A South African Winter of Shark Research: The Adventure Continues

by Madeline Jehenself

The first project I worked on at the South African Shark Conservancy (SASC) was a portion of their broader Marine Ecology Research Programme (MERP) which explores the biodiversity of Walker Bay. Walker Bay is a gorgeous piece of the Atlantic Ocean between Hermanus to the northwest, and Gaansbaai, renowned white shark cage diving destination, to the southeast. These shallow, temperate waters are productive, however the extent of the biodiversity they contain (particularly when it comes to sharks!) is unknown.

Walker Bay as seen from an overlooking mountain & the idyllic town of Hermanus

To investigate, SASC uses basic rod and reel shore angling. They fish at 5 coastal habitats (estuary, sandy bottom, rocky shore, harbour, and kelp forest) all year long in order to compare catch rates of shark species, sexes, stages of maturity, and sizes between different habitat types, seasons, and years. Once a shark is caught a timer is immediately started, recording the length of time the fish is out of the water. The shark is laid on a measuring mat and measurements such as total length and precaudal length (distance from the tip of its rostrum to the base of its caudal fin on the dorsal side) are recorded. Sexual maturity (applicable only to males) is determined by checking the level of calcification, or stiffness, of their claspers. A fin clip of the first dorsal fin is taken for genetic analyses, and the shark is tagged before release. SASC uses spaghetti tags, which are the most basic identification tags out there for sharks. These tags are inserted directly adjacent to the first dorsal fin.

Spaghetti tags. These two would be used to tag smaller, bony fishes. The SASC tags are about 10cm long. Courtesy of the California Department of Fish and Wildlife, image modified.

Each tag has a unique ID number on it, which is recorded alongside the shark’s measurements. After release, changes in size and condition with subsequent recaptures are tracked. This information is used in studies of aging, growth, maturity, population dynamics, and site fidelity (its movements in relation to the place it was originally caught). Inserting a spaghetti tag is simple, but can be a bit stressful due to the stubbornly tough nature of a shark’s skin, which is covered in tooth-like dermal denticles!

A scanning electron micrograph of spiny dogfish (Squalus acanthias) denticles.

 

Since 2010, SASC has tagged 6 species of shark in Walker Bay: puffadder shyshark, dark shyshark, brown shyshark, leopard catshark, pyjama catshark, and one smoothhound shark. There has been an 18% recapture rate, and the sharks generally seem to favour kelp forests and rocky shores. These are just rough, preliminary findings, and the whole picture of shark biodiversity and habits will gain ecological sense as the Walker Bay sharks project continues!

 

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