Bull Shark Recruitment Into The St Lucia Estuary And The First Observation Of A Bull Shark Preyed On By A Nile Crocodile
The St Lucia Estuary, South Africa, is a UNESCO World Heritage Site, and is the largest estuarine lake system in Africa. Estuaries provide important nursery habitats for bull sharks (Carcharhinus leucas), and St Lucia is no exception. Due to prolonged drought conditions, the St Lucia Estuary remained closed to the Indian Ocean since 2002, but in January 2021 the mouth was opened, and bull shark pups immediately started recruiting into the estuary.
About St Lucia Estuary
The St Lucia Estuary is situated within the iSimangaliso Wetland Park World Heritage Site. The Park consists of various conservation areas, the oldest being St Lucia Game Reserve established in 1895. The Park is South Africa’s first world heritage site and is recognized for its superlative beauty, ecosystem services, biodiversity and ecological processes. The Park covers 332 000 ha and includes a coastline of 142 miles. The Estuary itself (also known as Lake St Lucia) is 50 miles long and 14 miles across at its widest point. The estuary is home to hippopotamus and Nile crocodile, as well as wetland plant species.
The estuary became closed off from the Indian Ocean in 2002, due to upstream water extraction for irrigation. This increased levels of salinity in the river and resulted in low lake levels, causing most of the marine-dependent fish species to die.
On 6 January 2021, the sandy berm separating St Lucia Estuary from the Indian Ocean was artificially breached. This reconnected the historic system, in the hopes that estuarine ecological functioning would be reestablished.
The bull shark (Carcharhinus leucas) is a predatory shark species that lives predominantly in coastal waters, however they have the ability to move into freshwater. Bull sharks can grow to a length of 11 feet. They are highly aggressive predators, and due to their ability to move into freshwater, they are responsible for fatally biting more people than any other species.
Bull sharks do not only briefly swim into freshwater – they are able to travel far upriver. There is even a semi-permanent population of bull sharks in Lake Nicaragua. Some bull sharks birth their pups in freshwater habitats close to the coast, and estuaries are important juvenile bull shark nurseries. Bull sharks prefer estuaries for their nursery due to a reduced risk of predation and competition for food sources.
Bull Sharks in St Lucia Estuary
Historically bull sharks pupped at the mouth of the St Lucia Estuary, with juveniles and adolescents observed farther upstream in the main lake. Since the mouth closure in 2002, and reduced lake levels resulting in hypersalinity, it was assumed that most of the bull sharks died in the following years.
The St Lucia Estuary is also home to the largest population of Nile crocodiles (Crocodylus niloticus) in South Africa. They are keystone predators in the system, and feed on mullet, catfish and juvenile bull sharks.
A paper published in the African Journal of Marine Science studied bull sharks in the St Lucia Estuary following the opening of the mouth in January 2021. The study aimed to document the recruitment of bull sharks into the estuary to highlight the importance of the estuary as a nursery habitat; to observe Nile crocodiles preying on bull sharks; and to collate a list of bull shark occurrences in South African Estuaries.
Observations from Opening the Estuary Mouth
Within a day of the mouth being artificially opened to the Indian Ocean, Nile crocodiles were observed moving in and out of the mouth, foraging on prey washing out into the sea. Most of the prey were sharptooth catfish. Similarly, adult bull sharks were observed at the mouth since the day it opened, and the presence of adult bull sharks at the mouth and along the coastline was observed sporadically for three months after opening.
On 16 January 2021, ten days after opening the mouth, the authors of the paper witnessed something amazing. An adult Nile crocodile was photographed with a neonate bull shark in its jaws. When first spotted, the juvenile shark was still alive and moving, but after a few bites the shark went limp and the crocodile swallowed it head-first.
The authors of the paper were able to tag a total of nine neonate bull sharks with acoustic tags. It was assumed that this cohort of juvenile sharks were pupped after the opening of the estuary mouth.
During the almost two decades that the St Lucia Estuary was closed off from the Indian Ocean, the system underwent many changes and it was unclear whether any bull sharks survived. The observations made in the study suggests that bull sharks were able to almost immediately respond to the mouth opening and pupped within the first week of the reconnection between the estuary and the sea.
This rapid response by bull sharks highlights the resilience and adaptability of bull sharks, as well as the importance of St Lucia and other estuarine environments for nursery habitats for bull sharks. Bull sharks are known to exhibit natal philopatry (remaining in, or returning to their place of birth), so it is possible that adult sharks born at St Lucia returned to their natal estuary to pup, guided by environmental cues.
Although there are reports of crocodiles preying on sharks in Australia, it appears to be a rare occurrence. In South Africa there are some reports of Nile crocodiles feeding on bull sharks at St Lucia, however this study produced the first photographic evidence. Although bull sharks are not an important prey item to Nile crocodiles, a single neonate shark weighing 2.5 kg may constitute a large portion of a crocodile’s daily food intake and could be an important opportunistic food source.
It still remains unclear how the St Lucia estuarine ecosystem will respond over time to the artificial breaching of the sand berm. The study provides evidence to suggest that recruitment of a top predator into the estuary is rapid. The amazing observations by the authors, and the rich biodiversity of the estuary, warrant St Lucia and iSimangaliso Wetland Park its World Heritage status, and the authors hope that the results of their study further encourage conservation efforts into maintaining a healthy ecosystem.
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