The mystery surrounding the stranding of nearly 40 bottlenose dolphins on a Mozambican island has deepened with the discovery of Norwegian-made seismic survey equipment in the sea near Bazaruto.

The South African petro-chemical group Sasol is planning to conduct a seismic survey for hydrocarbon gases close to the Bazaruto Archipelago National Park in 2007, but denies it has done any preliminary survey work using underwater sound guns because of the uncertain safety risks for endangered sea creatures such as the dugong.

However, the Maputo-based environmental group Eyes on the Horizon has produced photographs of a large orange seismic float bearing the markings: "Scanmarin. Seismic float, type SSF 1000. Made in Norway by Bakelittfabrikken A/S."

A spokesperson for a Norwegian company previously involved in making these seismic floats told The Mercury he could not speculate on when the device was made or who owned it, since a large number had been made over the past two decades.

However, University of KwaZulu-Natal dolphin expert Vic Peddemors has suggested that several of the dolphins which died last weekend may have been dumped overboard as bycatch from a fishing vessel.

The former Natal Sharks Board scientist, who is working at Macquarie University in Australia, said he had examin-ed e-mailed photos of some of the stranded corpses, and had noticed peculiar marks on their skin that suggested they might have been caught in fishing nets and then dumped as bycatch.

These marks might have been caused by ropes or nets. What complicated this theory, he said, was the fact that several animals were reported to have been alive when they beached.

"However, it could be that they were weak, but still alive, when released from the nets and barely managed to stay afloat on release.

"The reason I'm still considering the case for bycatch is the number of animals that I believe show evidence of rope marks."

Nevertheless, the discovery of the Norwegian seismic float has also raised questions.

Commenting on photographs e-mailed to him by The Mercury, Dag Eirik Thomassen, of the Oslo-based Polimoon Group, said the float could have been made 10 years ago or within the past year.

Barnacle and algae growth, evident in the photos, could start quite quickly and be visible within five to six months.

He said Scanmarin and Bakelittfabrikken were previously associated with his company in manufacturing such floats. "We still have the name Scanmarin on some of the polyethylene moulds we use, even though that company no longer operates."

Thomassen said the device in question was probably a foam-filled flotation buoy, containing no specialised sound equipment.

They were used to prevent the long arrays of floating seismic equipment from sinking while being towed by survey vessels.

Seismic marine surveys involve blasting short bursts of sound bubbles from several air guns. These sound blasts are directed at the ocean floor and reflected back to floating hydrophones (recording equipment), which are then relayed to the survey ship for interpretation.

According to an environmental impact assessment report of the proposed Sasol gas exploration scheme, only limited scientific research was conducted on the effects of seismic surveys on dolphins.

While there is some evidence that dolphins had been seen swimming close to seismic survey vessels, other research indicated that dolphins and whales reacted by swimming rapidly away from the source of the gun blasts.

The report suggested that there was some risk of injuring the hearing systems of dolphins and whales, and the possibility of fatal strandings.

"The case studies suggest that both fatal trauma and behavioural avoidance (leading to fatal strandings) can occur from high-level acoustic impulses," the report said

It noted that sound from underwater seismic surveys (at about 250 decibels) could be heard up to 50km away by some creatures.

One study suggested that bowhead whales actively avoided seismic gun noises and began to swim rapidly away when the approaching survey ship was 24km away.

To protect humans from injury, international regulations recommend that divers or bathers leave the water when surveys are under way.

Sasol issued a statement last week in which they emphatically denied that they were involved at present in any seismic surveys around Bazaruto. There have been numerous reports of mass dolphin strandings throughout history, and researchers have said avariety of natural causes or human factors could be responsible.