Grouper populations face an uncertain future as new research shows that fishing that targets their spawning grounds is causing males to be repeatedly chased away from their territories during their brief mating encounters.
By fleeing for safety, individuals lose valuable time to attract attention and court female fish.
A study led by scientists at Lancaster University and published today in Biological lettersshows that the impact of fishing on grouper spawning grounds goes beyond the fish that are caught, causing widespread behavioral change among those left behind.
These changes affect reproductive capacity. If mating is disrupted, fewer offspring may be created, which could have potential long-term consequences for the survival of their population.
Squaretail groupers only gather at very specific spawning staging areas a handful of times a year for a few days – during the new and full moons between December and March.
Male groupers arrive at staging areas a few days before new or full moons and establish mating areas to attract females, which they would normally vigorously defend against rivals. Females arrive a day before the new moon and the fish will attempt to mate and mate in sync with the moon cycles before the females leave.
These gatherings at spawning grounds may be the only opportunity for these populations to reproduce. But fisheries, especially spearfishing and hook and line fishing, which involve free diving, have begun to intensively target and exploit some of these gatherings to ensure catches while diverting the fish.
India’s Lakshadweep Archipelago, located north of the Maldives, has restricted access to protect its people’s culture and ecology. However, with the opening of new fishing markets, fishing at some grouper spawning grounds around the archipelago has increased significantly over the past five years.
Aggregation density at the studied fished site has declined by 70% in ten years, with the fishery targeting grouper year-round.
The marine scientists behind the study had studied some of these sites more than a decade ago and first noticed a change in fish behavior when they tried to replicate aspects of their previous research.
“We found that we couldn’t film or study the fish at the collection sites as easily as in the past,” said Dr Rucha Karkarey of Lancaster University and lead author of the study. “We couldn’t get that close to them, they swam away as if we were predators. It was clear to us that there was a difference in their behavior and we realized that the main thing that had changed in the ten years since our previous study was the start of fishing at the site.”
The researchers compared the behavior of male grouper at fished and unfished spawning grounds, and compared it with data collected from their previous spawning site surveys.
They analyzed the fear responses of fish by simulating a threat. Divers swam horizontally toward spawning groupers to see how close they could get to a fish before it fled, as well as the time it spent defending territory and courtship when there was no immediate threat.
Although males who had already interacted with females at the fished site faced greater risks (of potentially being caught by fishermen) during courtship, fled later and returned earlier, the study found that single males at fished sites were twice as likely to to flee and flee twice as often. and a half times longer to return to areas than to fish in unfished locations.
Less than half of the single males returned to their territories at the fished site, while two-thirds of unpaired males at the unfished site returned to their territories.
Although early escape can ensure the survival of individuals, escape too early can lead to lost mating opportunities.
Squaretail groupers are a long-lived species and can live up to twelve years, so researchers think the behavioral change may be the result of fish fishing.
“Our findings suggest that the stress caused by fishing makes the fish more nervous and wary,” says Dr. Karkarey. “This in turn reduces the chances for single male groupers to find a mate and reproduce, as they are more vigilant and compromise territorial defense by fleeing. It also means that mated males, who are fitter and more successful at reproducing, are the ones being fished, which could impact the fitness of this population in the future.
“If a spawning aggregation event only lasts a short period of time, every hour counts.”
The researchers also found that male groupers at fished locations also spent significantly less time and energy aggressively defending territories against rivals.
The fish at the fished location traded aggressively and defended their territory before fleeing. Males engaged in aggressive behavior almost ten times longer at the unfished site compared to their counterparts at the unfished site.
Researchers think this may be due to a number of factors, including females becoming less choosy in high-risk situations (through fishing), or bolder fish being removed from the population through being caught by fishermen, although this is not has been investigated.
“Reduced aggression could have significant evolutionary implications in mating aggregations, particularly through a relaxation of sexual selection, which could impact mate choice, population fitness and long-term resilience,” said co-author Dr. Sally Keith from Lancaster University.
Dr. Idrees Babu from the Department of Science and Technology, UT Lakshadweep, said, “This study is the first of its kind in these atoll reef islands, and the findings could play an important role in the management of fisheries resources in this archipelago.”
The authors of the article are Dr Rucha Karkarey, Dr Lisa Bostrom Einarsson, Dr Sally Keith and Professor Nicholas Graham from Lancaster University and local field officers and collaborators Ibrahim MK from Mukkrikakuddi House, UT Lakshadweep; Mohammed Nowshad and Abdul Riyas of the Research and Environmental Education Foundation, UT Lakshadweep; and Dr. Idrees Babu from the Department of Science and Technology UT Lakshadweep.
