South Africa is renowned for hosting one of the world’s largest populations of great white sharks (Carcharodon carcharias). However, concerns have arisen due to significant declines observed in areas where these sharks typically gather along the Western Cape province’s coastline. These gathering sites serve as hubs for feeding, social interactions, and rest among the sharks.
In Cape Town, expert “shark spotters” recorded over 300 great white shark sightings across eight beaches in 2011, but astonishingly, no sightings have been reported since 2019. This decline has raised alarm bells regarding the conservation status of this species.
Preserving great white sharks is of paramount importance, given their crucial role in marine ecosystems. As apex predators, they play a vital role in maintaining the health and equilibrium of marine food chains. Their presence significantly influences the behavior of other marine creatures, thereby shaping the entire ecosystem’s structure and stability.
To address this issue, our team embarked on an extensive study using data collected by scientists, tour operators, and shore anglers. We analyzed trends in abundance and shifts in distribution across the South African range of these sharks over time.
Our investigation unveiled notable disparities in abundance at primary gathering locations. While some sites experienced declines, others displayed increases or remained stable. Overall, a pattern of stability emerged, suggesting that white shark numbers have remained relatively constant since their protection was instituted in 1991.
Regarding the potential change in shark distribution among locations, we identified a shift in human-shark interactions from the Western Cape to the Eastern Cape. However, further research is essential to determine whether the sharks that disappeared from the Western Cape are the same individuals now documented along the Eastern Cape.
While the stable white shark population is reassuring, the shift in distribution brings its own set of challenges, including increased risks from fisheries and the need for effective beach management. Consequently, improved monitoring is imperative to track the whereabouts of these sharks.
Factors influencing shark movements are multifaceted. The most significant changes were recorded between 2015 and 2020. For example, at Seal Island in False Bay (Western Cape), shark sightings declined from 2.5 sightings per hour in 2005 to 0.6 in 2017. Conversely, in Algoa Bay, the number of sharks caught by shore anglers rose from six in 2013 to 59 in 2019.
Understanding these site-specific changes remains a complex challenge, as various factors come into play. White sharks have a long lifespan, with distinct behaviors at different life stages. Juveniles, especially males, tend to stick close to the coastline, while sub-adults and adults, particularly females, venture further offshore. Environmental variables such as water temperature, lunar phases, seasons, and food availability also influence their movement patterns. Long-term shifts in climate and ocean conditions may also play a role.
Complicating matters further, specialist killer whales with a preference for shark livers have been observed preying on white, sevengill, and bronze whaler sharks. While a direct cause-and-effect link is not firmly established, observations and tracking data suggest a flight response among white sharks following confirmed predation incidents.
A study published in 2022 highlighted a notable overlap between white sharks and longline and gillnet fisheries, covering 25% of South Africa’s Exclusive Economic Zone. The sharks spent 15% of their time exposed to these fisheries. The highest white shark catches occurred in KwaZulu-Natal, averaging around 32 per year. This underscores the importance of combining shark movement data with reliable catch records to assess risks to shark populations.
With changing shark movement patterns, there is a need to consider potential shifts in risk. Increased overlap between white sharks, shark nets, drumlines (baited hooks), and gillnets could elevate the likelihood of captures.
The evolving distribution of sharks may also impact beach safety. While shark bites remain a rare occurrence, the presence of sharks can influence human activities, especially in popular swimming and water sports areas. As distributions change, adjusting existing shark management strategies may be necessary, which could include increased signage, temporary beach closures, or enhanced education about shark behavior.
In response to changing patterns, shark spotter programs have expanded to additional locations, such as Plettenberg Bay, following fatal shark incidents in 2022. Anecdotal evidence suggests that surfers and divers encounter more white sharks than before in certain Eastern Cape areas.
Continued research is imperative to comprehend the factors driving shark movements and their effects on spatial and temporal distribution. Our study underscores the importance of standardizing data collection methods to generate reliable abundance statistics across their entire range. This challenge is not unique to South Africa, and international collaboration may be essential.
Additionally, we advocate for the establishment of long-term monitoring programs along the Eastern Cape and ongoing efforts to reduce shark mortality. The complex landscape of shark movements necessitates a proactive and adaptive approach to their conservation and management.
Source: The Conversation