October 27, 2015

Hammerhead sharks have declined in number by more than 90% in parts of world, and their conservation is high on the global agenda. In Australia, three hammerhead species are being considered for listing under the Environment Protection and Biodiversity Conservation Act 1999.

A new Marine Biodiversity Hub project is supporting the listing process by investigating the sharks’ population structure in Australian and neighbouring nations. Where appropriate, the project will work with Indigenous Australian peoples to benefit from their knowledge of hammerhead sharks, which are a cultural icon in Torres Strait.

Although one of the more fecund species groups among sharks, hammerheads are long-lived and produce relatively few offspring compared with most fish: traits that make them vulnerable to exploitation. In fact the Great Hammerhead is the Methuselah of sharks, living up to 44 years.

But hammerheads are in trouble not just because of sluggish reproduction. Highly specialised anatomical, behavioural and physical traits that over 10-20 million years have sculpted a perfect ecological niche are now contributing to their demise.

What are these traits, and could understanding them offer clues for hammerhead conservation?

A genius among fish

Great, scalloped and smooth hammerheads have extremely large brains relative to body size, and the most complex brains of all sharks and rays. Large brains enable advanced social and sensory systems, super agility, and complex habitat use and migrations.

Hammerheads swim large distances, across the open ocean with no heed for international boundaries. This makes it difficult for scientists to track and provide advice on the status of discrete populations.

While many shark species form aggregations, the scalloped hammerhead is one of the only large sharks with highly organised and complex social schooling behaviour. Because fishers target aggregations, this makes the species susceptible to capture. Female hammerheads also head inshore to breed, exposing pregnant mothers and their young to coastal fishing and development.

Two sides to the hammer

The hammerhead’s unique sensory traits and body shape are added handicaps.

These hunters are wired for ‘burst’ swimming in their attacks on fish, squid and other prey. The downside of this excitability is that hammerheads appear to be the most vulnerable of sharks to human-induced stress, with 60%–80% unlikely to survive capture.

Their key weapon, the hammer, or cephalofoil, could be a contributing factor. Several theories exist for its evolution.

First, the hammer gives a wide beam to electrical sensors (a feature of all sharks) that detect nerve impulses from potential prey. Secondly, the 360-degree vertical view is great for tracking fast-moving prey. Thirdly, the hammer aids movement in the water by providing hydrodynamic lift, enabling a rapid turn away from the seabed after a downward attack.

But the hammer’s evolution may have resulted in a trade-off in the performance of other features, such as decreased mouth size, which may limit the uptake of oxygen. It may also make the sharks more susceptible to capture, particularly to non-intentional capture in gillnet fisheries where they quickly become enmeshed.

Hope for conservation

Despite their problems, hammerheads are better placed than shark species with smaller initial population sizes, fewer young, restricted ranges, and very specific ecological niches. Grey Nurse sharks and Australia's river sharks (also the subject of Marine Biodiversity Hub research) fit this category.

Hammerheads once were extremely abundant, and scalloped and smooth hammerheads in particular are some of the most widely distributed large coastal shark species. And elements of hammerhead life histories - though still poorly understood - may enable them to recover quickly once appropriate conservation actions are taken.

An understanding of hammerhead traits suggests that conservation strategies such as avoiding peak areas or periods of aggregation, improving shark handling, and reducing the sharks’ detection of baited hooks may be worth consideration.

For example, the hammerhead's sensory systems detect electric fields from greater distances than do those of other shark species.

US researchers have trialled baited hooks fixed with naturally electrogenic lanthanide metal alloy to determine their ability to repel sharks. The experiment that targeted juvenile scalloped hammerhead sharks resulted in a significant reduction in captures compared with experiments that targeted other shark species. (Adult hammerheads were not tested.)

Which hammerhead is which?

Ironically, hammerheads at the species level are not specialised enough. Scalloped, smooth and great hammerheads are difficult to tell apart, yet accurate identification is fundamental to estimating and monitoring populations.

Hammerhead species are still being discovered. Molecular analyses revealed a new species off the United States east coast in 2013, previously thought to be the Scalloped Hammerhead. Could more hammerheads be waiting in the wings?
 

Further reading

 

Captions

Hammerhead shark: Credit: Andy Murch, OceanwideImages.com

Hammerhead sharks have special significance to the people of the Torres Strait Islands where it is an important cultural symbol. Credit: Suzanne Long, Alamy Stock Photo