Wednesday, February 20, 2008

Shark Chronicles 003 - The elusive Zambezi (bull) shark of the Breede river


This week saw SAMPLA travel to the Breede River in the Western Cape Province to search for South Africa’s most southerly population of Zambezi (bull) sharks. SAMPLA joined a team of conservationists and researchers from Marine and Coastal Management, Shark Life, Shark Lady Adventures and the Breede River Conservancy to assist in a new project initiated by the South African Shark Conservancy.

The expedition was motivated by Meagen McCord (of the South African Shark Conservancy) stumbling across a series of photographs on a visit to the town of Witsands (White Sands). Not every corner shop displays photographs of long faced fishermen holding up dismembered carcasses of fish in front of the Breede river. Meagen realized that the photos were not advert for Breede sashimi. The obvious explanation was that sharks, attracted to the struggling, hooked fish, had expertly severed the bodies of the fish, leaving the frustrated fishermen to retrieve nothing more than fish heads … ergo the long faces

The obvious culprit was the Zambezi shark, one of the few sharks evolved to thrive in brackish and fresh water. Further investigation by the Shark Conservancy discovered a stash of old black and white photographs of landed Zambezi sharks all hooked within the Breede river system. Zambezi’s in rivers have been well documented, but not at the latitude of Breede river, which is in a temperate zone, a zone more commonly associated with the warm blooded great white shark. Without the thermo-physiological abilities of the great white, the Zambezi shark seldom makes forays into such cold waters, preferring the comparative warmth of the kwa-Zulu natal and Mozambique coastlines. ‘Cold water’ Zambezi’s are definitely something worth investigating if you are SAMPLA, or any other shark enthusiast.

So come 6.30 a.m. on Monday in February we found ourselves on the chilly banks of the Breede river, preparing bait, chum, boats and equipment for three days of shark searching. We split into three teams, one shore based and two boat based. The plan was to use a variety of techniques to attract the Zambezi’s (“Zams”), and then to hook and restrain the sharks. Once restrained, we would tag, measure, photograph (for visual identification) the sharks and inject the sharks with tetracycline (a vertebral marker that enables animals to be aged, and growth calculated, if they are recaptured at a later date). SAMPLA scientist in residence, Ryan Johnson was part of the shore based team, along with Fiona Ayerst of Sharklife, and an experienced tag-release shark fisherman from Hermanus. Both Sharklife and SAMPLA were initially reluctant to hook and restrain the Zam’s, preferring less intrusive methods of tagging, such as those they had used when tagging free swimming Zams using Scuba in Mozambique. However, the poor water visibility of the Breede rendered scuba diving with sharks too risky, and with limited options, they decided fishing was the only option. SAMPLA hoped that the experience gained from successfully capturing and releasing great white for satellite tagging would help minimize stress when capturing the Zams.

As it turned out, there was no reason for us to stress about shark stress as to cause capture stress to a shark, you need to hook it first! Hooking a shark is not as easy as sometimes implied by pictures that portray sharks as a ‘voracious, exponential eating machine’. Over the three-day expedition, the team became increasingly aware that locating Zambezi’s in the expanses of the Breede river and its estuary was a tough task. With chum flowing, shark lines deployed, scientific equipment set up, we waited, and waited, and waited…. but too no avail. Despite occasional reports of a shark fin breaching the surface, the waters surrounding our vessels and lines were still. Today, evidence of Zambezi sharks of Breede river remain in two dimensional black and white, hanging in a corner store in White Sands. We have returned to base to reflect, think and plan for the next expedition.

Understanding why Zambezi sharks travel so far south into temperate waters is an intriguing question, a question that all involved in this expedition are keen to answer. Is the degradation of northern estuaries and rivers behind this range extension? Could it be the impact of climate change, or are we simply learning more about the ecological capabilities of another enigmatic shark species? We will strive to get closer to some answers...

RYAN & TOBY

2 comments:

Fiona said...

Hi Ryan. I really enjoyed reading your blog on the Zambezis. Rather terrifying thought that they are crusing aounrd the Breede but a pity they didn't show up on the day! Better luck next time?

Fiona said...

Any chance of posting the fisherman's pictures?