Project: Determining feeding patterns and trophic relationships in white sharks at Guadalupe Island, Mexico, based on stable isotopes analysis

Introduction:

The white shark is rare along the Mexican coast but the occurrence at Guadalupe Island is more frequent than was previously supposed. Due to its extreme isolation, Guadalupe Island is an ideal location to get important information about some aspects of the biology and ecology of the white sharks.

Fig. 1. Photo ID's of two white sharks showing conspicuous features (right and left view): a) "Dark Cloud" b) "Zorro"

It is important to understand the diet and trophic position of this species in order to better understand their ecology and manage their population. Because white sharks are rarely captured, documentation of their food habits is scattered throughout the literature. Nowadays, stable isotopes analysis has become an increasingly useful method for feeding research. This method can be used to evaluate effects of ontogeny, habitat alterations, or trophic relationships. For the white shark, this is the first feeding study based on stable isotope analysis of carbon and nitrogen, which will help us better understand the feeding preferences and behavior of this species.

Objective:

The main objective of this project is to determine the feeding patterns and trophic relationships of the white sharks at Guadalupe Island based on 13C and 15N stable isotopes analysis. This essential information will be critical in the development of future management and conservation plans for this species at the island.

Fig. 2. M/V Islander

Methods:

Logistical support for this project has been provided by Absolute Adventures - Shark Diver, an ecotourism company that operates shark watching trips to Guadalupe Island. We expect to be on the island during May-June and September-December from the next 2 years. During this time we will collect biopsies of white sharks, fish and pinnipeds.

Sampling:

Sharks
We will identify individual white sharks on the basis of their conspicuous features in order to create a photo ID guide (Fig. 1). A muscle sample will be taken from either a cage deployed from the M/V Ocean Odyssey and M/V Islander (Fig. 2) or from a small boat by using a biopsy needle mounted on a pole spear (Fig. 3, click here for 2.7 Mb Quicktime video). The sample will be removed from the needle and kept in sterile vials. This technique will also be used for take fish and pinniped samples.

Fig. 3. Biopsy of white shark using pole spear (click for 2.7 Mb Quicktime video)

Fish
At least 10 individuals of the fish used for baiting (yellow fin tuna, Thunnus albacares, skipjack tuna, Katsuwonus pelamis, and yellowtail, Seriola lalandi), will be sampled per season. Approximately equal weights of dorsal muscle will be collected from each individual, pooled and then freeze dried.

Pinnipeds
Samples of at least 10 individuals of each species found at Guadalupe Island (California sea lion, Zalophus californianus, Guadalupe fur seal, Arctocephalus townsendi, and northern elephant seal, Mirounga angustirostris) will be collected. We will be able to visit the pinnipeds colonies periodically to take the samples.

Isotopic analysis

Each tissue sample will be cleaned of skin and defatted in a solution of chloroform and methanol (1:1). The sample will then be processed into a fine powder. Approximately 5 mg of each sample will be analyzed for stable carbon and nitrogen isotope abundances. The Stable Isotopes Laboratory at UC Davis, California, will perform the analysis of 13C and 15N.

Progress:

At the present we have collected 5 biopsies from white sharks, 11 from yellow fin tuna, 10 from yellow tail and 5 from skipjack. These samples are being processed for isotope analysis and we expect to have the first results in the middle of this year.