The first regional studies have been done concerning the population genetics of soupfin or topesharks, copper or bronze whaler shark and the endemic lesser sandshark found in South African waters. The research sheds light on how these species could be managed and fished sustainably. Soupfin fillets are for instance exported to Australia and used in fried fish and chips restaurants, while dried fins are shipped to Asia.
The research was conducted by Dr Daphne Bitalo, who received her PhD in Genetics from Stellenbosch University on 16 March 2016. Dr Bitalo, who hails from Uganda, has published some of her results in the scientific journal Fisheries Research and has presented at national and international meetings of shark experts. Her study supervisors were Dr Aletta Bester-Van der Merwe and Prof Rouvay Roodt-Wilding from the Molecular Breeding and Biodiversity Group of the Department of Genetics at Stellenbosch University. The group has a specific interest in commercially exploited marine species, such as abalone, kob and yellowtail.
The study was performed in collaboration with the national Department of Agriculture, Forestry and Fisheries and the South African Shark Conservancy.
Dr Bitalo focused on the soupfin shark (Galeorhinus galeus), the bronze whaler shark (Carcharhinus brachyurus) and the lesser sandshark (Rhinobatos annulatus). The latter is a guitarfish only found in southern Africa.
Little is known about the local patterns of population genetic diversity of these three species. All three are so-called demersal species that feed near or at the bottom of the sea. In the process, they can be easily caught as bycatch in the longline fishing industry, by commercial linefishermen, in gill nets or by trawlers. They are also not very fast breeders as they take time to reach maturity, and are also not very fertile. They can therefore easily be exploited if the dynamics surrounding their population growth and breeding patterns are not taken into account.
Soupfin and bronze whaler sharks already enjoy a vulnerable conservation status in the Southern Hemisphere, while too little is yet known about the populations of lesser sandshark to classify it accurately.
"As this is the first regional assessment for all three of these species, the findings of this study could help to improve the regional management and conservation of commercial and recreational sharks," says Dr Bitalo. "Information about how each shark's genetic make-up differs from region to region will help set out management units according to which better fisheries monitoring and specific conservation efforts can take place."
"Such information is important if one ultimately wants to make informed and integrated decisions to ensure sustainable shark fisheries in South Africa," adds Dr Bester-Van der Merwe.
Currently there are no specific catch regulations for these species.
Soupfin sharks
Tope sharks (locally known as soupfin sharks) are the second most commercially important shark species in South Africa. It was one of the first shark species to be exploited by the Western Cape fishing industry when commercial endeavours started in the 1930s. Up to 5000 tonnes were caught in 2013 off the western and southern coast of South Africa. In the 1930s and 1940s, its liver served as a valuable source of Vitamin A worldwide. Most soupfin harvested locally nowadays are exported to Australia where the fillets are consumed in the fish and chips industry.
Soupfin sharks have a long history of being present in the Southern Hemisphere, and this study confirmed three historically related but genetically very distinct populations across the South Pacific, South Atlantic and Indian Ocean expanses.
Dr Bitalo recorded significant differences between local populations caught off different spots along the South African coastline. "At least two, and possibly even three different genetic groups exist along the coastline, with the Eastern Cape population(s) being the most unique," she writes.
The SU-geneticists believe that site restrictive licensing and size restrictive licensing should be put in place to prevent the depletion of genetic diversity and consequently reproductive potential.
"This management approach should immediately be applied in regions with high levels of genetic uniqueness, as the Eastern Cape sites for example could well be the source for genetic variability of soupfin sharks found along the South African coastline," adds Dr Bester-Van der Merwe.
In practice this could mean reducing the number of fishing permits currently issued and implementing size limits on soupfin caught. Ideally this would also lead to soupfin sharks being protected in the same marine protected areas that are already set aside for commercial shark species such as the common smoothhound shark.
Copper or bronze whaler sharks
The copper shark or bronze whaler is also a wide-ranging species. It is the fifth most commercially important shark species in South Africa, and is mainly caught by linefishermen from the Western Cape to KwaZulu-Natal, or as a bycatch of prawn trawlers.
Based on Dr Bitalo's results, it seems as if all copper sharks are genetically similar across different coastal regions. Distinct nursery sites are likely to exist for this type of shark along the Southern Cape coast. Therefore bay areas such as Mossel Bay are of particular importance for conserving the species genetic diversity and wellbeing.
Lesser sandshark
Little is known about the distribution and population dynamics of South Africa's endemic lesser sand shark (also known as the lesser guitarfish). It is found from southern Angola to KwaZulu-Natal. Because it inhabits very shallow waters close to sandy beaches, it is a likely candidate to be caught by recreational fishermen and to be influenced by the effects of human settlements.
Based on her findings, Dr Bitalo believes that lesser sand shark numbers are not as abundant as are generally presumed. The number of breeding individuals per area sampled could effectively stand at a critically low number of below 500.
She found significant levels of genetic differences for lesser sand shark populations. This means that although they are of the same species, the exact genetic make-up of lesser sand sharks caught off Jeffrey's Bay's coastline for instance differ from those of ones caught at Die Plaat near Struisbaai. Interestingly, these genetic differences occur even though distance-wise the different areas are not very far removed from one another.
This implies that the species might be highly fragmented with unique gene pools, all containing a different genetic make-up, throughout its entire geographical range.
It is the first time that any genetic information for this guitarfish has been generated.
"Dr Bitalo's study therefore makes an important contribution towards our understanding of this species, and can be used to inform preventative measures to ensure the future sustainable fisheries and biodiversity conservation of this endemic species," says Dr Bester-Van der Merwe.
Media enquiries:
Dr Aletta Bester Van der Merwe
Department of Genetics
Stellenbosch University
aeb@sun.ac.za
021 8085835
