Mossel Bay is internationally recognised as one of the centres of abundance of white sharks in South Africa. During 2008 – 2010 there were four sites within the bay i.e. Seal Island, Hartenbos, Kleinbrak and Grootbrak, which were sampled to gain insight into the population dynamics of this species. Currently, life history information on white sharks in this area is limited. This study used a combination of mark-recapture using photographic identification techniques and sight per unit effort methods. Inter-annual, seasonal and spatial patterns in abundance are assessed. The effects of environmental parameters on abundance are also investigated. Photographic identification techniques were employed to identify unique individuals within the sampled population. This modified mark-recapture approach is therefore non-invasive and cost-effective. Open population POPAN parameterization was used to analyse the data in software program MARK. The total population was estimated at 389 sharks (351 – 428; 95 percent CI). Over the three year period, a marginal (yet non-significant) decline in numbers was observed, in terms of both monthly and seasonal population estimates. Sightings per unit effort data were collected during sampling trips. The relative abundance and body size composition of white sharks demonstrated significant spatial and seasonal variation. The highest and lowest relative abundance was observed at Seal Island and Hartenbos, respectively, and is likely attributed to prey availability. Although white sharks were present year-round in Mossel Bay, the highest relative abundance occurred during summer and the lowest relative abundance occurred during spring. White sharks were grouped into three main size classes based on estimated total length (TL): Young of the year (YOY) (125 – 174cm), juvenile (175 – 324cm) and adult (325 – 524cm). YOY white sharks were most prevalent at Grootbrak, with juvenile and adult individuals concentrating at Seal Island. Although most size classes were present throughout the year, seasonal differences were observed. YOY individuals were most abundant in the autumn months, juvenile size-classes appeared to concentrate in the study area during winter, and the adult individuals were most abundant in the spring months. Overall, there was a high concentration of white sharks ranging in size between 175 – 324cm TL, and it was thus hypothesised that Mossel Bay represents an interim nursery or grow out area for white sharks in South Africa. Data collected from 2008 and 2009 was used to investigate the relationship between specific environmental parameters, i.e. sea surface temperature and vertical water clarity, in relation to the relative abundance of white sharks. Sea surface temperature and vertical water clarity observed in this study ranged from 9.3 - 22.7°C and 0 – 10m, respectively. Sea surface temperature did not have a significant influence on the relative abundance of white sharks and this may be attributed to the thermoregulatory capacity of the species. Vertical water clarity, however, did significantly influence the relative abundance. Furthermore, the combined effect of site and season significantly influenced the relative abundance of white sharks and is probably linked to the distribution and abundance of inshore prey resources.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nmmu/vital:10717 |
| Date | January 2012 |
| Creators | Ryklief, Rabiah |
| Publisher | Nelson Mandela Metropolitan University, Faculty of Science |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Masters, MSc |
| Format | xi, 105 leaves, pdf |
| Rights | Nelson Mandela Metropolitan University |
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