The Endangered Species Act of 1973 (16 U.S.C. 1531 et seq.), declares the intention of Congress to conserve threatened and endangered species of fish, wildlife and plants, and the ecosystems on which those species depend. The Act provides that Federal agencies must utilize their authorities in furtherance of its purposes by carrying out programs for the conservation of endangered or threatened species, and by taking such necessary action to insure that any action authorized by that agency will not jeopardize the continued existence of such endangered or threatened species or result in the destruction or modification of habitat of such species which is determined by the Secretaries of the Departments of Commerce or Interior, as appropriate, to be critical.