In Harford County, Maryland, June 10, 2016
We’re at the base of the Conowingo Hydroelectric Dam, which spans the Susquehanna River near the Maryland-Pennsylvania border. Fish swimming up the River are blocked here by the Dam, except when the fish ladders are working; fish swimming down the River get sucked into the Dam’s turbines and are spit out on the other side, dazed or dead.
Fish predators wait for the fish on the downstream side of the Dam, where we wait. We focus on the Bald Eagles, Ospreys, Great Blue Herons, and Double-Crested Cormorants, each of which has its favored mode of hunting.
The Eagles often perch in the big trees on the ridge above the River, where they are virtually hidden in summer. It’s hard for an Eagle to hide completely, however: its wing span can exceed seven feet (that’s a professional basketball center rotated 90 degrees) and It can weigh up to 14 pounds.
Bald Eagles tend to circle above their scaly prey, swoop down, pluck the fish neatly from the water, and then arc upward. They can reach up to 99 miles an hour in a dive.
The Ospreys usually take their meals to nests outside the area, where the Eagles can’t steal from them. An Osprey is mostly wings – it weighs only about three pounds, but its angled wings can span up to six feet. It sometimes hunts by the circle-and-pluck method like an Eagle; however, an Osprey often hovers like a helicopter high above the water, tips over and power dives straight down, head first until the last second. This bird can reach up to 80 miles-a- hour this way, knock a big fish silly, disappear under the water while gripping it, swim up with its wings, and fly off with the prey.
Great Blue Herons also can pluck a fish from deep water while flying or while briefly floating like a duck, but usually that prey is a dead or dying fish on the surface. These birds, for the most part, fish from a still stance in shallow waters. They can reach four feet in standing height, weigh almost eight pounds, and have a wingspan of over six-and-one-half feet.
A standing Great Blue that sees a nearby fish will coil its long neck, slowly move its spear-shaped head toward its unsuspecting prey, and then strike like a snake into and below the water. As often as not, it will emerge from the imploded water with an amazed fish in its sharp beak.
Unlike the others, a Double-Crested Cormorant usually swims below the surface after its prey, using powerful webbed feet and, sometimes, its wings. This prehistoric-looking bird’s bright blue eyes pierce the murky depths and its hooked and serrated beak can grab and hold not only slippery fish, but ultra-slippery eels.
The Cormorant can weigh up to five-and-one-half pounds, be almost three feet long, and have a wingspan of up to four feet.
The waters below Conowingo Dam contain larger predators than those mentioned here, but these are far from self-sufficient and much less interesting.
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Barbara and Dick