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It was such an unusual sight that the commercial fishing crew in the northern Gulf of Mexico took an hour out of their work day to count the whale sharks swimming around and even rubbing their sandpaper-like backs against the boat.

The crew stopped at 44 to avoid double-counting the dark-bodied fish, some up to 50 feet long, opening and closing their wide mouths as they vacuumed in plankton, fish eggs or small fish southwest of Morgan City, La. It was the largest sighting in a record year for a study of the world's largest fish that began in 2002 at the University of Southern Mississippi's Gulf Coast Research Laboratory in Ocean Springs, Miss.

"As far as your eye could see in every direction, you just saw fish after fish after fish after fish," said David Wesley Underwood of Pensacola, Fla., a deckhand on his uncle's boat, the Norman B.

That pod, seen in June and reported Jan. 29 to USM scientists, was among 70 sightings of at least one whale shark during 2008 - by far the largest number for the project.

"We're getting the word out," said lead researcher Eric Hoffmayer.

He depends on non-scientist spotters because the sharks are seen most frequently where the water turns sharply from shallow to deep, and trips out there are expensive.

About two-thirds of the sightings have been within 100 miles of the Mississippi River's mouth. Hoffmayer believes the sharks are attracted by plankton blooms fed by fertilizer and other nutrients in the river water - the same phenomenon that creates an oxygen-depleted dead zone closer inshore every summer. Hoffmayer worries, however, that the sharks may be swallowing poison along with the plankton.

"What about all the other chemicals that are being washed out with all this runoff?" he said.

Researchers plan to look into that. First, they need a handle on how many sharks are swimming around the northern Gulf, where they come from, and where they're going. The one whale shark tagged last year in the northern Gulf was 260 miles south, in Mexican water, and more than a mile underwater, at 6,000 feet - probably the lowest the species has been recorded - when the electronic tag popped off, Hoffmayer said.

Little is known about whale sharks, including where they go after they leave gathering places near Australia and Yucatan, or where they give birth to their pups. Although they were added last year to the World Conservation Union's "red list" of threatened species, the Australian research indicates that - unlike most sharks, which are declining sharply - whale sharks appear to be increasing off Western Australia, said Jason Holmberg of Portland, Ore., information architect of the Ecocean Whale Shark Photo-identification Library.

They are a warm-water fish and all sightings in the Gulf of Mexico have been between April and November, with most from June through October - roughly corresponding to the seasonal heating of Gulf waters that feeds hurricanes.

It's possible the increased numbers around the river's mouth are just a fluke, because that's where most of the spotters are. Hoffmayer doesn't think that is the case. To find out, researchers are recruiting watchers along the Gulf from Texas to Florida.

"They may have found a new and important aggregation point," said Holmberg, who is an adjunct research associate with Murdoch University in Western Australia.

He is not a biologist, but analyzes population statistics for the Australian project.

Even before the Norman B's Capt. Russell Underwood called, Hoffmayer had received 68 sightings for 2008, more than in all six previous years combined. On Jan. 30, an offshore oil worker reported the 70th, from November.

"He saw one of our posters in a safety room at whatever rig he was at," Hoffmayer said.

The posters are one way he spreads the word about his search for data. He also goes to an annual safety meeting for helicopter pilots who fly over the Gulf and is trying to establish better ties with offshore oil rig workers.

Besides Hoffmayer's project, in 2003, the Georgia Aquarium and the Mote Marine Laboratory in Sarasota, Fla., began working with Mexico's National Commission on Protected Natural Areas to tag whale sharks off Yucatan, where an estimated 1,500 feed each summer. It's the largest known gathering, with estimates of 500 to 3,000, said Bob Hueter, director of Mote's Center for Shark Research.

"We really think 500 is low," he said. "We've had images of as many as 75 whale sharks in one photo."

In the northern Gulf, they're more spread out. Two-thirds of those seen last year were single sharks, with 21 sightings of two or more. A few groups of 100 or more have been reported over the years. The worldwide population may be as high as 500,000, "which sounds like a big number, but when you're talking about fish, it's not a big number at all," Hueter said.

A Coast Guard report of a whale shark in Mississippi Sound was one of the nearest inshore last year, and fits the idea the fish come for plankton nourished by riverborne nitrates and phosphates.

It showed up after the Army Corps of Engineers opened a spillway northwest of New Orleans, pouring river water into Lake Pontchartrain to avert the chance that high water might batter the river levees and endanger New Orleans. The release created an algae bloom intense enough to create a "dead zone" of oxygen-depleted water like the much larger one that forms every year off Louisiana, and the shark was "right up where the plankton blooms would have been," Hoffmayer said.

Skipper Russell Underwood has offered his boat for a week-long study trip of the sharks in June, to see if the same large group shows up in the same area as he spotted last year. The Gulf has given him a good living, he said.