Factbox: Possible environmental impact of Gulf oil spill

(Reuters) - A giant oil slick from last week's deadly offshore drilling rig explosion in the Gulf of Mexico threatens wide-scale coastal damage for four states.

Following are some possible impacts from the spill on the Gulf environment, commercial fisheries, wildlife and tourism.

FISHERIES

- A number of fisheries could suffer as a result of the spill. The Gulf menhaden fishery -- a species harvested mostly for fish meal and fish oil -- is America's third largest and in some seasons its second largest, according to Greenpeace. Menhaden are filter feeders and so they could be badly affected by the spill, as they pass tainted water through their filtering system. The season for Texas, Louisiana and Mississippi just opened on April 19th.

Omega Protein Corp -- which relies on the fishery for its core business -- said in a statement on Wednesday that: "We believe that the impact, if any, on our Gulf of Mexico fishing operations will be minimal."

- The northern Gulf of Mexico is a crucial spawning ground at this time of the year for the Atlantic population of bluefin tuna, which is critically endangered. Their eggs float near the surface and the larvae also stay near there after they first hatch. So, the spill has occurred at a critical time in their life-cycle.

"We expect a spill like this could dramatically decrease the amount of bluefin tuna larvae that are surviving," said John Hocevar, the Oceans Campaign director for Greenpeace USA.

- Losses could also be inflicted on the shrimp and oyster industries in Louisiana. Oysters are filter feeders and cannot swim to escape the slick. The prime oyster-gathering season in Louisiana starts on May 1.

BIRDLIFE

- Several areas that are important to bird populations could be potentially affected.

According to the National Audubon Society, places it has designated as "Important Bird Areas" or IBAs that could be threatened by the slick include, Chandeleur Islands IBA and Gulf Islands National Seashore IBA in Louisiana and Mississippi; also in Louisiana, the Delta National Wildlife Refuge and Pass-a-Loutre Wildlife Management Area.

Species at risk include, Louisiana's state bird, the brown pelican, which was only removed from the U.S. Endangered Species Act last year. They nest on barrier islands and feed near shore. Their breeding season has just started.

Other species that would not fare well, especially if the slick comes ashore, include the American oystercatcher and Wilson's plover.

TOURISM

- Depending on where the slick goes, a number of beaches could be adversely impacted in areas such as the northwestern part of Florida, which has been running televised ads aimed at attracting tourists to the area.

OTHERS

- Several species of sea turtles are currently moving through the Gulf, as their spring nesting seasons commences and they need to surface to breathe, so the slick at the water's top could damage their populations.