In 2013, the Two Oceans Aquarium agreed to assist with an exciting long-term research project on broadnose sevengill sharks (Notorynchus cepedianus – also known as cowsharks).
The project involves gaining a better understanding of their feeding, spatial (movement) and reproductive ecology, and ultimately aims to define their role as apex predators in South Africa.
It will use a combination of cutting-edge, minimally invasive techniques, including satellite and acoustic tracking, to study movements and diet, and to analyse blood and hormone concentrations in blood to investigate reproductive biology.
The broadnose sevengill shark is a large (up to 3m) coastal-associated apex predator that is found in temperate waters around the world. It lives in kelp forests around the Cape and increasingly attracts the interest of scuba divers, particularly off Miller’s Point in False Bay.
According to Steve Benjamin of Animal Ocean, “It is called a number of names [including] cowshark, sevengill shark or the full-length broadnose sevengill cowshark, which has one too many descriptive nouns for my liking. We mostly just refer to them as cows! Not a very frightening name to attract shark divers.”
The common name “sevengill” alludes to the fact that these sharks have seven gill slits instead of five, which is the norm in other shark species.
Apex predators and their conservation status
This species’ position in the food chain rivals that of other species considered important apex predators such as tiger sharks and white sharks. Yet, in contrast to the latter two shark species, considerably less information is available on sevengill sharks.
Sevengill sharks are a low-value fishery species across most of their global distribution. However, there are limited management policies or conservation considerations for this species in any country, and commercial exploitation in South Africa is currently unrestricted.
Ongoing unregulated exploitation is of concern, as previous targeting of sevengills in California and Namibia suggests that they are vulnerable to fishing.
Their low conservation priority may be attributed to the lack of available data (classified as data deficient on the IUCN Red List). One of the aims of this project will be to collect enough data to inform management as to whether any threats exist.
Sevengills in False Bay
False Bay is unique in that it is the only place in world where large numbers of sevengills (up to 70 on a single dive) can be consistently seen. In all other instances, the water is so murky that there is little visibility, or sightings are few and far between.
“We suspect that pregnant females reside here,” says Dr Alison Kock, Research Manager for Shark Spotters, who is leading the research project. “There is little known information on pregnant females elsewhere in world so this presents a great opportunity to collect more data, which will be done by using non-destructive methods, e.g. analysing hormones in the blood samples that are collected.”
Sevengill sharks are being tagged with acoustic and pop-off archival satellite transmitters (PAT tags). PAT tags are programmed to collect and archive data such as depth and temperature before the tag releases from the animal at a pre-programmed time.
The tag floats to the surface and the GPS position of the release location and archived data are sent to satellites passing overhead. The acoustic tags transmit a signal that can be picked up by one of 24 acoustic receivers that have been deployed in Cape Town waters.
The sharks are also measured, sexed, and biological samples collected. Blood and muscle samples allow for the analysis of genetics, for stable isotopes to determine diet without the need for lethal sampling, and for reproductive hormone analysis, which allows the team to assess maturity status and reproductive status (the stage of the reproductive cycle they are in) without the need for lethal sampling.
The team takes the utmost care of the sharks to ensure their welfare, and all methods have been approved by the University of Cape Town’s ethics committee. All surgical procedures are overseen by the Aquarium’s resident veterinarian, Dr Georgina Cole.
The research is being co-ordinated by Dr Kock, and Dr Adam Barnett from James Cook University (Australia), with assistance from colleagues and students from the University of Cape Town; the Department of Agriculture, Forestry and Fisheries; the South African Shark Conservancy; Bayworld Centre for Research; SAIAB’s Acoustic Tracking Array Platform; the Ocean Tracking Network; and the Oceanographic Research Institute.
Funding is primarily from the Two Oceans Aquarium, with additional financial or equipment support from the University of Cape Town, University of Tasmania, Save Our Seas Foundation and Ocearch.