Supersizing salmon farms in the coastal zone: A global analysis of changes in farm technology and location from 2005 to 2020

Abstract

Salmonid aquaculture, producing nearly 3 million tons per year, has expanded across temperate seascapes around the globe in recent decades. Cage technologies used to farm salmonids are thought to have changed in both size and location in coastal environments, yet remarkably little data exists to explain these major developments. Using satellite images from Google Earth, we mapped and measured sea-cages and their positions to quantify changes between historical and current use in major production regions. While cage numbers have remained similar across the major producing nations from 2005 to 2020, cage diameters have increased by 27–87%, which has resulted in large increases in farm sizes (total surface area enclosed by cages per site) of 61% in Chile, 84% in Scotland, 212% in the Faroe Islands and 221% in Norway. Cage type has changed from predominately square cages within steel platforms to plastic, circular arrays over the past 15 years, with notable exceptions such as Chile and the Pacific coast of North America. While farms in the top producing regions in 2020 were slightly further from the coast compared to 2005, salmon farming remains an activity that occurs close to the coast with farms on average sited just a few hundred metres from the nearest land. Distance to the nearest neighbouring farm increased for Norway, Chile and the Faroe Islands, but decreased for Scotland and the Atlantic coast of North America. Changes in farm technology used and their location have environmental and production consequences, with larger farms producing more waste at an individual site, bigger cages impacting the dissolved oxygen available to fish within them, and proximity of farms to their neighbours influencing the risk of disease incidence and spread. Characterising cage technologies in use across the major farming regions may allow for management practices to be critically analysed to improve fish welfare and production and reduce farm biosecurity threats and environmental impacts.