A new long-term study in Endangered Species Research presents the most detailed picture so far of the zebra sharks off the coast of Mosambik. The team analyzed data from 14 years: Between 2011 and 2025, a total of 631 sightings were recorded during 9,219 underwater surveys and 164 animals were distinguished based on their individual spot patterns.
The results do not focus on an evenly distributed population. A small cluster of reefs north of Tofo stood out as a seasonal hotspot. Individual sharks returned there over several years. At the same time, the team documented three courtship interactions, the first scientific evidence of this behavior in the species in the western Indian Ocean.
164 animals with unmistakable spot patterns
As adult animals, zebra sharks have a pattern of dark spots. Similar to fingerprints, these patterns can be recognized in photos. This allows an animal to be tracked without catching or marking it. For the study, the researchers combined standardized surveys with photo identifications over a long observation period.
Of the 164 identified sharks, 35 percent were seen at least once again. 27 percent even appeared in the records over multiple years. This does not prove a permanent presence at a single reef, but it shows that a significant part of the population repeatedly returns to the studied coast.
The recorded group was clearly female-dominated: around 70 percent of the animals were females. Adult and not yet fully grown sharks dominated; the average total length was 1.66 meters. Why the sex ratio was so heavily skewed cannot be conclusively explained from the abstract alone. Possible reasons include sex-specific space use, seasonal migrations, or differences in capture probability.
A few reefs account for a large part of the sightings
Sightings accumulated at a small reef cluster north of Tofo. This makes these sites particularly relevant for management: If an endangered species regularly uses only a few suitable habitats, local disturbances can affect a disproportionately large portion of the observed population.
For this reef area, an open Jolly-Seber population model estimated a so-called superpopulation of around 88 individuals. However, the 95-percent confidence interval ranged from 50 to 154 animals. The wide range is part of the result: it shows that a modeled population size is not an exact count.
The model resulted in a high apparent survival probability of 0.85 and a low detection probability of 0.17. “Apparent” means that an animal no longer seen may have either died or permanently emigrated from the studied area. The low detection probability also explains why many surveys are necessary to identify robust trends.
Zebra sharks appear especially in the southern summer
The observations followed a pronounced seasonal pattern and reached their peak in the austral summer. At the same time, soil temperature and underwater visibility changed. The study describes a joint seasonal occurrence of these factors without deriving a single cause from mere correlation.
For animals in the Indian Ocean, temperature, food, reproduction, and local current conditions can interact. A long-term dataset is therefore more valuable than individual sighting peaks: it can show whether seasonal windows are shifting, reefs are used less frequently, or certain individuals return over the years.
At less than five meters distance, escape reactions occur
More than half of the recorded encounters were not completely neutral: in 55 percent of the sightings, the sharks exhibited avoidance behavior towards divers. The likelihood of such a reaction increased significantly when the nearest diver approached within less than five meters.
The result does not make five meters a universal legal limit. However, it provides a concrete, locally gathered guideline for responsible encounters. Distance, calm movements, and a clear escape route reduce the pressure on an animal – especially in places that are regularly used by the same sharks.
For dive bases and protected area management, a simple principle applies: do not surround resting animals with groups, limit approaches, and train guides to recognize early avoidance reactions. At the same time, standardized observation protocols from dive tourism could provide additional data, provided the quality and photo assignment are checked.
Three rare insights into courtship
The researchers documented three courtship interactions. Such observations are rare in wild sharks because they can be brief in time, spatially limited, or occur in hard-to-reach habitats. For Stegostoma tigrinum these are the first described courtship observations from the western Indian Ocean.
Three events are not enough to fully determine reproductive timing, mating success, or the function of a particular reef. However, together with the seasonal concentration and the high proportion of adult animals, they provide a clear research question: Do zebra sharks use these reefs not only as a habitat but also for reproduction?
A critically endangered subpopulation
The species is under pressure in large parts of its range due to fishing and the loss of suitable habitats. The study classifies the subpopulation from the western Indian Ocean to Southeast Asia as endangered. Especially for animals with a low reproductive rate, local losses can only be compensated slowly.
The Marine Megafauna Foundation has been researching zebra sharks in the Inhambane Province for years using photo ID, telemetry, and tissue samples. The goal is to understand seasonal use, migrations, and genetic connectivity, and from this, to derive priority protected areas. The new study creates a quantitative baseline for this purpose.
What protection planning can derive from the study
The most important finding is the spatial concentration. A protection approach does not have to treat the entire coast equally, but can initially prioritize the regularly used reefs, the south-summer season, and disturbances from nearby approaches. Additionally, data on fishing in and between the reef areas is needed.
The numbers should still not be read as a complete stock assessment. The dataset comes from observable reef habitats; animals outside these areas or outside the surveys are more likely to remain undetected. The model range of 50 to 154 individuals also shows how important continuous monitoring is.
This is precisely where the strength of the work lies: it replaces spectacular individual sightings with a 14-year benchmark. In doing so, reefs north of Tofo become visible as repeatedly used habitats – and diver distance, season, and site fidelity become measurable values against which future protection can be assessed.
