Ocean wind turbines generate more than just electricity. They also appear to be creating a buffet for seals, say scientists.

When rooted in the ocean floor, a wind farm can become a sort of artificial reef, a home for invertebrate animals. These animals attract predators, which in turn attract species higher up the food chain, eventually leading to the fish that seals eat.

Researchers found this phenomenon wasn't limited to just the turbines. Underwater pipes also attracted the marine predators, who returned to feast multiple times.

The scientists observed eleven harbor seals outfitted with GPS tracking tags in the North Sea frequenting two active wind farms, Alpha Ventus in Germany and Sheringham Shoal off the southeast coast of the United Kingdom. One seal even visited 13 times, according to a report published this week in the journal Current Biology.

The wind turbines make up a grid. When foraging for food, the seals moved "systematically from one turbine to the next turbine in a grid pattern, following exactly how the turbines are laid out," says study author Deborah Russell of the University of St. Andrews. "That was surprising to see how much their behavior was affected by the presence of these artificial structures and how they could actually adapt their behaviors to respond to that."

At a glance this would seem like heaven for the seals, and a net gain for the natural world. But researchers say that such a feast could have detrimental effects on the balance in the food chain.

Dr. Russell explains that one of two situations could have created this buffet. First, the structures could have simply attracted prey that would otherwise have been spread sparsely across the region, leaving the overall mass of living animals in the ocean unchanged. Alternatively, the structures could provide a protected area, increasing overall amounts.

If the seals are encountering a concentration of prey rather than increased prey overall, says Russell, these snacks are "very vulnerable to be Hoovered up."

"Because everything is so linked, if the seals are Hoovering up a prey species, that will have effects on other predators, other prey of those prey," Russell explains.

But even if the opposite scenario is true and the prey animals are thriving in the protection of the structures, the balance in the ecosystem could still be thrown off. "It's all very linked and the food web is very complex," says Russell. "There can be quite a big effect of a disturbance in one area of the system."

A quiet neighborhood?

The structures actually limit the impact of boats, as large ships cannot maneuver through the turbines. Also, fishing activity is reduced around the wind farms.

But while external forces are kept out of this artificial paradise, maintenance vessels still must navigate the waters. Russell says "there's obviously also negative consequences of spending a prolonged time in a very manmade environment" for the seals. "There might be more interactions between these maintenance vessels and these seals because they are attracted to the area."

Also, "these operational wind farms do make quite a lot of noise," she says. Although some researchers have said that it is unlikely the noise level is high enough to cause hearing damage for the seals, Russell isn't quite sure. "Some seals actually preferentially spend a long period of time right at the structures, where it is obviously going to be the noisiest."

Although it is unclear whether prolonged exposure to the turbine noise will damage seal hearing, sound does likely perform an important function for the animals." We do know, in some cases, that the males make noises when they're displaying to the females," says Russell. "So it's important to be able to hear that. But we're not absolutely sure to what degree sound is used for prey or predator detection."

Tracking the seals

Researchers outfitted over 100 harbor seals with GPS tracking tags in order to examine their interactions with manmade structures underwater.

Of these seals, 11 spent time in one of the two wind farms in the North Sea. Most of these mammals demonstrated the grid-like movement patterns on recurring visits.

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One of the next steps for researchers is to determine how often and how many seals frequent these foreign structures. Russell says they also need to determine whether the prey are "simply being concentrated or are they actually increasing in biomass overall as a result of the structures."

"If it turns out that these are actually beneficial in terms of increasing the prey species of the seals and they're providing foraging opportunities for the seals, then it may be that the structures can be designed in such a way to maximize the chance of artificial reefs developing on those structures," says Russell. But if "it turns out that they overall have a negative impact, then maybe these structures could be designed to minimize the chances of an artificial reef."