A "Grouper Moon" Rises Over The Grammanik Bank

Allowing grouper the opportunity to spawn is critical for producing the next generation and the long-term sustainability of their population.  The Grammanik Bank is closed to fishing for 3 months each year, February 1 to April 30.  Photo courtesy of Jason Quetel

The Grammanik Bank

The Grammanik Bank, located just south of St. Thomas, is a unique marine environment where an abundance of large corals, which struggle to survive at near-shore locations, thrive in the deeper, cooler mesophotic environment. This deep-water or mesophotic reef environment represents over 60% of all corals in the US Virgin Islands. The Grammanik Bank is also the only known spawning location in the region for critically endangered Nassau grouper (Epinephelus striatus).  The groupers rely on its extensive and well-developed coral reefs for protection and food during spawning times. 

the "grouper moon"

Nassau and other groupers spawn in large groups or aggregations that are synchronized to a specific week of the lunar cycle, typically around the full moon, during winter months from November to April. The Grammanik Bank spawning aggregation is the only opportunity for these groupers to reproduce each year. When they aggregate, Nassau grouper perform complicated courtship rituals and display unique coloration before spawning where millions of eggs are released into the water column and are fertilized. The fertilized eggs hatch into tiny grouper larvae, the majority of which will perish, the the survivors will eventually replenish local fish stocks.  

Allowing grouper the opportunity to spawn is critical for producing the next generation and the long-term sustainability of their population.  The Grammanik Bank is closed to fishing for 3 months each year (February 1 to April 30) and no anchoring is allowed at any time to protect the corals.  During this time the grouper (and other species) engage in their annual spawning rituals. This is a very vulnerable time for them and history has shown that spawning aggregations with thousands of fish can be wiped out by fishing in a very short time.

hyperstability

Nassau grouper were once the most common large grouper inhabiting the coral reefs of the Caribbean. Prior to the 1970’s, their populations were able to sustain relatively heavy fishing pressure from traps, lines and spears in shallow nearshore waters throughout much of the Greater Caribbean region. However, in the 1970’s, economic development initiatives promoted the expansion of local fisheries to deeper off shore waters through subsidies for larger vessels, outboard engines and hydraulic trap haulers.  These exploratory fisheries lead to the discovery of large grouper spawning aggregation sites where hundreds to thousands of Nassau grouper and other species gathered to spawn during specific months and locations. These spawning sites were typically located at the edge of insular or continental shelves at reef promontories.  

The Nassau grouper in the USVI were fished to near extinction because people didn’t know that their population could be depleted. During the first few years of unregulated and intense fishing on the spawning aggregations, thousands of pounds of fish were landed during each “grouper moon” before fish had a chance to spawn. The spawning aggregations seemed limitless and the high catches flooded fish markets.  By the late 1970’s and early 1980’s the harvest rates suddenly plummeted, a condition known as hyperstability, where catches remain high until the spawning aggregation is suddenly depleted and the adult population collapses, effectively eliminating the production of fertilized eggs and supply of baby fish for the next generation.   At least six Nassau grouper spawning aggregation sites in the US and British Virgin Islands were eliminated.

This scenario occurred nearly everywhere in the Greater Caribbean region from the Greater Antilles, Eastern Caribbean islands, Bahamas, Bermuda, Cayman Islands, Cuba, Florida, Puerto Rico, Virgin Islands, Mexico and Central America.  Nearly 50 years later, and despite strict Federal regulations in the mid 1990’s prohibiting take and possession of Nassau grouper in the US Caribbean and elsewhere, the species populations continue to decline. In 2018 the global status of Nassau grouper was updated and listed as “Critically Endangered” in the IUCN Red List and is listed as “Threatened” under the Endangered Species Act. Today there are only a small number of sites scattered around the Caribbean that are known to have remnant spawning populations, including the Cayman Islands, Belize, Bahamas and the Turks and Caicos.  Today it is still a rare treat to see a large Nassau grouper on a coral reef in the Virgin Islands. 

The number of Nassau grouper showing up at the spawning aggregation site has continued to increase and is believed to be up to at least 500 fish. In February 2020 over 400 fish were counted in one large group, which is the first time such a large group of Nassau grouper has ever been seen at the Grammanik Bank. Photo courtesy of Jason Quetel.

research, regulation and hope

Dr. Richard S. Nemeth, Ph.D., at the University of the Virgin Islands, has been studying the Nassau grouper since 2003.  In 2005, Nemeth and Elizabeth Kadison, a technical diver at UVI, counted about 100 Nassau grouper on the Grammanik Bank often in small groups of 3-5 fish. The following year, the Grammanik Bank seasonal fishery closed area was established. This same year, the USVI also adopted the Federal ban on catching Nassau grouper in Territorial waters.  This means that Nassau grouper cannot be caught or eaten in the USVI. With these added protections, the number of Nassau grouper aggregating at the Grammanik Bank has more than doubled in the past 10 years with numbers reaching over 200 fish and groups of 30-50 Nassau seen in certain locations.

Scientists Elizabeth Kadison  and Sarah Heidmann prepare to dive into the deep waters of Grammanik Bank Photo courtesy of Jason Quetel

Since Nassau grouper take about 4 years to reach maturity, Nemeth believes the large increase in Nassau grouper in 2010 may be from a large pulse of baby Nassau that survived the planktonic stage and settled onto nearshore reefs a few years earlier in 2006 and grew up to become adults by 2010. In 2014 and 2015, two more large pulses of juvenile Nassau grouper were seen in shallow seagrass beds and rubble areas around St. Thomas. As anticipated, the number of Nassau grouper at the spawning aggregation site showed a sudden large increase and is believed to be up to at least 500 fish. In February 2020, over 400 fish were counted in one large group, which is the first time such a large group of Nassau has ever been seen at the Grammanik Bank.  

The Nassau grouper spawning aggregation at the Grammanik Bank is a very special place. The US Virgin Islands are very fortunate because this is one of only two location in the entire Caribbean where Nassau grouper have shown signs of recovering (the other being the Cayman Islands).   Understanding the life cycle of Nassau grouper and other commercially important species allows natural resource agencies to incorporate ecosystem-based fisheries research into management decisions that not only benefits species populations but also the livelihoods that depend upon them.

How far do Nassau grouper migrate to their spawning sites? See Part 2: The Secret Life of Nasau Grouper Revealed, where advanced acoustic tracking technology give scientists a closer look at the Nassau migration patterns during spawning.