Dyer Island Research Articles

Patterns in sightings of ocean sunfishes (Molidae) in the greater Dyer Island ecosystem, Western Cape, South Africa

This study aims to describe the distribution of ocean sunfishes (family Molidae) in the greater Dyer Island ecosystem off the southwest coast of South Africa. Between 2014 and 2022, local whale watching vessels operating under Dyer Island Cruises conducted 3 227 trips which resulted in 274 trips with Molidae sightings, with a total of 300 individuals sighted. Molidae sightings showed a significant correlation to sea surface temperature and a strong seasonal pattern. Sightings in the greater Dyer Island area increased significantly after 2016, suggesting a correlation with the decline of white shark Carcharodon carcharias sightings, with further increases in 2021. This suggests that the greater Dyer Island ecosystem has become a more favoured habitat for species of Molidae.

Influence of Environmental Factors on Prey Discrimination of Bait-Attracted White Sharks from Gansbaai, South Africa.

The influence of environmental factors on prey discrimination of bait-attracted white sharks was studied over a six-year period (2008-2013) at Dyer Island Nature Reserve (Gansbaai, South Africa). Across 240 bait-attracted feeding events observed in this period, both immature and mature white sharks were attracted by the seal-shaped decoy rather than the tuna bait, except for the years 2008 and 2011. Tide ranges, underwater visibility, water temperature, and sea conditions were, in decreasing order, the factors which drove white sharks to select the seal-shaped decoy. High tide lowered the minimum depth from which sharks could approach seals close to the shore, while extended visibility helped the sharks in making predatory choices towards the more energy-rich prey source, the odorless seal-shaped decoy. On the contrary, warmer water is associated with an increase in phytoplankton that reduces underwater visibility and increases the diversity of teleosts including tuna-a known prey of white sharks-driving the sharks to favor the tuna bait. Overall, sea conditions were almost always slightly rough, ensuring a good average underwater visibility. Recommendations for future research work at this site are presented.

Passive Prey Discrimination in Surface Predatory Behaviour of Bait-Attracted White Sharks from Gansbaai, South Africa.

Simple SummaryWhite sharks, in surface passive prey predatory behaviour, are initially attracted by the olfactory trace determined by the bait and then implement their predatory choices to energetical richer prey, especially thanks to their visual ability, which plays an important role in adults and immatures with dietary shifts in their feeding patterns. Gansbaai represents a hunting training area for white sharks who are changing their diet.Between the years 2008 and 2013, six annual research expeditions were carried out at Dyer Island (Gansbaai, South Africa) to study the surface behaviour of white sharks in the presence of two passive prey: tuna bait and a seal-shaped decoy. Sightings were performed from a commercial cage-diving boat over 247 h; 250 different white sharks, with a mean total length (TL) of 308 cm, were observed. Of these, 166 performed at least one or more interactions, for a total of 240 interactions with bait and the seal-shaped decoy. In Gansbaai, there is a population of transient white sharks consisting mainly of immature specimens throughout the year. Both mature and immature sharks preferred to prey on the seal-shaped decoy, probably due to the dietary shift that occurs in white sharks whose TL varies between 200 cm and 340 cm. As it is widely confirmed that white sharks change their diet from a predominantly piscivorous juvenile diet to a mature marine mammalian diet, it is possible that Gansbaai may be a hunting training area and that sharks show a discriminate food choice, a strategy that was adopted by the majority of specimens thanks to their ability to visualize energetically richer prey, after having been attracted by the odorous source represented by the tuna bait.

Analysis of sightings of white sharks in Gansbaai (South Africa)

In Gansbaai (South Africa), at Dyer Island Nature Reserve, a large White shark population is present and can be observed due to the support of local ecotourism operators authorised to reach the field observation sites. Between 2009 and 2019, it was possible to create a database including information about each individual observed. In total, 423 white sharks were sighted during 462 direct observation hours from the boat, that included 220 hours from the diving “cage”. The mean sighting rate was 0.91 (range 0.18–1.53) sharks per hour and sighting rates dramatically declined in the last three years of the study period. Ninety-nine unique Photo-Ids of the dorsal fin were collected and only five re-sightings occurred, which indicate a transient behaviour for the Gansbaai White shark population. The sex ratio showed that females were always prevalent over males throughout the duration of the observations: the ratios were 1:2.2:0.8 for males, females, and unsexed sharks, respectively, and showed the prevalence of immature female individuals (immature: 51 males, 201 females, and 40 unsexed; adults: 49 males, 14 females, and 1 unsexed; undefined maturity: 5 males, 19 females, and 43 unsexed sharks). The predominance of immatures only applies to the females; there were as many immature males (51) as mature (49). The total length for all the individuals was between 150 cm and 500 cm (mean 308 cm, n = 423) with few young-of-the-year and adults recorded, indicating that Gansbaai Area is not a nursery area nor an adult aggregation site, but a seasonal feeding ground. The interannual sighting trend showed a consistent long-term increasing peak (ca. 4–5 years) and this could confirm that, in Gansbaai, the White shark frequency is not affected by ecotourism but, since 2017, a consistent loss of sightings was also due to recorded transient killer whales’ unusual fatal attacks.

The influence of kelp density on white shark presence within the Dyer Island nature reserve, South Africa

Large-scale and small-scale natural barriers have the ability to mediate the ecological dynamics within a region. In some instances, these barriers greatly influence the presence of predators and prey on spatio-temporal scales. For this study, we aimed to assess how varying densities of sea bamboo (Ecklonia maxima), a kelp species found within South African waters, could influence the presence of C. carcharias. Using baited remote underwater video systems (BRUVS), thirty-one different C. carcharias were identified and a total of 135 h of video were collected in high, moderate, and low kelp densities to create an organismal sighting index. Generalized linear models illustrated that the best fit model included the main effect of kelp density, with a clear inverse relationship between kelp density and C. carcharias sightings. However, although zero sharks were sighted within the high kelp density regions, ten different C. carcharias were sighted within the moderate kelp density regions, illustrating that the broad notion that C. carcharias do not navigate through kelp ecosystems is false and requires more specificity (e.g. high density kelp areas may result in decrease C. carcharias presence). In contrast, kelp density had no significant influence on the presence of five other elasmobranch species detected by the BRUVS. This study demonstrated that highly dense kelp forests (i.e. ≥1 stalk of kelp per 1 m2) serve as a local natural barrier for large C. carcharias; however, the need for a more inclusive analysis (e.g. routine inclusion of current speed and multi-location assessment) is warranted.

Cape cormorants decrease, move east and adapt foraging strategies following eastward displacement of their main prey

Numbers of Cape cormorants Phalacrocorax capensis breeding in South Africa decreased by nearly 50% from approximately 107 000 pairs in 1977–1981 to 57 000 pairs in 2010–2014. Although four colonies had >10 000 pairs in 1977–1981, there was just one such colony in 2010–2014. Almost all the decrease occurred after the early 1990s off north-west South Africa, between the Orange River estuary and Dassen Island. South of this, the number breeding in the two periods was stable, with some colonies being formed or growing rapidly in the 2000s. The proportion of South Africa’s Cape cormorants that bred south of Dassen Island increased from 35% in 1977–1981 to 66% in 2010–2014, with the opposite situation observed in the north-west. This matched a shift to the south and east in the distributions of two of the Cape cormorant’s main prey species, anchovy Engraulis encrasicolus and sardine Sardinops sagax. In 2014, an apparent scarcity of prey in the north-west resulted in Cape cormorants attempting to take bait from hooks of fishing lines over an extended period, a behaviour not previously recorded. The number of Cape cormorants breeding in the south may be constrained by the absence of large islands between Dyer Island in the west and Algoa Bay in the east. If so, it may be possible to bolster the southern population through the provision of appropriate breeding habitat, such as platforms, or restricting human disturbance at suitable mainland cliff breeding sites.

System dynamics modelling of the Endangered African penguin populations on Dyer and Robben islands, South Africa

African penguins (Spheniscus demersus) in South Africa have been subject to rapid decline in the 20th century and are currently listed as “endangered” on the IUCN Red List. A stochastic, stage-specific system dynamics model with spatial components was developed to investigate the interaction of multiple pressures on penguin population development, and originally applied to study conservation management options for the penguin colony at Robben Island, Table Bay, South Africa. We here present an extension of the model to the nearby colony at Dyer Island. The modelled population was found to be strongly dominated by the effects of Cape fur seal (Arctocephalus pusillus pusillus) predation and immature emigration, which appear to be key drivers behind current declines in population numbers at this colony. Chronic low-level oiling also had a consistent impact despite ongoing mitigation measures, while kelp gull predation had a lower effect than expected. At current low population size, the pressure from either seal predation or immature emigration seems sufficient to mask any beneficial effects to penguins from possible improvements in available food biomass (e.g., from fishery restrictions). Results suggest that conservation management at this colony should focus on regular culling of predating seals combined with improving availability of prey (thus discouraging emigration) both in the foraging range of breeding penguins and in their general foraging area. Our findings demonstrate the use of site-specific scenario tools to explore conservation strategies in data-poor management situations.

Skin Biopsies as a Sensitive Non-Lethal Technique for the Ecotoxicological Studies of Great White Shark(Carcharodon carcharias) Sampled in South Africa

Skin Biopsies as a Sensitive Non-Lethal Technique for the Ecotoxicological Studies of Great White Shark (Carcharodon carcharias) Sampled in South Africa Top-predators may be extremely vulnerable to environmental contaminants, such as organochlorines (OCs) and polycyclic aromatic hydrocarbons (PAHs), mostly because of their position in the trophic web. In this study, the use of skin biopsy is proposed as a sensitive non-lethal technique for the toxicological assessment of white shark (Carcharodon carcharias) living off the South African coasts. In 2012, 15 specimens of great white shark were sampled in the waters off Dyer Island and Geyser Rock. Then OCs and PAHs were extracted from muscle and biomarkers techniques for the evaluation of the cytochrome P4501A (CYP1A), Vitellogenin (Vtg) and Zona Radiata Proteins (Zrp) in the skin have been developed for the first time. The results showed levels of OCs higher than those found in the literature, ranging in ng/g dry weight (d.w.) from 6.80 to 21.26 for hexachlorobenzene (HCB), from 86.72 to 1416.97 for DDTs and from 379.76 to 11284.31 for polychlorinated biphenyls (PCBs).

Pollution, habitat loss, fishing, and climate change as critical threats to penguins.

Cumulative human impacts across the world's oceans are considerable. We therefore examined a single model taxonomic group, the penguins (Spheniscidae), to explore how marine species and communities might be at risk of decline or extinction in the southern hemisphere. We sought to determine the most important threats to penguins and to suggest means to mitigate these threats. Our review has relevance to other taxonomic groups in the southern hemisphere and in northern latitudes, where human impacts are greater. Our review was based on an expert assessment and literature review of all 18 penguin species; 49 scientists contributed to the process. For each penguin species, we considered their range and distribution, population trends, and main anthropogenic threats over the past approximately 250 years. These threats were harvesting adults for oil, skin, and feathers and as bait for crab and rock lobster fisheries; harvesting of eggs; terrestrial habitat degradation; marine pollution; fisheries bycatch and resource competition; environmental variability and climate change; and toxic algal poisoning and disease. Habitat loss, pollution, and fishing, all factors humans can readily mitigate, remain the primary threats for penguin species. Their future resilience to further climate change impacts will almost certainly depend on addressing current threats to existing habitat degradation on land and at sea. We suggest protection of breeding habitat, linked to the designation of appropriately scaled marine reserves, including in the High Seas, will be critical for the future conservation of penguins. However, large-scale conservation zones are not always practical or politically feasible and other ecosystem-based management methods that include spatial zoning, bycatch mitigation, and robust harvest control must be developed to maintain marine biodiversity and ensure that ecosystem functioning is maintained across a variety of scales.

Core habitat use of an apex predator in a complex marine landscape

Dyer Island Conservation Trust, Marine Dynamics Shark Tours and Dyer Island Cruises, VW South Africa and the National Lottery

Processes influencing the population dynamics and conservation of African penguins on Dyer Island, South Africa

Dyer Island, South Africa, supported the largest African penguin Spheniscus demersus colony in 1979 (22 655 breeding pairs), but population dynamics of the species have not followed the trends of adjacent colonies in years of high fish abundance or shifts in prey distribution. Less than 1 500 pairs were breeding on Dyer Island in 2013. Available knowledge on demographic parameters was collated and ecological processes were quantified. Juvenile and adult survival probabilities estimated for birds ringed on Dyer Island between 2005 and 2011 were 0.247 (SE 0.06) and 0.545 (SE 0.05) respectively. Juveniles had a high probability (0.918; SE 0.077) of moving away from Dyer Island. Predation by seals and kelp gulls, as well as oiling, are currently important top-down pressures on penguins on Dyer Island. Numbers of birds breeding on the island were negatively correlated with purse-seine catches 20 nautical miles around it. However, it seems that once the colony size is <3 500 breeding pairs, the importance of fishing pressure diminishes, suggesting that once a colony has been reduced to a relatively small size, its growth is limited by pressures other than fishing. From the review of available knowledge presented here, we recommend a number of management options that should be tested in a suitable model.

A novel categorisation system to organise a large photo identification database for white sharks Carcharodon carcharias

The white shark Carcharodon carcharias was one of the first elasmobranch species where photo identification was used to identify unique individuals. In this study, we propose guidelines that improve the current photo identification technique for white sharks by presenting a novel categorisation system. Using this method, a high-resolution photograph of the dorsal fin is placed on a standardised three-section grid. Notches associated with the trailing edge of the fin are counted and scored within each grid section. The number of notches in each grid section is then used to produce a three-part code that is utilised to systematically organise the individuals into a database. Our proposed system was tested on sharks photographed in the sampling area over a 27-month period (4 398 photographs) and the method significantly reduced the search time associated with identifying resighted individuals. Using the notches code, we were able to identify 426 different C. carcharias within the Dyer Island Nature Reserve (South Africa). Due to the inherent ease of use, the accuracy associated with this method, and the ability to ‘resight’ individuals rapidly within a large photographic database, this non-invasive technique presents a validated and feasible alternative for future white shark photo identification studies.

An assessment of the impact of predation by Cape fur seals Arctocephalus pusillus pusillus on seabirds at Dyer Island, South Africa

At Dyer Island, South Africa, observations of predation of seabirds by Cape Fur Seals Arctocephalus pusillus pusillus were made during 2004 and 2006/07. It was estimated that seals killed about 7% of adult African Penguins Spheniscus demersus annually. This may have contributed to the penguin colony not increasing in spite of food becoming more available in its vicinity during an eastward shift of epipelagic fish resources off South Africa at the start of the twenty-first century. Adult penguins were most susceptible to mortality in the breeding season as they returned to feed chicks in the evening. Seals also killed substantial numbers (3–9%) of Cape Cormorant Phalacrocorax capensis fledglings as they left the island, mostly in the morning. The losses of Cape Cormorant fledglings accounted for about 95% of the overall mortality of seabirds attributable to seals. Mortality inflicted by seals on adults of four species of cormorant was negligible and no predation by seals on gulls and terns was observed.

Surface behaviour of bait-attracted white sharks at Dyer Island (South Africa)

Six research expeditions were conducted at Dyer Island to observe the surface behaviour of white sharks in the presence of bait. Observations were made from a commercial cage-diving boat. We observed 140 white sharks that exhibited 9 different types of behaviour: parading, bait following, visual inspection, breach, tail slap, tail stand, spy hop, repetitive aerial gaping, and head-up vertical emerging. Breach and tail slap were most often performed by male sharks, and tail slap and tail stand were more often performed by mature animals. The ethogram consisted of an average of 20 behavioural units, with a significant transitional pattern from bait follow to parading and from parading to bait follow. The individual surface behaviour of white sharks is not a simple stimulus–response reflex, but rather a complex tactical situation in which animals show plastic responses. Many aspects of white shark surface behaviour resemble those of other predator species that spend time at the surface.

Natural and human-induced predation on Cape Cormorants at Dyer Island

SummaryTo develop conservation strategies for vulnerable seabird species that need attention, it is important to know which factors influence their breeding productivity. Predation of eggs and chicks can have large influences on seabird reproduction, especially when human disturbance facilitates predation. On Dyer Island, Kelp GullsLarus dominicanusprey on Cape CormorantPhalacrocorax capensiseggs and chicks, whereas Cape fur sealsArctocephalus pusillus pusillusprey on Cape Cormorant fledglings in the waters surrounding the island. Kelp Gulls were estimated to predate 3.8% of the total number of Cape Cormorant eggs and 2.0% of the chicks on the island. These percentages can be expressed as a loss of 4.8% of Cape Cormorant fledglings, which is low compared to the estimated 24.3% mortality of Cape Cormorant fledglings by Cape fur seal predation. Human disturbance facilitated Kelp Gull egg and chick predation and increased the mobbing of cormorant fledglings by Kelp Gulls. Cormorant egg predation by gulls was more frequently reported in the late afternoon. Seal predation was more abundant at the northern side of the island compared to the southern side, was recorded more frequently in the morning, and increased through the breeding season. The altered abundance and distribution of prey, the availability of suitable breeding habitat and mortality from avian cholera, have also influenced the Cape Cormorant’s population size. Hence, the possibility that Cape Cormorants may be locked in a predator-pit, where seals and gulls prevent the population from increasing in size, needs further attention.

Social interactions among bait-attracted white sharks at Dyer Island (South Africa)

The goal of this preliminary research was to provide an ethogram of social interactions among white sharks at Dyer Island's Nature Reserve (Gansbaai, South Africa) in the presence of bait. Eighty-three social interactions were observed, both from a boat and from a cage. Seven different interaction types were recognized: swim by; follow give way; follow; parallel swimming; give way; splash fights; and piggybacking. The preliminary data analysis did not reveal any significant relationship between the behavioural patterns and the sex of the sharks, but a strong correlation between behavioural patterns and the size of the animals was found. The observed displays were more often performed by animals of the same length than by animals of different size.

Encounters between white sharks and Cape fur seals in a shallow channel

This communication presents observations of predatory and non-predatory encounters between white sharks and Cape fur seals in a shallow (3–6 m) channel between Geyser Rock and Dyer Island, South Africa. Within the channel Cape fur seals raft extensively for thermo-regulatory purposes, to play, or due to terrestrial competition for space. The channel's physical environment effectively limits a white shark's approach orientation to the horizontal plane, thus inhibiting it to effectively utilize depth, and associated stealth, to capture pinnipeds. In the absence of effective camouflage, sharks may patrol this area in search of unaware, incapacitated or dead seals. Here, predator mobbing is a behavioural strategy adopted by Cape fur seals to lower predation risk. Specific benefits of mobbing may include: (a) perception advertisement to sharks; (b) intra-specific communication of a shark's locality; (c) driving the shark away from the area; (d) increased vigilance; (e) advertisement of a mobber's good health to a shark; and (f) possibly learning about a predator's behavioural capabilities by inexperienced prey. Mobbing expression is further promoted by the channel's shallow nature which enhances a seal's ability to visually detect the shark, and therefore makes it easier for a seal to evade it (reducing immediate predation risk). This environment thus promotes the widespread use of mobbing amongst Cape fur seals when confronted with a patrolling white shark.

Divergent trends in bank cormorants Phalacrocorax neglectus breeding in South Africa's Western Cape consistent with a distributional shift of rock lobsters Jasus lalandii

In South Africa's Western Cape Province, numbers of bank cormorants Phalacrocorax neglectus breeding at 11 localities decreased from above 500 pairs during 1978–1987 to 350 pairs from 1995 to 2006. The most northern colony (Lambert's Bay) was extinct by 1999 and there were substantial decreases at the two largest colonies (Malgas and Dassen Islands), also north of Cape Town. There was long-term stability at three smaller colonies near Saldanha and fluctuating growth at Robben Island near Cape Town, where the colony was twice affected by oil spills. Between Cape Point and Cape Hangklip, two colonies were monitored and their numbers increased. Farther east, numbers at the Dyer Island group showed a slight decrease. These trends are consistent with a reduced abundance of West Coast rock lobsters Jasus lalandii in the north and an expansion of this resource to the east, but not yet as far as Dyer Island. Rock lobsters were the most important item in the diet of bank cormorants to the north of Cape Town.

Management of avian cholera Pasteurella multocida outbreaks on Dyer Island, South Africa, 2002–2005

In 2002 there was a widespread epizootic involving seabirds on five of the offshore islands of the Western Cape, South Africa. Since then, avian cholera Pasteurella multocida outbreaks have been occurring annually on one of these islands, Dyer Island. This paper reports on the three subsequent summers, 2003/04, 2004/05 and 2005/06, during which further avian cholera outbreaks were recorded. It focuses on the outbreak in 2004/05, which was the largest in extent and the most closely monitored. The mortalities during 2005/06 were not as extensive as expected. The management measures used to bring these outbreaks under control are described. Removal of all the carcasses from the entire island in one day is important in reducing mortality. Management intervention is required to reduce the negative impacts of disturbance due to kelp gull Lasus dominicanus predation on other breeding seabirds, primarily the African penguin Spheniscus demersus, during the carcass collection process.

Regionally coherent trends in colonies of African penguins Spheniscus demersus in the Western Cape, South Africa, 1987–2005

From 1987 to 2005, numbers of African penguins Spheniscus demersus breeding in South Africa's Western Cape Province increased by about 50%. Numbers decreased at the four northernmost colonies in the region: Lambert's Bay and the three colonies in Saldanha Bay, although at Jutten Island the decrease is inferred from an estimate for 1987, derived from interpolation. Numbers also decreased at Geyser Rock and Dyer Island on the South Coast. At five colonies between Saldanha Bay and Dyer Island there were large increases. At a sixth colony in that region, Seal Island, where Cape fur seals Arctocephalus pusillus pusillus limit breeding space, numbers remained stable. At two colonies that were initiated in the early 1980s, Robben Island and Boulders, increases were initially rapid (>20% per annum) and matched growth of the South African stock of sardine Sardinops sagax. Strong growth at Dassen and Vondeling islands, between Robben Island and Saldanha Bay, was observed from about 1996–2002, when there was a large increase in the biomass of pelagic fish off South Africa. Increases at colonies between Saldanha Bay and Boulders slowed after 2002, whereas the colony at Dyer Island stabilised at that time. In 2003, a new colony was initiated east of Dyer Island at De Hoop Nature Reserve. These latter trends followed an eastward shift in the distribution of sardine. Small penguin colonies may act as foci for growth in a period when the distribution of prey is changing. Hence, it is important that they be maintained, especially those that, if lost, would increase the isolation of regional populations. Some of the small colonies are less susceptible to oil spills than colonies in the proximity of harbours, and for that reason also are important.