The problem of estimating the number of sound-producing sources detected using a compact array of hydrophones is addressed. Closed form expressions representing the techniques of automatic detection and estimation of the number of callers are given. Their performance is evaluated on a year-long dataset (1 October 2015-6 October 2016) containing humpback whale and killer whale calls collected in the Strait of Georgia, near Vancouver, British Columbia. Manual verification of the automatic detections produced by the approach required ∼40 h.