Many shark species appear to display a high degree of fidelity to very specific sites.

The Migramar network is investigating the residency of these sites, and the degree of connectivity between sites. In this way, we can build up a model of spatial utilization for the different populations, and thus contribute to the management of protected areas (e.g. technical advice on coastal zonation processes around the islands).

By using the same technology at each Marine Protected Area, we are also able to determine whether tagged individuals migrate from one place to another.

Our research teams have studied over 250 scalloped hammerhead sharks in the Eastern Tropical Pacific since 2006, along with Galapagos sharks, whale sharks, silky sharks, whitetip reef sharks, bull sharks and sandtiger sharks.

Our array of underwater listening stations is strategically placed around key points at the oceanic islands of Galapagos, Cocos and Malpelo, and in coastal areas such as Coiba, Las Perlas and Isla de la Plata (where we’ve tagged giant devil rays too).

Site Fidelity

Site fidelity studies are being pioneered by James Ketchum (UC DAVIS) in Galapagos, specifically around Wolf Island in the far North of the archipelago. Preliminary results are now being analysed, to be published shortly.

The diagram above shows a number of “clock diagrams”. In these graphs, each segment represents two hours, and the shading shows us daytime and night-time.

Each concentric ring is one tagged shark, and each dot on the ring is a time that the shark was detected at that site.

These figures show us the time spent at different sites around Wolf Island in the month of August 2007.

Our first conclusion from looking at this figure is that there are more sharks using the eastern sites (Derrumbe and Punta Tiburon), than the western sites (Fondeadero and Elefante).

Not only that, but each shark spends more time at the eastern sites, whereas they seem to be only passing through when detected at the western sites.

Diel Behaviour

At all our sites, hammerhead sharks tend to display site fidelity almost exclusively during daytime hours. At these times, large schools of hammerheads can often be seen. Mostly made up of females, it is thought that schooling is an important social behavior. For more information about schooling, read Prof. Klimley’s article on hammerhead schooling for Behavioral Ecology and Sociobiology.

The figure above shows the detections of a female hammerhead tagged at Malpelo in 2006.

The date is shown along the horizontal axis, and the time of day is on the vertical axis. Throughout most of the year, the shark was only present at the island during daytime hours (from 6 am to 6 pm).

This is consistent with all our islands – the big schools tend to break up at dusk, and sharks move offshore, returning around dawn. However, sometimes, they may spend the entire night at the site, as this shark does for a few nights in March 2006.

Inter-island movements

Although hammerheads displayed fidelity to particular sites (we call them “hotspots”) within an island archipelago, they also moved between these sites at certain times.

Our studies of inter-island movement are focused mainly on the Galapagos Islands, where sharks are commonly seen in particular areas throughout the 13 major islands and over 100 islets and small rocks.

The figure above shows the number of tagged sharks present at Darwin (blue) and Wolf (pink) islands between 2006 and 2010. The arrows show tagging expeditions.

Most of the sharks were actually tagged at Wolf, yet we can see in the figure that at certain times of year many of these sharks appear at Darwin.

We found that sharks often make the trip of 40 km over the course of a single day. Sharks from Wolf have also been found at Roca Redonda, in the north of Isabela Island.

Migration between Marine Protected Areas

Our first evidence of connectivity between Marine Protected Areas came soon after our work began. In 2006 we found that two out of eighteen hammerheads tagged in the Galapagos Islands were detected at Cocos Island.

The sharks, both females, were detected on the same day. One had been registered only 14 days earlier at Darwin, and after a few weeks at Cocos, was registered once again in Galapagos.

This was the first definite record of a migration and return between the islands.

Since then, we have found several examples of movements of hammerhead sharks between Cocos, Malpelo and the Galapagos Islands, as seen in the figure above.

The big picture

Our combined results tell us that scalloped hammerhead sharks rarely make use of all the habitat available to them. They form large schools around specific points of the oceanic islands of the region. They spend much of the day at these sites then move offshore at night, presumably to feed. We have also found that where hammerhead sharks are abundant, there is often a high abundance of other pelagic species.

This has implications for the management of marine reserves – zonation schemes can be designed around shark movements.

In the case of the Galapagos Islands, there is already a coastal zonation but there is come controversy regarding how far it extends into the ocean.

This research, combined with our tracking individuals continually for several days to understand their local open water movements, will provide a technical basis to help resolve this issue.

By linking our site fidelity studies with the tracks of open ocean movements, we can build a regional image of hotspots and pathways, which can be overlaid on national and protected area boundaries, fishing areas, and used to study the feasibility of temporal and/or spatial open water management tools.