Sea Lions and salmon nets

The Fish Site
by The Fish Site
24 April 2007, at 1:00am

CANADA - There have reports of recent episodes where some marine mammals have been caught in salmon farm nets.

Here is some information that may be helpful.

Each farm consists of a system of nets designed to keep fish in and predators out.

  • The grower net houses the salmon.

  • The shark guard, attached to the bottom of the grower net, is a false bottom designed to keep dog fish out.

  • Grower nets and shark guards are surrounded by a larger net, called a predator net, which surrounds an entire farm.

  • Sea lions attack the salmon farms hoping to gain access to the fish. In one case, it is believed the sea lions chewed through the predator net and shark guard to get at the salmon in the grower net but drowned when they couldn’t get back out.

Increased populations create stiff competition for food. A 275 kg California bull requires about 10.6 kg of fish daily (based on the energy composition of herring), while a 900 kg Steller bull would eat up to 25 kg per day. A herd of the size and species composition of the one in Tofino Inlet this spring would be expected to consume as much as 24 tonnes of fish daily.

The recent incidents have prompted a review of the effectiveness of farm procedures for predator protection. A meeting will be held with farm managers, net manufacturers and dive contractors to determine what changes can be made to reduce the risk to sea lion populations and other marine mammals and to adapt to a larger number of animals while continuing to protect farm stock from one of their natural predators. In addition, there are discussions underway with federal government officials to increase reporting of any interaction with marine mammals.

Dr David Groves provides some perspectives on how changing climate patterns may be contributing to an increase in sea lion populations.

Sea Lions are also having an impact on seals in Alaska. According to a recent article, steller sea lions that thrash harbour seals to death in their powerful jaws have become surprise suspects in the mysterious harbour seal decline in Southeast Alaska's Glacier Bay National Park. Click here to read the article.