An increase in plastic debris floating in a zone between Hawaii and California is changing the environment of at least one marine insect, scientists reported.

Over the past four decades, the amount of broken-down plastic has grown significantly in a region dubbed the Great Pacific Garbage Patch. Most of the plastic pieces are the size of a fingernail.

During a seagoing expedition, researchers from the Scripps Institution of Oceanography found that a marine insect that skims the ocean surface is laying its eggs on top of plastic bits instead of natural flotsam like wood and seashells.

Though plastic debris is giving the insects places to lay eggs, scientists are concerned about the manmade material establishing a role in their habitat.

"This is something that shouldn't be in the ocean and it's changing this small aspect of the ocean ecology," said Scripps graduate student Miriam Goldstein.

The finding was published online Wednesday in Biology Letters, a journal of Britain's Royal Society.

Goldstein led a group of researchers who travelled 1,600 kilometres off the California coast in August 2009 to document the impacts of the garbage on sea life. For three weeks, they collected marine specimens and water samples at varying depths, and deployed mesh nets to capture plastic particles.

The team previously found that nearly 10 per cent of fish studied during the trip had ingested plastic. The voyage was partly sponsored by the University of California and National Science Foundation.

Confetti-sized flecks of plastic

Thousands of tonnes of plastic waste enter the oceans every year and break down into smaller pieces over time. Some wind up in the Great Pacific Garbage Patch, a vortex formed by ocean and wind currents.

The garbage patch cannot be seen by satellite. Most of the plastic pieces are confetti-sized flecks spread across thousands of kilometres of ocean and are hard to see with the naked eye.

A similar plastic trash patch was recently discovered in the Atlantic between Bermuda and Portugal's Azores islands, and new research suggests that both patches could be much larger than first thought.

Some experts believe the plastic fragments, which can be impossible for fish to distinguish from plankton, are dangerous in part because they sponge up potentially harmful chemicals circulating in the ocean.

Plastics have entangled birds and turned up in the bellies of fish. A paper cited by the U.S. National Oceanic and Atmospheric Administration says as many as 100,000 marine mammals could die trash-related deaths each year.

Another concern is that the plastic particles can carry bacteria and algae to new regions of the oceans where they could become invasive.

As much as 80 per cent of marine debris comes from land, according to the United Nations Environmental Program.

With files from CBC News