Effective anti-shark measures

Norwegian fish farms are experiencing growing conflicts with the common shark (Squalus acanthias), the spiny dogfish. During autumn and winter months, attracted to the smell of dead fish in the net pens, the dogfish manage to create sizeable holes in net pens. Not only do these attacks result in farmed fish escaping and significant costs in repairs, but the sharks sometimes also feed on the live salmon, causing a welfare dilemma. These issues are increasingly concerning for farmers, primarily in Southern and Western Norway.

Sharks are attracted to dead fish

It appears that dead fish within the cages draw in the spiny dogfish. Once they infiltrate the barriers, the sharks eat dead fish at the bottom of the cages but sometimes harm and consume the live, farmed fish. In response, farmers continuously remove deceased fish from the cages and examine the nets for signs of damage using divers and underwater cameras. Despite these efforts, it has been difficult to find a solution to avert these issues.

– To date there is no effective method to prevent spiny dogfish incidents in fish farms. Hence, it is crucial to test and develop active and passive methods to prevent these spiny dogfish incidents in fish farms. In the project we tested active measures that interact with the dogfish sensory system in both laboratory and field trials, explains Pradeep Lal, Senior Reseacher in NORCE.

Laboratory and field trials

Researchers in NORCE aimed to address this growing problem by trialing the use of an electromagnetic (EM) pulse and other sensory deterrents in both laboratory and field settings.

Spiny dogfish of size 60-85cm were caught and held in a lab environment in the aquatic facility at the University of Bergen, with conditions simulating the surrounding sea environment. The researchers introduced a series of stimuli to these captive sharks, including EM pulses, sound of orcas, and skin extracts from conspecific and mackerel (as an attractant).

Field trial was conducted in collaboration with Salarsafe AS. Salarsafe AS has developed an electromagnetic shark protection system for fish farms and is a supplier of the system. For information on the technical details and availability of the system, Stian Rennestraum at Salarsafe AS can be contacted.

The trials with Atlantic salmon were conducted at the Marine Research Centre at Mekjarvik, NORCE.

Electromagnetic pulses most effective

Pradeep Lal explains how results indicated that the sensory deterrents produced a change in the sharks' movement behavior:

– The exception was the orca sounds, to which the sharks surprisingly showed no reaction. The smell of mackerel prompted explorative behavior, while the EM pulses and skin extracts triggered an avoidance response. Similar results emerged in field trials: orca sounds didn't deter the sharks, while EM pulses repelled them.

Interestingly, EM pulses also showcased a promising result with salmon smolts. The pulses led to an increase in cortisol, glucose, and lactate levels immediately after administration, indicating a stress response, but no long-term stress signs were detected. Moreover, growth parameters remained consistent within the control and treatment groups, suggesting that the EM pulses did not induce chronic stress in these farmed fish.

Further research

These findings propose the EM pulses and extracts from conspecific skin could function as effective shark deterrents, while having minimal impact on the farmed fish. The next step is to research the nature of these deterrents further and to assess their long-term effectiveness in real-world fish farms. Furthermore, optimizing the EM pulses for different conditions, such as varying shark sizes and hunger states, is essential to ensure the strongest deterrent possible.