Aggregating groupers

Documenting and protecting spawning aggregations in the Lakshadweep

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

  • The square tail grouper gathers in large numbers over the new moon to spawn

  • Male groupers establish small territories defending them fiercely against competitors

  • Aggregations are particularly prone to fishing pressure and can be very rapidly overharvested

New moon rendezvous

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.

Conserving a behaviour

Over the course of our investigations since 2011, we noticed a nascent but worrying commercial fishery that had begun targeting reef fish at these northern reefs. While not directly focused on groupers, it was clear that, if unchecked, this fishery could result in devastating consequences for the aggregation. Because they are so predictable in space and time, they aggregations are particularly vulnerable to overexploitation. An average square tail grouper can live several decades and its long life span makes it additionally vulnerable to fishing. 

When we presented our findings to the local community, they agreed that this unique behaviour needed their protection.  With their support we approached the  Lakshadweep Fisheries Department and Administration to garner their cooperation.  In what is a first in Indian waters, these actors came together to establish a floating reserve across the atoll.  This reserve restricts commercial fishing across the reef and extends a few days before and after every new moon during the spawning season.  Encouragingly, the commercial fishers that operate at these reefs were keen to comply with the terms of the reserve and to become active partners in the management of this species and its unique behaviour.

People

Funding

Publications

  • Journal Article
    2017
    Alternative reproductive tactics and inverse size-assortment in a high-density fish spawning aggregation
    BMC Ecology, 17:10, DOI 10.1186/s12898-017-0120-5
    Download

    PDF, 1.44 MB

    Mating successfully at high densities often requires species to employ unusual reproductive tactics. We report unique courtship behaviours in an un shed, high-density spawning aggregation of squaretail groupers (Plectropomus areolatus) that are potentially associated with alternative reproductive tactics (ARTs). Aggregating males are typically known to court females in small territories (pair courtship), which is often associated with a pair-spawning tactic. However, we also observed the largest males simultaneously courting several females in mid-water shoals – a unique, high-cost-high-benefit courtship tactic which appears to result in a novel school-spawning tactic. Counter-intuitively we observed an inverse size- assortment in individuals–large males courted smaller females and vice-a-versa, likely linked to different pay- offs with competitive ability and local mate density. These unique, high-density behaviours are threatened to be lost, with increasing commercial fishing pressures on the P. areolatus aggregation.

  • Popular Article
    2015
    On the line
    The Caravan Magazine, September 2015.

    A remote island's fight to save a remarkable grouper spawning aggregation.

    http://www.caravanmagazine.in/reportage/on-the-line-island-lakshadweep-spawning-site

  • Poster
    2014
    The life-history of a grouper along a Lakshadweep time-line
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    PDF, 2.79 MB

  • Poster
    2014
    Larval connectivity in Lakshadweep
    Download

    PDF, 3.33 MB

  • Popular Article
    2012
    A thousand leopards in the Sea
    The Hindu in School, 29 September

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