Shrinking dolphin populations in UK waters may be causing different species to breed with each other, scientists have said.

Researchers from wildlife charity Whale and Dolphin Conservation identified three "atypical" dolphins with mixed characteristics during fieldwork off the Isle of Lewis.

Evidence of the hybridised dolphins, published in the journal Marine Biodiversity Records, suggests bottlenose dolphins in the area have been mating with resident Risso's dolphins.

The three dolphins, photographed in 2011 and 2012, had body features such as pronounced, broad-based, curved dorsal fins consistent with Risso's dolphins and short, defined beaks consistent with bottlenose dolphins.

A fourth dolphin was spotted that was also suspected to be atypical, the scientists said.

It is the first evidence of wild dolphin cross-breeding in the UK, though it has occasionally been documented elsewhere, and the occurrence of several dolphins with hybrid features in one small geographical region is highly unusual, the charity said.

Bottlenose dolphins are rare on the west coast of Scotland, with a known population of around 45 individuals off the west coast and another 10 or so off the Isle of Barra, the WDC said.

Nicola Hodgins, WDC head of science and the paper's author, said: "One species mating with another may be down to a lack of suitable mates within their own individual species group.

"But further research is needed to understand the implications of this unusual activity.

"The conservation implications of hybridism are unknown, but demonstrate the importance of effective management for these individuals and also for the wider populations found in the area. Only continued monitoring will help us to understand the extent and significance of hybridism in wild dolphins."

She also said the findings have ramifications for the proposed marine protected area (MPA) in the area.

"All four of the atypical dolphins were sighted within the proposed boundaries of the MPA and therefore they would hopefully benefit from increased protection."

Meanwhile a team of dolphin experts from Aberdeen University has shed new light on the effect of marine tourism on the behaviour of the mammals.

The team examined the impact of tour boats on the foraging efficiency of an endangered population of bottlenose dolphins off the coast of New Zealand.

Their findings suggest the presence of these boats could be reducing the dolphins' ability to feed themselves.