Any Mississippi catfish farmer can tell you, double-crested cormorants eat fish - lots of fish. In fact, documented prey species for cormorants include channel catfish, gizzard shad, green sunfish, bluegill, white crappie and largemouth bass. This is bad news for the hundreds of fish farmers in the southern United States whose profits can be gobbled up by these aquatic birds.
Research has shown that during the winter months in the South, cormorants usually forage within 16 km of their roosts. Cormorants roosting near aquaculture facilities may include these facilities (and their fish) in their daily foraging routines.
Current management approaches to reduce cormorant damage at fish farms include dispersing cormorant roosts near the farms with the hopes of moving the birds to areas where their feeding activities will not impact aquaculture. But how do wildlife managers know if they are targeting the most appropriate roosts?
One way to answer this question is to study the diets of birds at various roosts to see what they've been eating. In fact, if scientists can distinguish between farm-raised and wild caught prey in the diet of cormorants, they can establish feeding histories for depredating birds at various roosts. This will help target wildlife damage management activities at roosts that are causing the most damage. Traditional methods for determining cormorant diets have relied on visually inspecting the contents of the birds' gastrointestinal (GI) tract. As you can imagine, this is not only a very dirty job, but it can also be complicated by the degree to which the items are already digested.
In hopes of developing a more efficient and accurate method for identifying the food items in a cormorant's gut, NWRC scientists recently analyzed the fatty acid profiles of both wild and farm-raised fish to determine whether it is possible to distinguish between the two groups. With the cooperation of several commercial catfish producers, the Mississippi Wildlife Services Operations, Mississippi Agricultural and Forestry Experiment Station, and Noxubee National Wildlife Refuge, NWRC scientists collected fish for a total of 12 sample groups. Fish species included farm-raised channel catfish (from three different farms and one research facility), gizzard shad, and green sunfish, plus wild caught channel catfish, gizzard shad, green sunfish, bluegill and largemouth bass. Lipids were extracted from the fish and the resulting fatty acids were identified using gas chromatography/mass spectrometry.
Results showed it is possible to not only distinguish between farm-raised channel catfish and wild catfish in the diet of cormorants, but also that it may be possible, based on fatty acid analysis of gut contents of cormorants, to assess the actual impact of birds from a given roost or colony on a specific channel catfish producer.
Future research will examine the fatty acid profiles of other diet items that may be eaten by cormorants at aquaculture farms and examine profiles in cormorants during controlled feeding studies.
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