Investors in Japan might keep a close eye on sales of the homely horse mackerel as an early warning of any trouble ahead.

The good news is that in the more confident Japan of 2013, tuna remains king.

So says Mizuho Securities economist Kenta Ishizu, who believes he has found a leading indicator of Japanese consumer demand that relies on two of the most common items on a sushi menu: Japanese horse mackerel and tuna.

Ishizu was looking for patterns in Japanese consumer behavior that would point to the impact of "Abenomics" -- the monetary and fiscal stimulus central to the government of Prime Minister Shinzo Abe.

Horse mackerel, also called jack mackerel, or "aji" in Japanese, is an oily fish that is far cheaper than bluefin tuna or "maguro," the mainstay of sushi menus around the world.

Using data on household spending, Ishizu has developed a "Tuna-Mackerel Index", which shows how much the average Japanese family is spending on tuna relative to mackerel based on government-compiled data.

Remarkably, the index appears to have both anticipated the sharp decline in spending that followed the global financial crisis of 2008 and the stock market boom that came in late 2012 with hopes for Abenomics.

The sushi index has risen for two consecutive months, based on the most recent data for the Ministry of Internal Affairs and Communication. The March reading -- the most recent available - showed the largest percentage gain since August last year.

"There is a possibility here that consumer psychology is proving more resilient," Ishizu said in a research note for clients published this week.

A beef and pork pairing might have worked as well, but fears of mad cow disease a decade ago led to import restrictions on beef and health concerns that skewed the data, Ishizu said.

Meanwhile, consumer spending on "small extravagances" like sea urchin, or "uni", and musk melons have also been trending up since the middle of 2012. Even so, Ishizu said the gains could prove short lived.

"If consumers don't see higher wages and economic recovery that they can feel, there's a real risk that the tuna-mackerel index could come tumbling back down," he said.