IT'S A mystery that would drive even Sherlock Holmes to distraction - the curious affair of the missing listeners. Why have thousands of Scots apparently stopped listening to their radios?

Last week's figures from audience measurement body Rajar showed that weekly reach - the way the industry measures the number of listeners tuning in for at least five minutes in an average week - had dropped year-on-year for all but three of the country's stations, leading to fears that Scots are abandoning the medium.

The total number of radio listeners fell by 2.4% from 3.79 million to 3.68 million in the first quarter of 2008, an overall drop of 90,000 people. The reasons behind the fall are still puzzling those in key positions in the industry.

Overall UK listening to all radio stations was up by 0.8%, meaning Scottish listeners are acting differently to those in other parts of the UK and making the mystery all the more problematic.

Alison Winter, head of audience insight at commercial radio marketing body the RadioCentre, says the figures are puzzling everyone.

"We have had a good look and we cannot see anything that can definitely explain them," she says.

Winter argues there are often seasonal shifts in radio and the bad weather over the first three months of the year might have played a role.

"If there has been a heavy winter then what we find is people stay indoors and watch their televisions, rather than being out and about and listening to their radios. We are looking at this as very much a short-term seasonal shift, we need to wait to see figures from the next few quarters before we can establish if there is a trend."

According to the Rajar figures, only GMG-owned Smooth radio and Bauer's Northsound 2 and Tay FM posted a growing weekly reach. Smooth Radio and Northsound 2, which both target older listeners, and Tay FM grew by 14.6%, 13.2% and 3.7% respectively.

Of the nation's 16 other commercial stations measured by Rajar, their weekly reach fell between 1.7% and 28.2% year-on-year, with Fife's Kingdom FM the biggest loser.

And it wasn't just the commercial stations: BBC Radio Scotland's reach dropped by 4.5% too.

XFM Scotland and Talk 107 - both stations hit by a spate of sackings and internal change over the past three months - also reported big drops.

Kevin Brady, Kingdom's managing director, said there were several reasons for the fall. He said the way Rajar measured audiences had changed last year and that had disproportionately affected smaller local stations such as Kingdom.

Brady says Rajar's new way of measuring audiences pays closer attention to those areas on the edge of a local station's broadcast area. Brady says the nature of local radio means the further away listeners are from the town where the station is broadcast from, the less likely they are to tune in.

"There are several factors. We changed breakfast show presenters three times over the last 18 months and that might be coming back on us. Also, we know the overall number of people actually listening to radio has dropped and we feel that is to do with the competition from other media - how many television channels do people now have to choose from, for example?"

Measuring radio audiences is a complicated process, taking account of several different measurements of which weekly reach is but one. It is not the most important figure to advertisers, but the picture last week's reach figures paint of an overall decline in radio use is concerning, with senior radio executives keeping their fingers crossed that the figures are a blip rather than the beginning of a longer trend, Mark Mulligan, senior analyst from JupiterResearch, says the changing way people use the internet is one reason radio stations are hurting.

"With the spread of high-speed broadband people are now doing different things online," he says. "Before they would be using the web to check emails and to carry out tasks where they could have the radio on in the background. Now, people are increasingly using the web for entertainment. If you are watching videos on YouTube then you don't want the radio on in the background."

"I don't want to paint a doom-laden picture but the radio industry is going to have to deal with this changing trend."

Rajar uses a team of diarists who write down their listening habits to compile its figures. Mulligan says that means figures for any one quarter should not be taken overly seriously.

However: "If they are repeated next quarter then we may be seeing the start of a trend."

The radio picture is somewhat rosier for several stations if one looks at the percentage of potential listening time each station has managed to attract. Bauer's Forth One and Northsound One did well, posting year-on-year growth of 2.9% and 5.7% respectively, and GMG's Real Radio and Smooth posting 4% and 1.5%.

Radio expert and Glasgow Caledonian University senior journalism lecturer Ken Garner says the figures do not paint a "doomy picture for Scottish radio". He points out that stations with a younger target audience have done less well than others: "The message you have to take is that those listeners are spending less time with their radios."

Richard Muir, Radio Clyde marketing director, said Bauer believes its stations had done "very well" but admitted competition from other forms of media, such as the internet, was fierce.

Asked why Scots appeared to be turning away from radio, he says: "There is no one answer, media is changing so fast."

Muir says he is not concerned by the falling total number of listeners over the first three months of the year.

"We would prefer to concentrate on the positives," he says. "In Scotland people prefer commercial radio over the work of the BBC. I'm pleased with Clyde's performance and the work of the team but there is no room for complacency. We never take a listener for granted."

John Simons, group programming director for GMG Radio - the company that owns Scottish stations Smooth and Real radio - is also bullish. If there is an as yet undisclosed predator stalking Scottish radio, then Simons believes it is still far too early to call in the detectives.

"Between 85% and 90% of the people in Scotland tune into radio every day. Those are figures to make anyone working in other forms of media jealous.

"Yes, there has been a slight drop but it is my opinion commercial radio in Scotland is in rude health. As for BBC Scotland - they should rename it Jurassic Park FM because there is nobody under 65 who listens to it."

Jeff Zycinski, head of radio at BBC Scotland, counters: "John Simons is starting to sound like a grumpy old man but he better not let my Dad catch him saying things like that about older audiences.

"He's 87 and I bet he could still take him in an arm-wrestling contest. The truth is that 51% of our listeners are under 55, a third (36%) are aged between 35-54 and a quarter are aged 25-34."

Zycinski will be hoping when the Rajar's are next published in three months' time they will show the number of listeners increasing again, for if that does not happen then the industry post-mortem will be far from elementary.