Project Title: Spatio-temporal dynamics of Zambezi sharks and their prey in the Breede River, South Africa
Zambezi (bull) sharks, Carcharhinus leucas, are targeted by recreational anglers and landed within a suite of poorly identified Carcharhinids in several commercial fisheries in South Africa. The Breede Estuary, situated on South Africa’s southwest coast, is heavily utilised by recreational anglers targeting dusky kob Argyrosomus japonicus and spotted grunter Pomadasys commersonnii — the primary prey of C. leucas in the system. Depredation events have caused anglers to become progressively agitated by the presence of sharks in the estuary, resulting in growing user conflict. This project aims to mitigate user conflicts and promote a multi-species approach to conservation and management by examining inter- and intra-specific interactions, short-term, seasonal and broad-scale movement patterns of C. leucas and its primary prey. Since 2009, five Zambezi sharks and eleven spotted grunter have been fitted with acoustic tags. Active tracking of four sharks indicated sex-specific differences in habitat utilisation and movement patterns exist, although both sexes foraged opportunistically from the lines of anglers.
Some of the major objective of this work include:
- Quantify habitat use patterns for Argyrosomus japonicus, Pomadasys commersonnii and Carcharhinus leucas; Quantify physico-chemical habitat relationships (e.g., salinity, temperature, tide, freshwater flow);
- Quantify seasonal and inter-annual fluctuations in habitat use;
- Identify & quantify inter-specific relationships in habitat use;
- Quantify fishing effort for kob, grunter & sharks;
- Quantify interactions between fishing effort, fish habitat use & environment;
- Establish the importance of the Breede Estuary as essential habitat for juvenile & adult kob, grunter and sharks on a regional & National scale;
- Integrate spatio-temporal dynamics, environmental and fisheries data to predict effects of micro- and macro-scale ecosystem changes on these species.
Since Zambezi sharks commonly occur in areas heavily utilized by people (i.e., inshore coastal areas, rivers, estuaries), several studies throughout the species’ range have focused on movement and behaviour. Increasingly, studies are focusing on the role of these apex predators in maintaining the health and integrity of ecosystems through, for example, predation effects. Very little is known about the behaviour and ecology and Zambezi sharks in southern Africa as studies here have focused on age and growth using opportunistic samples from the bather protection nets in KwaZulu-Natal. There is increasing concern about the health of this population however as data from the bather protection nets indicates the local Zambezi shark population is declining.
All field work currently occurs in the Breede River — a warm-temperate estuary situated on the southwest coast of South Africa. To examine fine-scale habitat use sharks and fish are fitted with internal acoustic transmitters. Movement is passively monitored using a series of 18 acoustic receiving stations situated from 0km-21km upriver. Temperature-pressure loggers are attached to several receivers to enable monitoring of fish movement according to water temperature and tide state. These data are supplemented with 20 years of environmental data collected by conservation officers, including salinity, dissolved oxygen and turbidity. Fisheries surveys are conducted to determine frequency of interaction with sharks and quantify depredation events, and public perception surveys are concurrently conducted to quantify state of knowledge, attitudes and beliefs about Zambezi sharks. Broad-scale movement is being examined using two different tagging techniques: pop-up archival satellite (PAT) tags and smart positioning tags (SPOTS).
Models to quantify spatial use and examine species macroecology will be designed using Visual Basic. The data will be used to inform multi-species approaches to conservation and management and to mitigate human-shark interactions.
RJD In Focus
This project aims to mitigate user conflicts & promote a multi-species approach to conservation and management.
Two male sharks and one female shark have been tagged with external acoustic and conventional dart tags, and have been actively tracked for over 460 hours. Initial results indicate sex-specific and individual-specific differences exist in movement and behaviour. 11 spotted grunter and one shark were fitted with internal acoustic tags and are being passively monitored by the acoustic array, which will be serviced — and data downloaded — every three months for the project duration.
The female shark exhibited tidally-dependent fine-scale movement, swimming upriver on the incoming tide and downriver on the outgoing tide. The maximum distance travelled by the female shark during a 24hr tracking period was 93km. Movement patterns of both males were independent of tide and appeared to be based on physico-chemical parameters such as salinity and turbidity. The male sharks appeared to have distinct territories in the estuary, with Male 1 (M1) spending 75% of the time in the 11km-17km range of the estuary and Male 2 (M2) spending 100% of the time between 3km and 11km. The remaining 25% of the time, M1 made forays into the zone occupied by M2. Both males preferred less turbid areas of the estuary, although M1 exhibited a preference for lower salinity and M2 preferred higher salinity areas. Maximum distances travelled during a 24hr tracking period were 37km (M1) and 24km (M2), respectively. We hypothesize sex-specific differences in behaviour are related to reproduction (i.e., pupping) while individual-specific differences in behaviour could be due to differences in foraging strategies.
Tracking during winter (April to October) has revealed Zambezi sharks are absent from the Breede River estuary. Prior to the 2011 field season, however, we were uncertain whether these sharks remained in the cooler coastal waters surrounding the Breede River estuary or undertook migrations to the sub-tropical waters further east along the South African coast. In March 2011 we successfully deployed a PAT tag on a 3m male shark, which subsequently undertook a migration of approximately 2000km, from the Breede River to Bazaruto, Mozambique, in 53 days. This animal was one of the male sharks (M2) captured in 2010. The recapture indicates Zambezi sharks exhibit a degree of philopatry — returning to specific estuaries for reproductive and/or feeding purposes — in South Africa. The discovery that this species undertakes large-scale migrations across international borders has critical trans-boundary management implications.
The South African Shark Conservancy (SASC) runs an internship program for students and non-students interested in marine science and conservation. Interns join SASC on field trips and have the opportunity to partake in all related activities, including fishing, tagging, tracking, etc. Research results are shared with the public through several forums such as Facebook, newsletters and blogs.
• M McCord
• Dr. S Lamberth
• Dr. S Kerwath
• C Erasmus
• C Wilke
• Dr. N Hammerschlag
McCord, ME and SJ Lamberth. 2009. Catching and tracking the world’s largest Zambezi (bull) shark Carcharhinus leucas in the Breede Estuary, South Africa: the first 43 hours. Short Comm., African Journal of Marine Science. 31(1): 107–111.