It's a macabre case of less is more: spilled diesel is more toxic to fish when it has been broken up into small droplets and dispersed in the water than when it is concentrated in large pools on the surface, Canadian researchers have found.

"Diesel, when it floats in nice calm water with no turbulence actually is not particularly toxic, but what we showed with dispersant studies was that as soon as you mix it, materials in the oil get into the water much more readily and therefore harm fish," biologist Peter Hodson told CBCNews.ca Friday.

Hodson conducted the study with other researchers at Queen's University in Kingston, Ont., and Victoria, B.C.-based environmental consultant Lizzy Mos. The results, published in the March issue of Environmental Toxicology and Chemistry, suggest that spilled diesel could be much more dangerous to fish if it gets into rushing rivers and other situations where it might get mixed into the water.

Detergents are often used as chemical dispersing agents during oil spills on the ocean. The detergents break the oil up into tiny droplets, allowing it to be more easily diluted and broken down by light and bacteria. It also makes the oil less harmful to seabirds, which can end up coated in oil concentrated on the surface.

Hodson and his group used detergents to disperse simulated diesel spills in his lab, and found that young rainbow trout were much more likely to sicken or even die when a dispersing agent was used than when it wasn't. He believes aquatic organisms are likely to be similarly affected.

Fresh water often affected

Diesel spills are common in fresh water, because diesel is often transported by truck, rather than by ship on the sea, and thus released during collisions or rollovers from the truck's fuel tanks, Hodson said.

Detergents aren't usually used to disperse freshwater diesel spills, as some dispersing agents used in the past were known to be toxic. However, Hodson's study found the detergent he used wasn't the problem, as the fish were unharmed by detergent mixed with non-toxic mineral oil. Instead, the results suggest that it's the diesel that is toxic and the mixing that makes it more harmful to the fish.

Hodson hopes to undertake further studies in which the diesel is mixed mechanically rather than with a dispersant.

But for the moment, the results seem to show that cleaning up diesel spills is extra urgent if there is a risk it could flow in choppier waters, he said.

"The biggest single solution to this problem is not to spill oil at all," he said.

Where that's impossible, he added, "You really want to contain it on site and prevent it from getting into turbulent water."