by Ruth H. Leeney
As part of the Sawfish session at the Sharks International conference this June in Durban, Bernadine Everett spoke about sawfishes in South Africa. On South Africa’s west coast, the Benguela current brings cold, productive water from the Antarctic as far north as Namibia’s border with Angola, excluding sawfishes at least from inshore waters. However, the warmer waters of South Africa’s eastern coast once supported sawfish populations – most likely largetooth sawfish (also known as freshwater sawfish, Pristis pristis), and green sawfish (Pristis zijsron). In the 1930s, sawfishes were amongst the most commonly-reported elasmobranchs in KwaZulu-Natal on South Africa’s east coast*, but it has now been 15 years since a sawfish was last captured in the region. So what happened to them?
Bernadine examined records dating back to 1964, from three sources: the KwaZulu-Natal Sharks Board (KZNSB) bather protection nets (providing information from 1964 to the present), various research projects carried out by the Oceanographic Research Institute in the St Lucia Estuary (1967-1970) and the Oceanographic Research Institute’s Tagging Project (1984-present). Bather protection nets were first installed at popular tourist beaches in KZN in the 1950s and the KZNSB started maintaining the nets and keeping records of the species caught in these nets only in the 1960s. The nets are surface gillnets, anchored to the seafloor and set approximately 400 m offshore. Between 1967 and 1970, the Oceanographic Research Institute (ORI) used 45-metre gill nets, set across an area called The Narrows, to intercept fish migration routes between the ocean and the lake in the St. Lucia estuarine system. Sawfishes were caught as bycatch (i.e. they were not the target of these studies) and were tagged and released. The third set of catch data came from ORI’s Tagging Project, whereby recreational hook-and-line anglers tag and release their catches along the entire southern African coastline. Through subsequent catches of tagged fishes, we can gain insight into the different habitats they use and how long they might live. Few sawfishes have been caught by anglers, as sawfishes are easily able to cut through fishing lines with their rostra.
|Two green sawfish captured in the St. Lucia estuarine system.|
These three sources have allowed Bernadine and her colleagues to better understand past sawfish abundance and distribution in South African waters. All the sawfish caught in the St. Lucia estuarine system by the ORI researchers were green sawfish. Of those caught by KZNSB nets, 11 were identified as green sawfish and three as largetooth sawfish. The rest of the KZNSB sawfishes, and all of those caught by the ORI Tagging Project, were not identified to species level. Over 50 years, the KZNSB bather protection nets caught a total of 91 sawfish, of which 23 were released alive and 13 were found dead (there was no available information on the fate of the remaining 55). ORI’s gillnetting in the St Lucia estuarine system caught 115 sawfishes, of which only 3 died and the rest were released with tags. Only six sawfishes have been caught by anglers involved with the ORI Tagging Project, and all have been released alive.
Catches of sawfishes in the KZNSB nets peaked in 1966 (17 animals), but became sporadic from the early 1970s onwards. Anglers only caught sawfishes in the 1980s. The last recorded capture of a sawfish in KZN waters occurred in 1999. More sawfishes were consistently caught off particular parts of the coast, including Richards Bay and Zinkwazi (see map below), suggesting that these areas may have been important habitats for sawfishes. And whilst sawfishes caught in the bather protection nets ranged in size from 1.23 to 5.3 m (average length of 3.1 m), those caught in the St. Lucia estuarine system were far smaller, averaging 1.5 m. The prevalence of juveniles in the estuary suggests that this was an important nursery ground, at least for green sawfish. We know that in other parts of the world, adult largetooth sawfish swim into estuaries and even upriver into freshwater, in order to give birth to their young, and that those young stay in estuarine and mangrove areas for some time, where perhaps they are safer from predators, before they venture into the open ocean. Only a handful of shark and ray species can use both freshwater and marine environments, which makes sawfishes even more unique.
|Total number of sawfishes caught along the KZN coast, 1964-2013. Map by B. Everett.|
So, why have sawfishes, once common along the KwaZulu-Natal coast, disappeared? There are various factors that may have caused their decline. Farming in the St. Lucia catchment area resulted in the release of sediment and nutrients (from the soil and fertilisers) into the river, and diversion of water for irrigation. In addition, the Mfolozi River was diverted away from the mouth of the St. Lucia estuarine system in the 1950s and a dredging operation was then implemented in order to keep the river mouth open. These activities likely had serious implications for sawfishes by changing the environment so important for their young. Furthermore, sawfishes are extremely vulnerable to entanglement in fishing nets of any type. The use of nets on the KZN coast began in the late 1800s and in the 1930s, an industrial-scale shark fishery was launched in the inshore waters around Durban. These activities must invariably have taken their toll on sawfishes. The installation of bather protection nets in the 1950s, and illegal gill and seine netting activities in the St. Lucia estuarine system most likely caused increased mortality in already reduced populations of sawfishes, and led to their local extinction. It is unlikely that sawfishes now remain in the waters of KwaZulu-Natal, but given recent improvements in the state of the St. Lucia estuarine system, it may now offer suitable sawfish habitat once more. Given the findings of my recent study in Mozambique, which suggest that sawfishes are still present there, perhaps we might one day see a return of sawfishes to South African waters. And that it is even more incentive, if we needed it, to find and protect Africa's remaining sawfish populations, before a similar fate befalls them.
Many thanks to Bernadine Everett for providing the details of her study for this article.
This study was funded by the South African Association for Marine Biological Research. A paper summarising this study will be published as part of the Sharks International 2014 conference proceedings: Everett B, Cliff G, Dudley S, Wintner S, van der Elst R. Do sawfish (Pristis spp.) represent South Africa’s first local extirpation of a marine elasmobranch in the modern era? African Journal of Marine Science (in press).
*von Bonde C. 1934. Shark fishing as an industry. Investigational Report No. 2. Pretoria: Department of Commerce and Industries, Fisheries and Marine Biological Survey Division. p 19.