Biogeographic variation in the diet of juvenile blacktip sharks across the Galapagos Archipelago.

Abstract

Juvenile blacktip sharks (Carcharhinus limbatus) are highly abundant in the mangrove-fringed bays throughout the Galapagos Archipelago, utilizing these habitats as nursery grounds. Due to spatial variation in factors including temperature and larval connectivity, fish species richness is higher in the eastern portion of the archipelago. Predator diets often reflect regional prey availability, leading us to hypothesize that juvenile blacktip sharks in the eastern Galapagos would have greater prey taxa richness than those in the west. Here, we used DNA metabarcoding of fecal matter collected via non-lethal cloacal swabs to describe and quantify juvenile blacktip shark diets. We collected samples from 107 sharks across 240 km, spanning 14 bays on four islands, and released all sharks alive. The most common prey taxa identified were Thoburn's mullet (Mugil thoburni; 20.0%, based on percent of occurrence), followed by fishes in the genus Abudefduf (sergeant-majors; 13.4%), Galapagos ringtail damselfish (Stegastes beebei; 12.7%), scalloped hammerhead shark (Sphyrna lewini; 11.7%), and yellowtail damselfish (Stegastes arcifrons; 10.7%). Prey taxa richness was highest among sharks sampled in the bays of San Cristobal, possibly due to geographic variation in prey availability. Additionally, sharks sampled during a La Niña event in 2021 had higher prey taxa richness than those sampled during an El Niño event in 2023, which could be due to El Niño-induced declines in species richness and abundance resulting from shifts in primary productivity and upwelling. A deeper understanding of shark diets can improve our ability to predict how changes in food web dynamics and prey availability affect shark populations. This knowledge is especially vital in the face of overfishing and climate change, helping to inform more effective conservation strategies and support shark survival.