The ETS dolphins appear to be year-round residents of the coastal waters of central western Taiwan where dedicated surveys have resulted in sightings from April to August (Wang et al. 2007a). Opportunistic sightings have been made in other months; as of December 2007, the only months with no confirmed sightings were January, February and March, when conditions and opportunities for observations are poor (J.Y. Wang pers. comm., 13 December 2007). In late winter and early spring, grey mullet (Mugil cephalus) fishermen report seeing humpback dolphins near their nets (trammel and gill nets that are commonly used as encircling nets as well). Recreational shore fishermen report that the dolphins are seen most commonly in the winter months in the DaduRiver estuary. Although reports by fishermen need to be viewed skeptically because of the possibility of misidentification, other species of dolphins are generally not present in the near-shore waters of western Taiwan so the chances of confusion are relatively small in this instance (Wang et al. 2007b). All sightings have been in waters less than 25 m deep, most in less than 15 m and within 3 km of shore. The few measurements of sea surface temperatures at sightings have varied from about 24 to 30oC (Wang et al. 2007a).
Schools of dolphins often patrol parallel to the coastline just off the surf zone and large sandbars. Estuaries are likely where most of the foraging occurs. Feeding behind active trawlers (as in Hong Kong and Australia) has not been observed but dolphins move along the length of set trammel or gill nets, possibly searching for injured or net-entangled fish. In general, they appear to be indifferent towards boats (at least the research vessels that have been used to study them – a fishing boat and a large raft made of plastic tubing).
Indo-Pacific humpback dolphins appear to be opportunistic feeders. They take a wide variety of nearshore, estuarine, and reef fishes. They also eat cephalopods in some areas, although crustaceans appear to be rare in the diet (Jefferson and Karczmarski 2001, Ross 2002, Ross et al. 1994). Little is known about the specific feeding habits of the ETS subpopulation but these dolphins have been observed feeding on croakers (Sciaenidae), mullets (Mugilidae), threadfins (Polynemidae) and herring (Clupeidae) (Wang et al. 2007b).