Expert topic: Predators: Seals
by Rebecca Sherratt, Features Editor, International Aquafeed
In this new series of articles for International Aquafeed's Expert Topic section, we are delving into the predatory species that can cause many a farmer to pull out their hair in frustration: predators. Ranging from sharks, herons and seals, these are the species that pose potential hazards to your fish farming operations and here we help provide more insight into these species and the best preventative measures to take towards them. In our first article of the series we take a look at what is perhaps the most troublesome threat: seals.
Coastal and marine fish farmers will no doubt be familiar with the threat of seals invading their fish farms. Serious damage can occur to both fish and farming equipment in the hands of rogue seals, ranging from lethal and non-lethal wounding, disease transfer and escape of fish out into the wider ocean. Not only does this significantly negatively impact fish, but also often results in economic losses for the farmer.
Whilst such damaging attacks from seals could be considered quite rare, there have been cases of seal attacks on fish farms that have costed farms millions in rectifying. Risks like these mean that farms must ensure to take the appropriate precautionary measures to ensure that their stock are kept safe.
However, such precautions can also be difficult to execute, as the control methods must also not place the predator, a valued species in its own right, to be placed at risk. Many federal and provincial laws in place in various countries and states place restrictions on the specific kinds of methods that farmers can utilise to keep their farms secure, so there is no one solution to the problem.
The enemy of the fish farmer
Seals are renowned in the aquaculture industry for their ferocious appetite. They eat a variety of fish such as hake, sand lance, rockfish and flounder, but most farmers will know that they are especially big fans of salmon. They do, on rare occasion, also eat crustaceans and krill. Seals can be either nocturnal or diurnal, so it is not always easy to predict when they will feed. Adult seals consume between four-to-six percent of their body weight every day, which equates to a significant amount of fish- approximately 4kg.
The Common seal (also known as the Harbour seal) can swim up to 50 kilometres in search of feeding grounds and can also swim upstream into large rivers in search of prey. They can reach depths of up to 50 metres (154 feet), but their average dive is closer to 20 metres (66 feet). Common seals swallow their prey whole underwater or bring larger catches up to the surface, where they shake and tear them into more manageable pieces.
Seals use their whiskers to locate prey in turbulent waters. They are remarkably adept at sensing fish, with the ability to determine both the shape and size of fish from a distance. They can reach impressive speeds of up to 35 kilometres per hour in the water and they can lower their heart rates during diving to reduce the need to return up for air, making them an especially deadly enemy of fish everywhere.
The fish farming technology industry is evolving at a rapid rate, and this means that predator deterrents are also becoming increasingly more refined and effective. Some of the most popular methods to deter seals from your fish farms include:
Cages: The first stage to preventing seals from taking a bite out of your fish is using barriers to prevent them approaching your stock. These usually take the form of cages and perimeter netting in open-water aquaculture.
Netting: A variety of companies now offer special netting constructed from refined rope fibres that are highly resistant to seal and shark attacks. These often involve a mix of plastic and steel rope fibres combining to result in a resilient and powerful barrier. For nets to proves especially effective against predatory attacks the holes must be small, to ensure that the seal cannot enter the cage, nor can stock escape.
Between 2009-2012, it was reported that 17.5 percent of salmon escape cases from fish farms were attributed to predator interference. Whilst this figure shows that predators are not the leading cause for escapes, the figure still highlights that seal attacks can cause a great deal of concerns for farmers who cannot afford to lose their stock.
Predator nets, of a skirt or curtain type, are also often used to isolate fish from potential predatory attacks. These, however, can come with their own difficulties. Predator nets are notoriously difficult to install and can be prone to entanglement with other parts of the cage system. Water flow is also reduced through the use of predator nets which negatively impact the water quality the fish are subject to.
Seal blinds: Evidence suggests that seals tend to attack the bottom of nets where morts will reside. Holes found towards the bottom of nets can likely be attributed to seals trying to knaw at morts. Due to this, effective mort removal systems must be implemented in all fish farms. Seal blinds are also used to prevent this issue from occurring. Seal blinds are thicker materials at the base of nets, that obscure the base of nets where mort build-up typically occurs. Seal blinds are usually 5x5m squares of netting.
Acoustic Deterrent Devices (ADDs): Often used as a combination solution in conjunction with other preventative methods, certain auditory aids can be used to drive seals away from places where they pose a hazard. These are also sometimes called 'seal scarers'. Noise-emitting deterrents have been proven to help reduce seal attacks on fish farms, but it has also been recorded that seals can eventually learn that such deterrents are only auditory in nature and, in fact, pose no real threat and will be largely ignored.