Wednesday 31 October 2012

To cull or not to cull? The Goliath Grouper in Floridan waters.

The Goliath Grouper is a large (up to 2.5 m and 363 kg) predatory reef Fish, formerly found in coastal waters from Florida to Brazil in the west Atlantic and Caribbean and from Senegal to Congo in the East Atlantic, but which has become extinct, or close to extinct, over much of this range due to overfishing. The species has been protected in US waters since 1990, and appears to have made a good recovery, though populations have not yet reached historical records. However this recovery has coincided with a decline in stocks of commercially important Spiny Lobsters (Panulirus argus) and Grey Snappers (Lutjanus griseus), as well as some other Snapper species, leading many commercial fishers to call for a cull of the Groupers.

A breeding aggregation of Goliath Groupers off Jupiter, Florida. Walt Stearns in Frias-Torres (2012).

In a paper published in the journal Orynx on 21 October 2012, Sarah Frias-Torres of the Ocean Research & Conservation Association examines the cases for and against the culling of the Goliath Grouper in Floridan waters, utilizing evidence from the National Marine Fisheries Service, the Florida
Fish and Wildlife Conservation Commission and the REEF Environmental Education Foundation to assess the vulnerabilities of the Fish and its likely impact on Floridan ecosystems.

Frias-Torres found that the Goliath Grouper is exceptionally vulnerable to overfishing, being large, long lived (possibly over 40 years), slow breeding (fish do not breed till they are at least eight years old), having high site fidelity and forming large, easily visible breeding aggregations. She also noted that the decline in Lobster and Snapper stocks began when the Grouper population was in sharp decline, and continued while the Fish were more-or-less extinct, suggesting that the decline in these species numbers was independent of the Grouper population.

Next Frias-Torres looked at the diet of the Groupers. She found that the Groupers did eat Lobsters, but as part of a varied diet that included other invertebrates, including some that themselves ate Lobsters (such as Crabs and Octopus), as well as a variety of large, slow-breeding poisonous Fish (Goliath Groupers appear to specialize in eating poisonous and venomous Fish, a diet that most other animals shun), many of which also consume Lobsters. It was therefore far from clear what impact culling the Groupers would have on the Lobsters; potentially it could trigger an explosion in the populations of some poisonous Lobster-eating Fish with no other predators, impacting the Lobsters badly. Frias-Torres could not find any evidence that the Groupers ate Snappers at all. 

Finally Frias-Torres examined the potential benefits of having the Groupers in Floridan reef ecosystems. She notes that the Fish are large, attractive animals that are naturally curious about divers, and that similar Fish in other parts of the world have proved highly lucrative fro SCUBA-diving ecotourism if managed well. The high site-fidelity of the fish means that commercial diving companies can more-or-less guarantee an encounter with Goliath Groupers, and the Fish's curious disposition makes a friendly reception likely.

In addition Floridan Reef ecosystems have been invaded by Indo-Pacific Red Lionfish (Pterois volitans/Pterois miles, Scorpaenidae) in recent years. These are voracious predator of native juvenile reef fishes and invertebrates, making them potentially extremely destructive to reef ecosystems. They are also spiny and highly venomous, and therefore have few natural predators. The preference for consuming venomous Fish seen in the Goliath Grouper makes this species an excellent potential natural control of the Lionfish, though the Grouper has not to-date been observed consuming the invasive Fish (this is not altogether surprising, as most studies of the Grouper's diet were carried out before 1990, when it was possible to study the stomach contents of commercially caught Fish; modern studies using non-invasive methods are underway, however).

Red Lionfish, Pterois volitans. Eat The Invaders.

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