{"id":25478,"date":"2018-11-08T00:05:12","date_gmt":"2018-11-08T08:05:12","guid":{"rendered":"https:\/\/hakaimagazine.com\/?post_type=custom_news&p=25478"},"modified":"2018-11-08T07:06:35","modified_gmt":"2018-11-08T15:06:35","slug":"some-seabirds-thrive-at-the-margins","status":"publish","type":"custom_news","link":"https:\/\/hakaimagazine.com\/news\/some-seabirds-thrive-at-the-margins\/","title":{"rendered":"Some Seabirds Thrive at the Margins"},"content":{"rendered":"

Seabirds and other ocean predators know the hunting is best at the subtle boundary between mixed and stratified water. Diving seabirds forage for prey at these intersections, as do certain fish and cetaceans. Researchers have known this for decades, too, but it\u2019s never been clear why these boundaries\u2014known as tidal fronts\u2014are good places to hunt.<\/p>\n

Stratified water is layered\u2014its physical and chemical qualities changing through the water column. Maybe it\u2019s warm at the surface and cold at the seabed, for instance. Stratified water is common where the ocean is dead and the current is slow. On the other side of a tidal front, the water is well mixed, its properties consistent no matter how deep you go. Mixed water is common in shallow areas of the sea with fast currents.<\/p>\n

To investigate why hunting seabirds seek out tidal fronts, James Waggitt, a marine ecologist at Bangor University in Wales, and his colleagues crisscrossed the northern Celtic Sea in a boat and counted diving seabirds of two species: the common guillemot<\/a> and the Manx shearwater<\/a>. They recorded how often they saw the birds plunging into the water to hunt or resting on the surface. They also measured the abundance of fish and used the water\u2019s depth and the speed of the current to map where the ocean was likely to be stratified or mixed.<\/p>\n

The researchers found that in mixed water, shoals of fish were big and close to the surface\u2014both good things for a diving predator. Yet these large groups of fish were rare; finding one would be difficult. In stratified water, by contrast, fish shoals were more common, though the fish swam deeper and in smaller groups. Neither water type was perfect from a seabird\u2019s perspective.<\/p>\n

\u201cBut at the interface of these two water bodies,\u201d Waggitt says, \u201cyou tend to get the best of both worlds.\u201d Within the tidal front, mediumly abundant, medium-sized groups of fish swim at medium depths<\/a>.<\/p>\n

\"illustration<\/a>

Fish tend to congregate in schools of different sizes and swim at different depths depending on the conditions in the water. These differences in fish behavior affect the success of predatory diving seabirds. Illustration by Femke Vangent<\/p><\/div>\n

Waggitt speculates fish behavior within tidal fronts may also factor into the seabirds\u2019 hunting success. \u201cPerhaps when they\u2019re moving between the two water bodies, the behavior of the fish is changing,\u201d he says. Perhaps the shoals become disorganized when transitioning between habitats, for instance. Or, they may pay less attention to predators.<\/p>\n

Jonathan Green, a seabird biologist at the University of Liverpool in England who was not involved in the study, says the work \u201cdefinitely adds insight into how and why predators such as seabirds use tidal fronts.\u201d Many studies have shown the pattern of predators hunting at these boundaries, he says, but few have given insight into why.<\/p>\n

Predators at the tidal front, however, may face a future where prime hunting spots are less fruitful than they once were. Waggitt says renewable energy infrastructure, such as wind or tidal turbines, can change the speed of currents or the properties of the seabed and alter ocean stratification patterns around these installations. Tidal fronts could shift, and if seabirds don\u2019t shift their behavior in response, they might start coming up dry.<\/p>\n","protected":false},"excerpt":{"rendered":"

Seabirds and other ocean predators know the hunting is best at the subtle boundary between mixed and stratified water. Diving seabirds forage for prey at these intersections, as do certain fish and cetaceans. Researchers have known this for decades, too, <\/p>\n

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