Total stocks were 309,000 tonnes in August.
The September figure was also down by 8.8% from 311,500 tonnes in September 2020.
Yokohama warehouses logged the largest outflow of material, with stocks slipping by 21,100 tonnes, or 14.7%, month on month to 122,300 tonnes and by 26.1% compared with last September’s 165,400 tonnes.
Stocks in Osaka warehouses fell to 14,600 tonnes, or 8.8%, in September from 16,000 tonnes in August, and were down 27% from September 2020's 20,000 tonnes.
Stocks in Nagoya warehouses fell by only 2,500 tonnes, or 1.7%. to 147,100 tonnes, from 149,600 tonnes in August, but were up by 21,000 tonnes, or 16.7%, from 126,100 tonnes in September last year.
Falling stocks in Japan - the world's second largest importer of aluminium - signals a pick up in domestic demand and the drawing down of available stocks after the seasonal demand slump seen in August that had slowed negotiations and capped spot buying.
In early September, market participants reported more inquiries immediately after the robust initial offers for fourth-quarter delivery aluminium into the main Japanese ports at $230-250 per tonne.
Expecting a jump in quarterly premiums, most buyers were scrambling to secure volumes at favorable spot prices, which were assessed at $175-190 per tonne in mid-August, before the conclusion of MJP negotiations.
Those who found the wide bid-ask spread difficult to stomach, amid the still-soft demand, turned to available volumes on hand instead, hence the drawing down of MJP aluminium stocks in September.
Fastmarkets has settled its quarterly premium for MJP aluminium supplies in the fourth quarter at $215-220 per tonne over the LME cash price.
The fourth-quarter premium was 21.85% higher than the third-quarter benchmark of $172-185 per tonne and almost three times the $88-per-tonne premium in the fourth quarter of 2020.
Stocks of aluminium held at main Japanese ports (MJP) slipped by 25,000 tonnes, or 8.1%, month on month in September to 284,000 tonnes, according to data compiled by the trading house Marubeni.