Several times a year, squaretail groupers gather at spawning aggregation sites to mate, typically around the new and full moons between December and March. The males show up several days before to establish their territories, while the females arrive a day before the new moon. Then, both groups will attempt to pair and spawn in synchrony with the moon’s cycles.

For this species, these brief encounters are their only opportunities to reproduce.

The Lakshadweep archipelago in India is one of the spawning aggregation sites. Although it is restricted to protect its culture and ecology, the opening of new fisheries has led to increased fishing activities in the last five years. 

Fishing activities take place all year round. However, the fisheries take advantage of the squaretail grouper's predictable gatherings by increasing their fishing activities, particularly spearfishing and hook-and-line fishing, when the fish gather there. As a result, the aggregation density at the site dropped by 70 percent in just a decade.

Changes in fish behaviour

In the study, published in Biology Letters, researchers compared the behaviour of male squaretail grouper at fished and unfished spawning sites, alongside their data from their earlier studies. 

The differences were obvious from the start: the researchers could not film or study the fish as easily as they did in the past. “We couldn’t get as close to them, they would swim away as though we were predators. [...] we realised that the major thing that had changed in the decade since our previous study was the start of fishing at the site,” said lead author Dr Rucha Karkarey of Lancaster University.  

According to the press release, males that had already mated at the fished site took greater risks during courtship, by fleeing later and returning earlier. The single males at fished sites were twice as likely to flee and took two and a half times longer to return than those at unfished sites.

At the fished site, fewer than half of the single males returned to their territories, while at the unfished site, two-thirds of unpaired males returned. “Our findings suggest that the stresses caused by fishing are making the fish more nervous and on guard,” said Karkarey. “In turn, this is reducing the opportunities for single male groupers to find a mate and reproduce."

Decreased aggression

Due to the fishing threat, the males were more vigilant and nervous, and spent more less time and energy defending territories from rivals. Instead, they spent more time fleeing. At the unfished sites, the males engaged in aggressive behaviour nearly ten times longer.

Paired males, those that were fitter and more successful at reproducing, were the ones being fished; and this may have implications for the species in the future.

Co-author Dr Sally Keith of Lancaster University, explained, “Decreased aggression can hold significant evolutionary implications in mating aggregations, specifically through a relaxation in sexual selection, which could have implications for mate choice, population fitness and long-term resilience.”

“This study is the first of its kind in these atoll reef islands, and the findings can play an important role in fishery resource management in this archipelago," said Dr Idrees Babu from the Department of Science and Technology, Union Territory of Lakshadweep.