Hilo Bay is an important reproductive area for two shark species.

Fishing surveys in the area have revealed the regular presence of neonates and young-of-the-year (YOY) Blacktip Sharks (Wallingford 2014; D Bartz unpubl. data 2024). Between March–November 2013, 11 fishing trips were conducted in the area. During each trip, two longlines with 30 hooks were set for 1–2 hours. Of 117 sharks caught, 92 where Blacktip Sharks measuring between 65–110 cm total length (TL) with 26 neonates having open or partially healed umbilical scars and the other fully healed scars (Wallingford 2014). The majority of individuals (n = 80) were <90 cm TL, which is close to the reported size-at-birth for the species (38–72 cm TL; Ebert et al. 2021), confirming they were likely to be neonates or YOY individuals. Further, fishing surveys using hook and line conducted between 2022–2024 captured 14 Blacktip Sharks measuring 60–100 cm TL (D Bartz unpubl. data 2024). Another 14 individuals were observed but broke the line and could not be measured; however, sizes were similar to those measured and were thus also likely to be neonates or YOY individuals. Sharks were captured year-round (February, April, June, July, August, September, and November) at depths of 0–15 m in sandy, muddy, slit, rocky, and reef substrates. In addition, Blacktip Sharks, including neonates and YOY, have been reported as being commonly caught by recreational and commercial fisheries in this area since the 2000s (D Bartz unpubl. data 2024).

Local ecological knowledge has revealed that neonates and YOY Scalloped Hammerheads regularly occur in the area (D Bartz unpubl. data 2024). Interviews with 32 fishers showed that in the 1950s and 1960s, they caught between 10–12 small hammerhead sharks per day. The majority of catches were made from shore, with fishers stationed either along the Hilo bayfront, ‘Isles’ (just outside of the Wailoa River mouth), or along the breakwater. Sharks were commonly caught in mud flats in front of the bayfront at 0–5 m depths, in rocky, reef, and sandy substrates in Isles, and in reef and sandy patches near the breakwater. All these sites within the estuary make a suitable habitat for these early life stages. Sharks were commonly caught between July–September. Fishers reported declines in catches of Scalloped Hammerheads since the 1990s with catches of Scalloped Hammerhead not very high but still regularly reported (D Bartz unpubl. data 2024). In addition, anecdotical observations indicate that pregnant female Scalloped Hammerheads come to the bay to pup (Division of Aquatic Resources Hawaii 2024) as they do in other shallow estuaries in Hawaii (e.g., Kaneohe Bay; Duncan & Holland 2006). Recent monitoring using environmental DNA and citizen science has shown that the area is still important for these life stages even if their abundances are not as high as in the 1960s (D Bartz unppubl. data 2024). From fishing surveys conducted between 2022–2024, one neonate Scalloped Hammerhead (~45 cm TL) was sampled near the Wailuku River mouth (D Bartz unpubl. data 2024). In August 2024, local fishers incidentally hooked three neonates using a dunking setup (weighted lead line attached to main hook like) and cut squid as bait near the opening of the Wailoa River mouth and the harbour entrance to the bay. Individuals measured ~45 cm TL and one had an open umbilical scar.