University of Tasmania
Penaherrera_Palma_whole_thesis.pdf (7.82 MB)

Abundance, distribution and conservation value of sharks in the Galapagos Marine Reserve

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posted on 2023-05-27, 11:27 authored by Penaherrera Palma, CR
The Galapagos Marine Reserve (GMR) is the largest reserve in the Eastern Tropical Pacific (ETP), covering approximately `138000 km^2` of the pelagic environment surrounding the Galapagos Islands. The GMR hosts 33 species of sharks, of which two are endangered, four are near threatened, and eleven are vulnerable. In recognition of the importance of sharks to the reserve's marine ecosystem, a precautionary management framework was adopted to protect sharks from fishing and trading since 1989. Almost two decades have passed since the creation of the reserve, and yet knowledge on population levels and spatial use of the reserve by sharks is still limited. This information is critical to review the efficacy of the reserve in protecting mobile species and provide decision-supporting tools to improve shark management at the local and regional scale. The aim of this thesis was to assess and contrast the population status and habitat preferences of the endangered scalloped hammerhead shark (Sphyrna lewini) and other commonly occurring shark species in the GMR. Specifically, my work aimed to: i) assess the current population size of the scalloped hammerhead in the northern GMR; ii) compare the historical abundance trends of hammerhead sharks and five other common shark species; and, iii) assess the differences in spatial and temporal use of the GMR between the scalloped hammerhead shark and the coastal blacktip shark (Carcharhinus limbatus). Whilst there are many techniques to evaluate the population size of wide ranging terrestrial wildlife, constraints imposed by challenging underwater conditions have halted their wider application to evaluate wide ranging marine species. To investigate the population size of hammerhead sharks I applied a novel mark-resight approach by combining acoustic telemetry and visual counts. During September 2011, September 2012, and October 2013 field trips deploying acoustic tags and undertaking underwater visual surveys (USV) were conducted at Darwin Island, north GMR. Resident population (`N^‚Äöv†v¿`) and super-population (N*) size were modelled using the Immigration-Emigration Logic Normal Mark-Resight Estimator (IELNE) implemented in the Mark 8.0 software. Although the resighting probabilities based on acoustic detection were generally low average 2011=0.25; 2012=0.19; 2013=0.16), I found strong evidence of time> and session-dependent resighting probabilities and individual heterogeneity in the resighting probability, particularly for 2012. Model estimations were less variable and with narrower confidence intervals for years 2011 (`N^‚Äöv†v¿``~487` \\(ind.;\\) `N`*`~590`) and 2013 (`N^‚Äöv†v¿`` ~391` \\(ind.;\\) `N`*`~574`), compared to 2012 (`N^‚Äöv†v¿` `~642` \\(ind.;\\) `N`* `~1672`). Temporary emigration of marked and unmarked hammerheads affected estimations for some days in 2012, yet such emigrations become increasingly likely when assessing highly mobile species with gregarious behaviour. Despite this, my analysis provided a reliable approximation of the population size of hammerhead sharks in this ecological hot-spot, and gives support to the use of combined approaches in underwater mark-resight experiments. Further work should explore the inclusion of environmental covariates with increased numbers of tags to improve estimations of resighting probability and population size in case of temporary emigration of marked and unmarked individuals. Abundance is commonly used to assess the status of wildlife populations and their responses to changes in management frameworks. Unfortunately, monitoring abundance trends often requires long>term data collection programs, which are not always carried out. One alternative to scientific surveys is to utilise local ecological knowledge (LEK) to evaluate abundance trends in data deficient regions. To assess the historical population trends of six shark species in the GMR, I developed a LEK-based virtual abundance change (VAC) model by using the ecological knowledge of experienced divers. This model assessed the decadal deficit or surplus in shark abundance since the beginning of the dive tourism industry in the 1980s. In general, dive guides showed consensus on negative trends for all shark species, and suggested fisheries as the major factor driving trends in shark abundance in the GMR. VAC model results suggest that hammerhead and whitetip reef sharks (Triaenodon obesus) have experienced the most consistent perceived decline over the last four decades. Silky sharks (C. falciformis) and Galapagos sharks (C. galapagensis) also experienced perceived declines but later stabilized. Whale shark (Rhincodon typus) abundance was perceived as stable across the study time frame, while blacktip sharks were perceived to have increased in abundance in recent decades. Comparing the VAC results with empirical datasets from the GMR and the ETP provided corroborating evidence that the dive guides' ecological knowledge has described the abundance fluctuations of all species except silky sharks. Spatial management through the implementation of marine protected areas (MPA) is one strategy to limit the extraction of commercial and/or sensitive species. Yet, its use to protect wide ranging marine predators is still unproven, mostly as their movements quite often exceed the established boundaries of MPAs. Understanding the area used by marine life is thus a key step towards the evaluation of the management framework and efficacy of a protected area. To provide information of the protective coverage of the GMR I assessed the habitat utilization distribution (UD) of hammerhead sharks and blacktip sharks in and around the GMR. My results show important aspects of the extent and seasonality of the UD of hammerhead sharks and blacktip sharks inhabiting the GMR. Nearly 90% of hammerhead shark's UD was enclosed by the reserve boundary during the cold season (June-October), yet this progressively decreased to only ~30% with the advent of the warm season (December-April). Conversely, blacktip sharks' UD was 100% enclosed by the reserve boundaries in all seasons. Season and depth were the most important environmental parameters defining the core UD of hammerhead sharks; whilst year and eddy kinetic energy were the most important parameters for blacktip sharks. These findings suggest the size of the GMR may result in seasonally variable protective coverage for sharks inhabiting either pelagic or coastal pelagic environments. This study suggests that the current population size of the endangered scalloped hammerhead sharks within the GMR reflects only approximately 50% of the abundance seen four decades ago. The area used by this species exceeds the reserve size especially during the warmer months, making it vulnerable to fisheries operating around the reserve boundaries. While the GMR is providing important protective coverage for other species, arresting the decline of the scalloped hammerhead shark in the GMR may rely on revisiting the spatial management of fisheries beyond the reserve boundaries. The population assessment tools developed through this research could aid the future evaluation of the status of not only this but other wide-ranging species.


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