Federal biologists spray mineral oil on cormorant eggs to reduce hatch success and manage populations.

In past years, otters were the greatest threat to a fishery.  Today, protective status for the double-crested cormorant, water turkey, has significantly increased their numbers and ensuing damage to fish populations among private ponds and commercial fish farms.

Cormorants are migratory species that typically winter in the south.  From daylight to dark, fish farmers attempt to chase them from hatchery farms.  Some assign employees to continually drive levees. Unfortunately, many operations are so large, birds criss-cross to other zones and resume feeding.  Small firms cannot sustain the never-ending strife and experience severe financial losses.  In extreme cases, federal fish and wildlife authorities authorize predation permits to reduce numbers.

Folks with recreational property who do not reside on their land, suffer great damage.  A scout bird finds a pond with abundant baitfish, brings a large flock, and they feed undisturbed.  Given a bird’s age, estimates speculate they eat one to three-pounds of fish daily.  In a short-time, large groups consume hundreds of pounds of bluegill, shad, and related species that rob bass of critical forage.  Sharp beaks also inflict severe injuries in missed attempts to catch them.  Cormorant feathers are not waterproof.  After extended diving, feeding excursions, you see them perched on a tree or stump with wings outstretched to dry.

Many years ago, cormorant numbers were kept in-check by agriculture use of the chemical DDT.  Birds ingesting DDT were unable to produce hardened egg shells, thus limiting hatch rates.  When DDT was banned, bird numbers soared.  Today, researchers use a technique called oiling to manage populations. The procedure is applied among nesting sites at the bird’s summer home around the Great Lakes and Canada.   At strategic times, mineral oil is sprayed on eggs.  Oiling has been highly successful in reducing hatch success.