A fish ladder
, also known as a
, fish pass
, is a structure on or around artificial
barriers (such as dams
) to facilitate diadromous
. Most fishways enable fish
to pass around the barriers by swimming and leaping up a series of
relatively low steps (hence the term ladder
) into the waters on the other side. The
velocity of water falling over the steps has to be great enough to
attract the fish to the ladder, but it cannot be so great that it
washes fish back downstream or exhausts them to the point of
inability to continue their journey upriver.
Fish ladders are known to be very effective. Written reports of
rough fishways date to 17th-century France, where
bundles of branches were used to create steps in steep channels to
bypass obstructions. A version was patented in 1837 by Richard
McFarlan of Bathurst, New Brunswick who designed a fishway to bypass a dam at his
water-powered lumber mill. In 1852–1854, the Ballisodare Fish Pass
was built in County
Sligo, Ireland, to draw
salmon into a river that had not supported a
fishery. In 1880, the first fish ladder was built in
Island on the Pawtuxet Falls Dam. The ladder was removed in 1924, when the
Providence replaced the wood dam with a concrete one.
As the Industrial Age
and other river obstructions became larger and more common, leading
to the need for more-efficient fishways.
There are five main types of fishways:
- Pool and weir
- Baffle fishway (Denil, Larinier, Alaskan Steepass, or other
- Fish elevator
- Rock-ramp fishway
- Vertical-slot fish passage
- See also Eel Ladder and Fish migration.
A pool and weir
is one of the oldest styles of
fish ladders. It uses a series of small dams and pools of regular
length to create a long, sloping channel for fish to travel around
the obstruction. The channel acts as a fixed lock
to gradually step down the water level; to
head upstream, fish must jump over from box to box in the
A baffle fishway
uses a series of symmetrical
close-spaced baffles in a channel to redirect the flow of water,
allowing fish to swim around the barrier. Baffle fishways need not
have resting areas, although pools can be included to provide a
resting area or to reduce the velocity of the flow. Such fishways
can be built with switchbacks
minimize the space needed for their construction.Baffles come in
variety of designs. The original design for a Denil fishway was
developed in 1909 by a Belgian scientist,
Denil; it has since been adjusted and adapted in many
Alaskan Steeppass, for example, is a modular prefabricated
Denil-fishway variant originally designed for remote areas of
A fish elevator
or fish lift
its name implies, breaks with the ladder design by providing a sort
to carry fish over a barrier.
It is well suited to tall barriers. With a fish elevator, fish swim
into a collection area at the base of the obstruction. When enough
fish accumulate in the collection area, they are nudged into a
hopper that carries them into a flume that empties into the river
above the barrier.
Connecticut River in Holyoke, Massachusetts, for example, a fish elevator lifts up to 500 fish
at a time, 52 feet (15.85 m), to clear
the Holyoke Dam.
In its first year of operation, 1955, the
Holyoke fish elevator carried 4,899 shad
the dam; by 2004, the typical annual number of fish lifted had
risen to more than 500,000.
A rock-ramp fishway
uses large rocks and timbers
to create pools and small falls that mimic natural structures.
Because of the length of the channel needed for the ladder, such
structures are most appropriate for relatively short
A vertical-slot fish passage
is similar to a
pool-and-weir system, except that each "dam" has a narrow slot in
it near the channel wall. This allows fish to swim upstream without
leaping over an obstacle. Vertical-slot fish passages also tend to
handle reasonably well the seasonal fluctuation in water levels on
each side of the barrier.
Day Dam fish ladder.jpg|John Day Dam fish ladder, viewed from the Washington (north)
side of the Columbia River, United States.
elevatorImage:Grave vistrap inlaat
scheepswerf.jpg|Fish ladder in Meuse River, Grave, the Netherlands.
Image:Grave vistrap laag.jpg|Fish ladder in
Meuse River, Grave, the Netherlands.Image:CapilanoRiverRegPark-salmonladder.jpg|Fish
ladder in North Vancouver, British
of fish ladder in Uppsala, Sweden.File:Dart at Salmon leap 2.JPG|Lower part of
the fish ladder on the River Dart in
Kingdom.File:FishLadder7.JPG|At the Charles
River Dam in Boston, United States.