By Stephen Beech
Sharks vary their movements by season, reveals new research.
Activity of the apex predators around a reef off South Africa was monitored using thousands of sightings by "citizen scientist" scuba divers.
The findings, published in the journal PLOS One, showed that shark sightings displayed "distinct" seasonal patterns, and mostly showed positive trends over six years of records.
The study was conducted by researchers at the University of Sussex and Aquaplanet Dive Centre in Margate, South Africa.
Lead author Dr. George Balchin, of the University of Sussex, said: "Many sharks move through different habitats as they follow food or search for mates.
"Since they are major predators, they change the shape of the ecosystems they visit.
"Examining these movements is key to understanding the health of ocean habitats as well as the impacts of human activity, but it is notoriously difficult to get the long-term data needed."
Dr. Balchin and his colleagues compiled citizen science data on shark sightings from professional scuba divers working in diving tourism.
The data included more than 5,300 sightings of six shark species - including hammerhead, tiger and bull sharks - between 2013 and 2019 in the Protea Banks reef.
Dr Balchin said: "The data revealed that all six species came and went with the seasons, though they followed their own schedules, with hammerhead sightings more frequent in the spring and tiger sharks more abundant in autumn, for example."
The researchers also identified changes from year to year.
Dr. Balchin said: "During the studied period, for instance, hammerhead shark, blacktip shark and ragged-tooth shark sightings increased; tiger shark sightings were stable, and bull shark and dusky shark sightings decreased.
"In some cases, these changes are likely to reflect broad-scale changes in the behavior or population of these species - for example, population growth in those shark species which now have protected status.
"This information regarding which sharks are present at which times is very valuable for planning fishing guidelines or conservation efforts throughout the year.
"Furthermore, this study demonstrates the potential of diving tourism as a source for citizen science data."
The research team says that future studies might expand their reach to new areas, as well as account for potential biases in the study's data by involving more varied habitats or types of dives.
Dr. Balchin added: "Our study provides a really good example of how shark diving operators and scientists, working together, can provide the important long-term information on shark population dynamics that's needed for shark conservation."