Fishing is causing frightened fish to flee when they should flirt

Populations of squaretail grouper face an uncertain future as new research shows fishing that targets their spawning sites is causing males to be repeatedly scared away from their territories during their short mating meet-ups.

By fleeing for safety, individuals are losing valuable time to catch the eye and court female fish.

A study, led by scientists at Lancaster University and published today in Biology Letters, shows that the impacts of fishing that targets squaretail grouper spawning sites goes beyond those fish that are caught, causing widespread behavioural change in those left behind.

These changes impact ability to reproduce. With disrupted mating, fewer offspring might be created, setting up potential long-term consequences for the survival of their population.

Squaretail grouper gather at very specific spawning aggregation sites for a few days only a handful of times a year – during the new and full moons between December and March.

Male groupers arrive at aggregation sites a few days before new or full moons and establish mating territories to attract females, which they would normally defend vigorously from rivals. Females arrive a day before the new moon and fish will attempt to pair and spawn in synchrony with the moon’s cycles before females leave.  

These gatherings at the spawning sites can be the only opportunities for these populations to reproduce. But, fisheries, particular spearfishing, and hook and line fishing, which involve free-diving, have begun targeting and exploiting some of these gatherings intensively to guarantee catches while fish are distracted.

India’s Lakshadweep archipelago, situated north of the Maldives, has restricted access to protect the culture of its people and ecology. However, the opening of new fisheries markets has seen a sharp increase in fishing at some grouper spawning sites around the archipelago over the last five years.

Aggregation densities at the studied fished site has plummeted by 70% in a decade with fisheries targeting groupers all year round.

The marine scientists behind the research had studied some of these sites more than a decade ago and first noticed a change in fish behaviour when attempting to replicate aspects of their earlier study.

“We discovered that we couldn’t film or study the fish as easily at the aggregation sites as we could in the past,” said Dr Rucha Karkarey of Lancaster University and lead author of the research. “We couldn’t get as close to them, they would swim away as though we were predators. It was obvious to us that there was a difference in their behaviour and we realised that the major thing that had changed in the decade since our previous study was the start of fishing at the site.”

The researchers compared male squaretail grouper behaviour at fished and unfished spawning sites, as well as comparing with data gathered from their earlier studies of spawning sites.

They analysed fish fear responses, by simulating a threat. Divers would swim horizontally towards spawning groupers to see how close they could get to a fish before it would flee, as well as the time it spent defending territory and engaged in courtship in the absence of a direct threat.

Although males who had already partnered with females at the fished site took greater risks (from potentially being caught by fishers) during courtship, fleeing later and returning earlier, the study found that single males at fished sites were twice as likely to flee and took two and a half times longer to return to territories than fish at unfished sites.

Fewer 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.

While escaping early can ensure survival for individuals, escaping too early may mean lost mating opportunities.

Squaretail groupers are a long-lived species, living up to 12 years, so researchers believe the behaviour change may result from fish individuals experiencing fishing.

“Our findings suggest that the stresses caused by fishing are making the fish more nervous and on guard,” said Dr Karkarey. “In turn this is reducing the opportunities for single male groupers to find a mate and reproduce because they are being more vigilant and compromising territorial defence by fleeing. It also means that paired males, that are fitter and more successful at reproducing, are the ones being fished, which can have a consequence for the fitness of this population in the future.

“If a spawning aggregation event only lasts for a short period of time then every hour counts.”

The researchers also found that male groupers at fished sites also spent significantly less time and  energy aggressively defending territories from rivals.

The fish at the fished site traded aggressively defending their territories for fleeing. Males spent nearly ten times longer engaged in aggressive behaviour at the unfished site compared to their counterparts at the unfished site.

Researchers believe this could be due to a number of factors including females becoming less choosy in high risk (from fishing) situations, or bolder fish removed from the population by being caught by fishers, though this was not studied.

“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,” said co-author Dr Sally Keith of Lancaster University.

Dr Idrees Babu from the Department of Scient and Technology, UT Lakshadweep, said: “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.”

The findings are detailed in the paper ‘Do risk-prone behaviours compromise reproduction and increase vulnerability of fish aggregations exposed to fishing?’ which is published in the journal Biology Letters.

The paper’s authors are Dr Rucha Karkarey, Dr Lisa Bostrom Einarsson, Dr Sally Keith and Professor Nicholas Graham or Lancaster University and local field collaborators and associates Ibrahim MK of Mukkrikakuddi House, U.T Lakshadweep; Mohammed Nowshad and Abdul Riyas of the Research and Environmental Education Foundation, U.T Lakshadweep; and Dr Idrees Babu of the Department of Science and Technology U.T Lakshadweep.