St. Helena Whale Shark Project

An aggregation of adults

Photo-identification, based on each shark’s unique spot pattern, recorded 277 individual whale sharks at St. Helena between April 2013 and May 2019. Of the 257 whose sex could be determined, 53% were male and 47% female, close to an even ratio. That makes the aggregation unusual in two respects: most known whale shark aggregations are dominated by young sharks, and most are dominated by males, at ratios from 3:1 to 10:1 or more.

The sharks were estimated at 5 to 12 m (16 to 39 ft) long, averaging 8.1 m (27 ft). Of the 108 males whose claspers (the paired male reproductive organs) were assessed, 86% were mature. About half of the females assessed were judged to be mature, using a conservative threshold of 9 m (30 ft) and excluding sharks in the uncertain 8–9 m (26–30 ft) range. Visual estimates tend to underestimate the length of large sharks, however, and laser measurements of 15 sharks in 2016 suggested that the sharks may, on average, be larger than they appear.

Between one and 40 whale sharks were seen around the island on any given day, with more than 30 feeding together on a few occasions. During one such event in January 2015, plankton samples contained large numbers of fish eggs, associated with nearby schools of skipjack tuna that were thought to be spawning at the surface. At most other times, however, the sharks showed no obvious feeding behavior and swam calmly 1 to 5 m (3 to 16 ft) below the surface.

Residency and the season

Whale sharks begin to arrive around St. Helena in November and December, with the aggregation peaking in January and declining by May. January to March accounted for 85.5% of all encounters in the sightings database.

A lagged identification rate model, which estimates the probability of seeing the same shark again after a given interval, put the number of sharks present at any one time at about 102, each remaining for an average of 19 days. The probability of resighting a shark fell to zero after about 75 days. The abundance estimate was considerably higher than at coastal sites in the Indo-Pacific analyzed the same way, including Mafia Island in Tanzania (35 sharks). It was similar, however, to the 116 sharks estimated offshore in Qatar.

Most sharks were transient: 65% were photographed only once. However, 34 sharks were identified in more than one season, and three acoustically tagged sharks returned to St. Helena at the start of a new season. Acoustic receivers detected sharks around the whole island, with activity concentrated on its calmer, northwest-facing leeward side. Two receivers there, at Barn Cap and the wreck of the Darkdale, recorded most of the detections.

Protection and shipping

By 2016, new environmental regulations fully protected whale sharks (opens in a new tab) in the waters around St. Helena. The island was also included in two global studies of collision risk between whale sharks and large ships, both co-authored by MMF scientists. The first, published in 2022 (opens in a new tab), used satellite tracks from around the world and identified St. Helena as one of the Atlantic’s hotspots of whale shark space use.

The second, a 2024 study (opens in a new tab), gave St. Helena’s surrounding waters the lowest danger rank, a measure based on the relative number of large ships passing within 100 km (62 miles) of the sharks’ core habitat. Within that core habitat, the small area where the sharks concentrate, local vessel traffic was the greater concern, and passenger vessels posed the main collision risk. As a result, measures to reduce ship strikes in that small area would likely benefit whale sharks across the wider region.