• rapid and extreme pressure changes (water pressures within the turbine may increase to several times atmospheric pressure,
  then drop to subatmospheric pressure, all in a matter of seconds),
• cavitation (extremely low water pressures cause the formation of vapor bubbles which subsequently collapse violently),
• shear stress (forces applied parallel to the fish's surface resulting from the incidence of two bodies of water of different velocities),
• turbulence (irregular motions of the water, which can cause localized injuries or, at larger scales, disorientation),
• strike (collision with structures including runner blades, stay vanes, wicket gates, and draft tube piers),
•  and grinding (squeezing through narrow gaps between fixed and moving structures).

Aside from "grinding," each of these threats can also be attributed to the propeller of an outboard or inboard/outboard motor or the impeller of a personal watercraft at speed.

Impacts of small boats

Consider a boat traveling at thirty miles per hour. It will have a propeller with a diameter of perhaps 15 inches rotating at somewhere between 1500 and 2500 revolutions per minute. As small, rapidly rotating propellers do, as it rotates it cavitates, vaporizing the water it passes through in what can best be described as an ongoing explosion. If the propeller is rotating at 2,000 rpm, at their periphery the blades are moving through the water at almost 100 miles per hour.

At a vessel speed of thirty mph the propeller will pass through approximately a million and a half gallons or 5 acre/feet of water in an hour of operation.[1]  Thus forty boats traveling at 30 miles per hour impact as great a volume of water, and the organisms in it, as a base load generating station.

A "target" estuary

Barnegat Bay is a large, shallow estuary lying behind New Jersey's barrier islands. Approximately 40 miles long, approaching 5 miles at its widest, it has an area of 47,615 acres. With an average depth of 4.6 feet, it has a total volume of 220,000 acre feet. A decade ago Barnegat Bay had over 200 commercial marinas with a capacity to store 12,000 boats. Its shores are heavily developed, primarily with single family residences. Many of these have one or several boat slips as well. There were also 45 launching ramps on the bay. According to estimates by the New Jersey Department of Environmental Protection, over 50,000 vessels (90% power boats) used Barnegat Bay each year.