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.
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