Essential waters: young bull sharks in Fiji’s largest riverine system

Written by Kerstin Glaus. Read the full article here.

The bull shark (Figure 1) is one of the few sharks that can freely swim between fresh, -and saltwater environments. Although the bull shark occurs pantropically, there is a large knowledge gap in their distribution and habitat use patterns of neonate, young-of-the-year (YOY) and juvenile bull sharks between regions. So far, such information has been gathered primarily in the northern Gulf of Mexico, in Florida and on the east coast of Australia. According to previous studies, young age classes of bull sharks occupy environmentally heterogeneous habitat and age-associated habitat transitions have been documented with YOY bull sharks occupying locations with lower mean salinities than juveniles, while sub-adults were more abundant in nearshore marine areas. Within coastal environments, juvenile bull sharks reportedly have an affinity for mesohaline areas. However, even within the same species, such habitat requirements can differ between and across regions and alter due to changing environments. To date, distribution and habitat use patterns of young bull sharks, are largely lacking from historically data-poor regions such as the South Pacific.

South Pacific Islands are among the least known or understood regions in the world. This lack of knowledge is highlighted by the fact that two aggregation sites for the young of several shark species have been discovered in just the last three years. Fiji is the South Pacific’s economic center. The archipelago consists of more than 330 islands, but the vast majority of the population inhabits the two main islands Viti Levu and Vanua Levu. Located off Viti Levu’s south coast, adult bull sharks can be studied year-round in the Shark Reef Marine Reserve (SRMR), the country’s first national marine park. Contrastingly, the exact location of essential habitats for young bull sharks and associated environmental parameters are either only preliminary investigated or virtually unknown. As a result, we have little data about essential fish habitats (EFH) for young age classes of bull sharks in Fiji.

Figure 1: A bull shark (Carcharhinus leucas) photographed in Fiji’s Shark Reef Marine Reserve. Copyright Valerie Taylor

To bridge this knowledge gap, a team of four researchers started a two-year vessel based survey in the Rewa River to the east, the Navua River to the south and the Sigatoka River to the west of Viti Levu (Figure 2). We aimed to confirm the occurrence of young bull sharks in several riverine systems, to determine their distribution and abundance in the rivers, and to collect environmental parameters at capture sites.

Our surveys usually started at low tide and typically lasted between two to six hours per day depending on weather conditions. We placed captured bull sharks in an on-board tank filled with river water (Figure 3). The following parameters were recorded for each individual caught: total straight length (Figure 4), umbilical scar condition (open, semi-healed, healed), and sex. Also, captured bull sharks were tagged with an internal Passive Integrated Transponder below the first dorsal fin for individual identification (Figure 3) prior to release (Figure 5,6). In addition, using a water quality meter, surface and bottom water temperature, dissolved oxygen and salinity were recorded at the respective sampling locations in the Rewa and Sigatoka Rivers at the beginning and end of each fishing survey.

Figure 2: The Rewa, Sigatoka and Navua Rivers in southern Viti Levu. Dashed inlets denote the stretches that were sampled
Figure 3: A neonate bull shark captured in the Rewa River, placed in an on-board tank filled with river water. The specimen is tagged with an internal Passive Integrated Transponder below the first dorsal fin for individual identification
Figure 4: A neonate bull shark captured in the Sigatoka River and placed on a board for total straight length measurement.
Figure 5, 6: Bull sharks are released after length measurement, sex determination, PIT tagging and assessment of the umbilical scar conditions. The whole procedure does not take more than 40-70 seconds.

After more than two years of extensive sampling, we captured 159 neonate and YOY bull sharks in the Rewa River and are now able to show that the Rewa River may be a hot-spot for the study of neonate bull sharks in Fiji. The study covers the first multi-year assessment of young bull shark’s occurrence and distribution across several rivers in a Pacific Island Country. Also, we examined and compared environmental conditions of two rivers, showing that the environmental profile with the highest bull shark abundance in the Rewa River typically was oligohaline and that young bull sharks are more likely to occur in the Rewa than in the Sigatoka River.

The poor knowledge of population trends in bull sharks in this unique upwelling region, together with habitat alterations and an increasing local demand for shark products for domestic consumption may lead to a potential decline of some age-classes of different elasmobranch species that may go unnoticed. Our data helps us to learn more about the bull shark’s distribution and abundance, information that is essential for studies of the species life-cycle. These new insights can provide a foundation for the urgently required assessments of essential shark habitats within the South Pacific.

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