Note 10 - Share-based Compensation - Summary of Option Activity Under Stock Plans (Details) - USD ($)	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Outstanding (in shares)	539,800	589,400	503,000
Outstanding, weighted average exercise price (in dollars per share)	$ 4.06	$ 4.06	$ 4.10
Outstanding, weighted average remaining contractual term (Year)	3 years	4 years 328 days	4 years 328 days	4 years 255 days
Outstanding, aggregate intrinsic value		$ 675,300	$ 675,300	$ 145,946
Granted (in shares)	5,000	50,000	120,000
Granted, weighted average exercise price (in dollars per share)	$ 2.11	$ 3	$ 3.53
Exercised (in shares)	(2,112)	(49,600)	(29,150)
Exercised, weighted average exercise price (in dollars per share)	$ 2.08	$ 2.17	$ 2.59
Forfeited (in shares)	(104,388)		(3,700)
Forfeited, weighted average exercise price (in dollars per share)	$ 4.58		$ 4.50
Expired (in shares)	(120,000)	(50,000)	(750)
Expired, weighted average exercise price (in dollars per share)	$ 3.53	$ 4.82	$ 1.60
Outstanding (in shares)	318,300	539,800	589,400	503,000
Outstanding, weighted average exercise price (in dollars per share)	$ 4.08	$ 4.06	$ 4.06	$ 4.10
Exercisable (in shares)	280,800
Exercisable, weighted average exercise price (in dollars per share)	$ 4.22
Exercisable, weighted average remaining contractual term (Year)	2 years 109 days
Exercisable, aggregate intrinsic value