They come silently, these hitchhikers from other estuaries.

Sometimes they are growing on the hulls of ships or swimming in ballast water. They can be stuck to the bottoms of aluminum fishing boats or dumped by bored aquarium hobbyists.

For researchers trying to pinpoint exactly how invasive species make it into Coos Bay or other bodies of water, it's not always an exact science.

Dr. Ian Davidson soon may offer some answers. Davidson is a researcher with Portland State University's Aquatic Bioinvasion Research and Policy Institute. Over the past few months, he's been collecting data on two mothballed ships for the U.S. Maritime Administration.

“The reason these ships are of interest is because they have sat around so long,” Davidson said.

Scientific surveyors

A hull-fouling community can be from a couple inches up to a meter thick. On some ships, it can account for an estimated 60 tons of debris, according to one study.

In MARAD's contracts with the university, divers were to survey the hulls of two 50-year-old ships, the ex-Florence, a former tanker, and the ex-Point Loma. They would identify all organisms stuck to the hulls before the ships left Suisun Bay near San Francisco bound for a Texas shipbreaking yard. Then, they would find what remained after the tow. The report done in partnership with the Smithsonian is due out this summer.

In the commercial shipping industry, ship owners want hulls kept as clean as possible. They use special hull paint chemicals to dissuade organisms from grabbing hold. Active military and commercial vessels are routinely pulled into dry dock so the hulls can be kept clean.

“If there's a lot of fouling on the ship, it increases the drag so it increases the fuel costs,” Davidson said.

With the Suisun Bay fleet, after decades many have collected a reef of cling-ons.

The goal is figure out whether hull species die in transit or whether something must be done to eliminate them.

He said, however, no one's sure what to expect. Suisun Bay's winter water is low salinity.

“If these things are dragged out to sea, it might be an effective barrier,” Davidson said.

It depends on the organism. Maybe the hull colony will die because it's out in saltier water longer. The other factor is that since these ships are being towed, they aren't moving very fast. Maybe more will make it to Texas because they don't get sheared off as would happen with a faster vessel.

The salinity issue is one researchers fasten on. Davidson has worked on studies looking at hull fouling on ships that sail into Portland. The challenge has been surveying enough vessels for valid analysis. They do know the vessels that move frequently in and out of the Columbia have less hull growth. Sail much past Astoria and up the Columbia toward Portland and the water becomes fresh, killing saltwater species. The challenge is to know where to draw the line in the water.

During an experiment in 1998, scientists drew it wrong.

Floating reef

They were caked to the hull of the USS Missouri.

The historic World War II battleship was being moved from Washington's Puget Sound to Honolulu to be put on display.

“Part of their plan was to tow the ship into the mouth of the Columbia River,” recalled Dr. Brenden Holland, assistant researcher at the Center for Conservation Research and Training at the University of Hawaii.

The ship soaked in fresh water for about 10 days to kill the 45-year encrustation of hull critters - at least that was the plan.

After a two-week tow, the ship arrived at Pearl Harbor. Marine biologists did a hull survey. They found a lot of species still alive.

“A couple months later ... Navy people brought me in what they thought were clams. They found these in a nuclear sub ballast tank,” he said.

The shellfish weren't clams, but blue mussels.

The submarine's movements were top secret, but eventually Holland learned the sub had been at Pearl Harbor. It was directly across the channel from Ford Island and the USS Missouri. The sub's ballast tanks had been cleaned when the sub came in and again a few months later. That's when they found what proved to be the Mediterranean blue mussel.

That kind of mussel isn't found in the warm Hawaiian waters. It lives in Puget Sound. It was on the USS Missouri. But that knowledge alone didn't prove any theories.

“Since ships and people and fishermen have moved these around, we can no longer tell you what species we have on hand based on where it came from,” he said.

Biologists investigated. Ultimately, using a molecular-based approach, they analyzed genetics of mussel spat (young) and tissue from adults from various places around the world. They found the submarine spat were the same as the Missouri spat - meaning they jumped ship.

“We're 100-percent certain of that,” Holland said.

Biologists believe Hawaii's water is too warm for the species to survive, but in a last burst for survival the mussels spawned. The spat were able to survive long enough to settle somewhere else.

“That submarine could act as a vector for this species to take off somewhere else,” he said.

Ultimately, researchers also learned a fresh water soak wasn't necessarily a sure method of prevention. In hindsight at the Columbia River, researchers discovered there was a substantial marine wedge at high tide. Salt water is heavier. On the bottom of the harbor at Astoria, there is full saltwater down about 30 feet, Holland explained.

“In other words, that idea to expose the ship to freshwater didn't really happen,” he said.

The bigger issue

The question researchers haven't answered is whether it's possible to clean a mothballed ship's hull so it can be towed to places such as Coos Bay without bringing along hitchhikers.

“I don't think it's possible to deliver a sterile ship to Coos Bay. What's clean?”

That was the question from Dr. Mark Sytsma, an invasive species expert who oversees PSU's Center for Lakes and Reservoirs, who spoke at a ship recycling forum in North Bend in April.

Some people have wondered whether the mothballed ships could be pulled into dry docks and cleaned. Others have denounced the idea as too expensive and too unlikely given the lack of dry docks around San Francisco. Others have suggested having divers scrape the hulls, an idea biologists have found amusing and dangerous. It's dark underwater, with little visibility along the ships' hulls and ripping currents. And, biologists have suggested, the stern would be resettled by the time divers would get to the hull.

The fears over foreign species potentially moving into Coos Bay are many. Tiny invaders could target the oyster industry as has happened in Wilapa Bay, Wash. They could damage the salmon industry. Or, they could drill into banks and cause erosion, as is happening now in Coos Bay.

That's why the scientific debate is simmering, as the government looks for ship recyclers to get rid of the fleet anchored at Suisun Bay. That's why Oregon officials want assurances should a ship recycler want to come here that there won't be invasive species.

“Whether or not they are going to come on these ships is a question that has to be answered,” Sytsma said.