Aggregating groupers

Team members: Rucha Karkarey & Rohan Arthur

Project timeline: 2014 to 2020

Every year, thousands of coral groupers gather at northern reefs in the Lakshadweep over the spring new moon to spawn. Our studies are documenting one of the largest aggregations of the square tail grouper in the tropics to understand this unique behaviour and highlight emerging threats to the population

It was in the course of an archipelago-wide survey of the Lakshadweep that we first chanced upon an aggregation of the square tail grouper (Plectropomus areolatus) at one of the northernmost atolls of the Lakshadweep. Across a small current-swept reef area were, by our estimation, several hundreds of groupers defending tiny patches of the benthos, while close by, in midwater, several smaller and darker individuals of the same species were gathered in a large shoal. We were intrigued enough to take a closer look.  Our studies over the last few years are uncovering a unique spawning aggregation of this species with some of the highest numbers reported anywhere in the tropics. We are also documenting novel mating behaviours for the species which we believe may be seen only in the most pristine and unfished aggregations.

A few days before the new moon, male groupers stream to the reef in large numbers and establish small territories.  There is fierce competition for the choicest locations and males jostle and display, chase and snap, and even occasionally engage in serious jaw locking combat to appropriate the best spaces. The females arrive in large shoals over the new moon and what follows is a complex series of mating rituals that our research is still busy unpacking as males and females attempt to maximize their reproductive success.  Two days after the new moon and the groupers disperse back to their natal reefs.

In early 2014, we observed an increase in reef fishing in Lakshadweep. Worried by this trend, and the threat it could pose to the squaretail grouper aggregation, we worked closely with the local community, the Lakshadweep Fisheries Department and Lakshadweep Administration to set up a seasonal fishing closure which restricted commercial fishing of the spawning site during critical spawning periods.

Last year (four years after the seasonal fishing closures were initiated), we evaluated the effectiveness of the closure. We found that there is high compliance of the seasonal fishing closure and no fishing activity was reported at the site during the aggregation periods since 2014. 

However, despite high compliance, there is a 60% decline in the overall density of the aggregating population since we first documented it in 2012.  In addition to this dramatic population decline, we observed that the novel mating tactic of ‘shoal spawning’ was absent in the aggregation since 2015. 

Our work in fisheries indicates that this might be due to an overall increase in reef fishing pressure in general, although there is no fishing on the aggregation. This is showing us that in addition to seasonal fishing closures, extra steps need to be taken to regulate reef fisheries during non-spawning periods to ensure the behavioural and numerical integrity of the unique squaretail grouper populations is maintained.