A University of Southern Mississippi researcher believes the population of the endangered yellow-blotch sawback turtle has dropped 47 percent.

A Pascagoula River survey taken by biology graduate student Will Selman this summer shows dramatically different results from a survey taken in 2005 just after Hurricane Katrina. He counted 514 per river mile this year after finding 969 per mile last year.

Selman said he couldn’t be sure what caused the decline, but theorized that Katrina many have reduced part of the sawback’s food supply — clams, snails, mussels and aquatic insect larvae.

He said the sawback, which is only found on the Pascagoula and its tributaries, the Leaf and Chickasawhay rivers, plays an important role in the rivers’ ecosystem. The scavenger helps clean the waterways.

“Without turtles, you’re not going to have fish,” he said.

The turtles have a distinct saw-tooth ridge along the spine of their shells, a yellow spot behind the eye and lime green stripes.

Selman said he will conduct surveys in 2007 and 2008 to see if the decline represents a trend.

The student did his survey work three days a month from April to October. He captures the turtles, takes measurements, a global positioning reading and blood samples, then marks them with fluorescent paint.

Selman found his first sawback hatchling this summer on a sandbar, though he also found a dead one nearby. The sawback female lays eggs in May and the hatchlings are left to fend for themselves, leading to a high mortality rate.

The hatchlings often serve as golf ball-sized prey for herons, raccoons and largemouth bass.