Filmed at Scottish Sea Farms' processing plant at South Shian on 15 September 2018
Category 3 wastes are classified as "low risk" whereas Category 2 wastes are classified as "high risk" due to "having infectious diseases" and include "carcasses of animals killed for disease control purposes" and "carcasses containing residues from authorised treatments": https://www.gov.uk/guidance/animal-by-product-categories-site-approval-hygiene-and-disposal#abp-categories-explained
Read about problems at South Shina in 2012 via:
FDA Health Warning for Scottish Farmed Salmon: http://donstaniford.typepad.com/my-blog/2012/10/fda-health-warning-for-scottish-farmed-salmon-.html
Salmon Farming is Sickening: FDA warning for "adulterated" Scottish farmed salmon "injurious to health": http://donstaniford.typepad.com/my-blog/2012/10/salmon-farming-is-sickening-vomiting-in-scotland-leads-to-staff-walkout-.html