The shads or river herrings comprise the genus Alosa, fish
related to herring in the family Clupeidae. They are distinct
from others in that family by having a deeper body and spawning
in rivers. Several species can be found on both sides of the
Atlantic Ocean and Mediterranean Sea. Alosa can also be found
throughout the Caspian Sea. 2] Many are found in freshwater
during spawning and some are only found in landlocked
freshwater. Alosa are generally dark on the back and top of the
head with blue, violet, or greenish tints. However, some species
can be identified as having a grey or green back. Spots are
commonly found behi nd the head and the fins may vary from
species to species or individually. Most species of Alosa weigh
300 grams or less with one species, Alosa pontica, weighing up
to 2 kilograms. Shads are thought to be unique among the fishes
in having evolved an ability to detect ultrasound (sound at
frequencies above 20 kHz, which is the limit of human hearing) .
This was first discovered by fisheries