Non-invasive methods characterise the world's largest tiger shark aggregation in Fuvahmulah, Maldives

Abstract

Tiger sharks are apex predators with a circumglobal tropical and warm-temperate distribution, with a general lack of population data for the central Indian Ocean. In Fuvahmulah, Maldives, tiger sharks display frequent use of the harbour area, attracted by discarded fish waste. Here, we document the population structure, residency, and reproductive characteristics of the world's largest known tiger shark aggregation in a geographically-restricted area. Using non-invasive methods, photo identification and laser photogrammetry, we identified 239 individual tiger sharks over a 7-year study period. The aggregation was female-dominated (84.5%), with both large juveniles and adults present. Adult females were resighted over the entire study period displaying strong inter- and intra-annual site fidelity. Modelled residency using maximum likelihood methods suggests they spent 60.7 ± S.E. 7.5 days in Fuvahmulah, with a larger aggregation size, shorter residence periods and longer absence periods compared to juvenile females. Prolonged abdominal distensions of adult females indicate they likely stay near Fuvahmulah during gestation and reproduce biennially. Fuvahmulah seems to provide suitable conditions for gestation given the year-round provision of food and warm waters, exhibited by strong site fidelity and temporal residency. Our results show indications of a thriving population within the confines of protected waters.

Keywords: Gestation; LIR; Marine megafauna; Maximum-likelihood models; Photo ID; Site fidelity.

Fig. 1

Location of Fuvahmulah within the Maldives Archipelago in the Indian Ocean. The dive sites, where tiger sharks are frequently spotted and where most footage originates from, are marked in the map (Map was created using QGIS 3.28.1-Firenze, URL: https://qgis.org/download/).

Fig. 2

(a) Countershading delineation in six different tiger sharks. The six different individuals (F-001, F-002, F-018, F-009, F-011, F-025) display intraspecific variation of the countershading delineation anterior to the pectoral fins. This feature was most useful in differentiating the individuals. (b) Cumulative number of identifications of tiger sharks by sex per month visiting tiger harbour. The orange line indicates the cumulative number of dive surveys per month.

Fig. 3

Total length (TL) of visual size estimates from 213 sharks incremented by 0.5 m. All females 3.5 m and larger were considered adults. All males 3.0 m and larger were adults.

Fig. 4

(a) Standardised width of presumably pregnant sharks over time. Day 0 indicates the last measurement before the sharks’ absence period, where we presume parturition may take place. Sharks scored 1 or 0 were visually assessed as ‘pregnant’ or ‘not pregnant’, while ‘not scored’ refers to a shark’s appearance, where we did not infer pregnancies based on their visual appearance (see Methods for more details). A linear regression model, including standard error, is fitted to the data until Day 0. (b) An example of one individual’s presumed pregnancy (F-049). Number in brackets provides a corresponding day value to a). The standardised width of this shark increased until day 0 followed by a period of absence for 93d. Upon its return, the standardised width of this shark had significantly declined.

Fig. 5

Lagged Identification Rates (LIR, mean ± S.E.) for all tiger sharks (top panel) and for juvenile (red) and adult (blue) female tiger sharks visiting Fuvahmulah. Model H (pale lines) is represented in all cases including emigration, reimmigration and mortality as model presets.