It's small, but it's a keeper

California is a thirsty state. You don't mess with its water, even in a good year, unless you have an excellent reason. Which is why many Californians are shaking their heads in dismay over a federal judge's recent decision to cut by as much as 30% the water sent south from the San Joaquin-Sacramento River Delta this winter. The judge's reason: to save a French-fry-sized fish called the delta smelt.

The delta smelt makes no heroic journey across the ocean or up river rapids to reproduce. Once superabundant, Chinese fishermen used to harvest the fish by net, but the little thing, a weak swimmer, wouldn't put up any fight at the end of a line. And a smelt would not even make a decent snack. Frankly, on first glance, the fish just isn't much to look at either.

So why should millions of Californians who rely on water pumped south from the delta make economic and social sacrifices -- including the possibility of rationing -- for a basically unremarkable fish?

There are at least four good reasons.

First, it is the law. The Endangered Species Act prohibits the government from doing anything that jeopardizes the continued existence of endangered or threatened species, and it forbids any government agency, corporation or citizen from harming, harassing or killing endangered animals without a permit. It is a sound law, put in place by the Nixon administration in 1973 to protect imperiled plants and animals "from the consequences of economic growth and development untempered by adequate concern and conservation."

By drawing a bright legal line this side of annihilating whole kinds of creatures, the law is to thank for saving the bald eagle, the gray whale, the California condor and the Pacific green sea turtle, among other animals. And it's a law that will be especially important in California and beyond as climate change, human population growth, habitat conversion and invasive species increasingly degrade the natural world.

But obeying even a good law may seem unjustified when it comes time to make sacrifices for a ghostlike fish that conveys no clear benefits to mankind. That common perception brings us to the second reason to save the smelt: The goal of the Endangered Species Act is not just to protect single species but also the ecosystems on which they depend. The delta smelt is what Peter Moyle, a fisheries biologist at UC Davis, calls an indicator species: Its condition reflects the overall health of an ecosystem.