A matter of taste: Spatial and ontogenetic variations on the trophic ecology of the tiger shark at the Galapagos Marine Reserve

Abstract

Sharks are top predators across ocean food webs and have a major ecological role in marine ecosystems. Investigating the trophic ecology of this group of species is thus essential to understand ecosystem functioning and inform specific management actions aimed at shark conservation. The Galapagos Islands represent one of the last ocean wildernesses, where populations of sharks and other top marine predators come close to a pristine status. Here we provide the first study on the trophic ecology of the tiger shark (Galeocerdo cuvier) within the Galapagos Marine Reserve (GMR), using a combination of stable isotope analysis, satellite tracking, and passive acoustic telemetry to investigate ontogenetic and spatial variations at two regions. The mean estimated δ13C and δ15N at Isabela island (western region) were -13.9 ± 0.5‰ and 13.7 ± 0.7‰; and for Santa Cruz island (central region) were -13.8 ± 0.3‰ and 13.4 ± 0.7‰, respectively. Green sea turtles (Chelonia mydas) were the main prey item for large tiger sharks (>280 cm TL), while smaller sharks mainly fed on squid and pelagic fish. Tiger sharks exhibited a high degree of philopatry around green sea-turtle nesting areas, with the majority of sharks detected around green sea-turtle nesting areas for at least 10 months after their capture date, and some individuals were even present during the entire three-year study period. Although we did not report statistically significant differences between the two regions, isotopic and electronic tagging data suggest that tiger sharks in the Galapagos could be segregated into specific populations separated by geographical scales of <100 km. The high productivity of the archipelago, along with the protection from industrial fishing granted by the GMR, result in abundant and predictable sources of prey. This high food abundance, combined with the presence of suitable habitats throughout the tiger shark life cycle, might result in a reduction of migratory behaviours when compared to movement patterns of tiger sharks in other ocean basins. Additional studies using genetic tools could provide further evidence on the presence of separate management units, as it has been recently revealed for other shark species inhabiting the GMR.

Priorities for ecological research on cetaceans in the Galápagos Islands

“…Our review confirmed that this ecosystem supports a large and diverse cetacean community, but beyond presence and distribution information, for most species not much more is known about habitat use, population structure, health, and social ecology. In contrast, significantly more efforts have been devoted to the study and monitoring of Galaṕagos invertebrates (e.g., Edgar et al, 2004;Edgar et al, 2008;Edgar et al, 2011), fishes (e.g., Schiller et al, 2014;Lamb et al, 2018;Salinas-de-Leoń et al, 2019), pinnipeds (e.g., Paéz-Rosas et al, 2012;Brock et al, 2013a;Brock et al, 2013b;Paeź-Rosas et al, 2016;Paéz-Rosas et al, 2018;Urquıá and Paéz-Rosas, 2019;Paéz-Rosas et al, 2021), and seabirds (e.g., Anchundia et al, 2014;Tompkins et al, 2017;Jimeńez-Uzcategui et al, 2019;Arauco-Shapiro et al, 2020). Admittedly, cetacean research is comparatively more expensive and more logistically demanding.…”

Hormone‐mediated foraging strategies in an uncertain environment: Insights into the at‐sea behavior of a marine predator

“…Similar correlations of foraging habitat density and TEST have been observed in grizzly bears ( Ursus arctos ), in which bears living in coastal habitats with high conspecific overlap for spatially constrained salmon resources also had the highest TEST levels when compared to more dispersed, generalist bears (Bryan et al., 2013). It should also be noted that, specific to the Galapagos archipelago, the highest abundance of shark species which prey on sea lions are found around near‐shore islets and seamounts (Acuña‐Marrero et al., 2018; Salinas‐de‐León et al., 2019), strongly overlapping with the foraging areas of benthic divers. TEST is a known inhibitor of apprehension toward predators in a variety of contexts (King et al., 2005; Wingfield et al., 2001).…”

Correction: A matter of taste: Spatial and ontogenetic variations on the trophic ecology of the tiger shark at the Galapagos Marine Reserve

“…For 18 sharks captured and fitted with satellite tags between 2014–2015, the satellite data used in this study was obtained from Acuña-Marrero et al (reference [23] of the article [ 1 ]), as indicated in the table caption. For five sharks captured and tagged with satellite tags between 2016–2017, satellite data were newly collected.…”

“…For five sharks captured and tagged with satellite tags between 2016–2017, satellite data were newly collected. For 18 sharks captured and tagged with acoustic tags between 2014–2015, the acoustic data used in this study is a combination of re-used data collected 2014–2015 (obtained from Acuña-Marrero et al, reference [23] of the article [ 1 ]) and new data detected by the acoustic receiver array September 2015-October 2017. For 10 sharks captured and tagged with acoustic tags between 2016–2017, acoustic data were newly collected.…”

“…Subsequent stable isotope analyses of samples from these sharks were carried out under the GNPD permit PC-38-16 issued to Dr. Diego Páez-Rosas. Acoustic data collection September 2015-October 2017 was carried out under GNPD permits issued to CDF; some acoustic data, including on the aforementioned four sharks, were downloaded as publicly available from Acuña-Marrero et al (reference [23] of the article [ 1 ]).…”