Dara Kerr, Oakland North

The Port of Oakland is the fourth busiest port in the United States. Nearly 2,000 ships arrived at its shore last year. In order for those ships with their deep hulls to pass through the waterway, the Port of Oakland has to dredge.

Every year, up to 6 million cubic yards of sediment are removed from the San Francisco Bay. How to deal with these hundreds of tons of dredged material has been a controversial issue for the government and environmentalists for decades. Since the 1970s, it's been tougher for developers and others to get permits and dispose of sediment because of concerns about the welfare of wildlife. At the same time, dredging is necessary for trade.

"It’s important to take into account the economic needs of dredging along with managing the environmental needs of the bay," said environmental analyst Kat Ridolfi, who works for the San Francisco Estuary Institute.

She was one of the scientists who met last week for a symposium at the State of California Building in downtown Oakland. This group of scientists was made up of representatives working for organizations including the U.S. Environmental Protection Agency, the U.S. Army Corps of Engineers and the San Francisco Bay Regional Water Quality Control Board, and they listened to research about the impact of dredging on wildlife in the bay, and discussed how to best minimize its effects.

Fish are changing migratory pattern

The scientists were a casual bunch and some were sporting short-sleeve button-ups with fish or bird prints. They showed PowerPoint presentations displaying the results of their field work.

Invasive species and toxins were flagged as one problem of dredging. But the main issue is that a smaller – and siltier – bay may be changing the migratory paths and feeding locations for some fish, say scientists.

Today the bay is one-third smaller than its original size. When mining began in the 1800s, sediment was poured into the rivers, settled in the bay and reduced its size. Around 40 percent of California’s water drains into San Francisco Bay, flowing in from the Sierra Nevada.

When dredging of San Francisco Bay began in the 1900s, dumped sediment filled in shallows and wetlands and destroyed the shallow bay flats where animals lived and fed. It also shrank the bay further.

While scientists couldn't give definitive answers about how dredging is affecting the bay’s fish, they did say it may be one of the factors that have changed the species’ behavior over the past few years.

The North American green sturgeon at risk

Like something out of the prehistoric ages, the green sturgeon is covered in large bony plates and has exceptionally long and flexible "lips" to suck up food. The fish migrates down the Sacramento-San Joaquin Delta, into the bay and out toward the Pacific Ocean. They feed near the shore and travel in deep water. But the sturgeons are spending little time around dredging sites, Ridolfi says.

The North American green sturgeon has been on the list of threatened species under the federal Endangered Species Act since 2006.

"They have a low reproductive rate because they live a long time," said Ridolfi. "While we know there are indirect negative impacts of dredging like the introduction of invasive species and propeller strikes," little is known about the exact effect it's having on the fish.

Chinook salmon and steelhead smolts avoiding dredged sites

Marine biologist Alex Hearn of UC Davis is working on tracking data and is focusing on chinook salmon and steelhead smolts. Like the sturgeon, both of these types of fish are born in fresh water, migrate to the ocean, and return to spawn in the fresh water again.

Hearn's group has tagged thousands of fish to study migratory movement and survival. To tag the fish, the scientists do a small surgery. “We cut them up, stick in the tag and sew them up again,” said Hearn.

Once released, the tags are detected by dozens of listing stations throughout the delta and the bay. The results show that the fish are spending little time in dredged sites although Hearn, who is still in the initial phase of his research, couldn't explain why.

Dredging could threaten Pacific herring

Another group of scientists found that dredge sediment could be affecting the survival rate of Pacific herring. The fish is unique because the females release eggs that are sticky for the first two hours of their lives, says research biologist Fred Griffin, who works for the Bodega Marine Laboratory at UC Davis.

When Griffin’s group exposed these adhesive eggs to sediment they found that particles attached and stayed there permanently. When compared with a control group, they found that the sediment reduced the larvae hatching rate and size. There was also an increase in the number of abnormal larvae that hatched earlier than normal and a reduction in survival of the just-hatched fish, Griffin explained.

Find out more about the studies here.