Fish abundance in Changuu Island (Unguja Island, Zanzibar Archipelago, Tanzania)

We deployed underwater cameras (GoPro Hero3) to measure the abundance of predators of T. gratilla. According to previous literature, the main fish predators of T. gratilla in this region were Balistapus undulatus (Triggerfish, family Balistidae) and Cheilinus undulatus (Wrasse, family Labridae) (McClanahan and Muthiga, 1989; McClanahan and Shafir, 1990; Eklöf et al., 2009). Abundances of these predators in our study site were the main focus of these surveys. We deployed the cameras in four sampling events: 22.07.2017 (three cameras), 07.08.2017 (seven cameras), 28.08.2017 (six cameras) and 30.09.2017 (six cameras). We deployed three cameras simultaneously, attached to a 50 cm tall wooden pole. Each camera recorded a mean (± standard error) of 53 ± 2 minutes and an area of 10 m2, resulting in a total of ~20 hours of video and a total surveyed area of ~220 m2. We used the maximum count method for counting fishes in the video surveys (Conn, 2011). We counted the fishes as the number of times they appear in the recorded area (i.e. if one fish left the area recorded and appeared again, it was counted as a new fish, as defined by Conn, 2011). This method was used to ensure that any predatory fish of T. gratilla would be detected despite the likely over-estimation of fish counts of abundant fish families. The fishes that appeared in the videos were screenshotted and identified using the fish identification guide developed by Allen et al. (2003). We created one category per fish family identified, two individual categories for the fish species that are predators of T. gratilla (B. undulatus and C. undulatus) and one category for unidentifiable fishes (generally due to suboptimal visibility). The final number of fishes is the individuals count.