefficient 0.9–1.0), which suggests that they stem from a common source.
Toxic elements will accumulate in fjords where farming takes place
Arsenic, lead, cadmium and mercury are natural elements and as such do not expire. As salmon and trout are farmed in an ongoing process with only a three-month break between each production cycle, toxic elements will accumulate in fjords where farming takes place.