An underwater study on the mangrove habitats of the Florida Keys has revealed the habitat preferences of the endangered Goliath grouper.
Goliath groupers, Epinephalus itajara, can live for up to 40 years and are one of the world's largest groupers reaching a length of over 2m/6'6" and weighing a wopping 400kg/63 stones. But its large size and slow growth rate means that it's vulnerable to the effects of fishing.
The species doesn't become sexually mature until its five years old, so decades of fishing, which have seen the removal of many fish before they have spawned, have led to its commercial extinction and a dramatic decline in the numbers of fish remaining in the wild. It's now protected by a fishing ban in the US, and is listed by the World Conservation Union's Red List as critically endangered.
Goliath groupers are one of the few Epinephalus species able to live in brackish water and tolerate the warm, low oxygen environment of the mangrove swamp, and it's these habitats that act as nursery areas for the young fish which can remain there until they reach a size of around 1m/39" in length. But little was known about the nursery habitats of the species.
Sarah Frias-Torres of Miami's Southeast Fisheries Science Center undertook a visual underwater census in the Florida Keys to evaluate the importance of the mangrove habitat structure and complexity in determining the distribution of the species in the area.
Her findings, which have just been published in the journal Endangered Species Research, show that the species is strongly associated with certain environmental features, and not randomly distributed around the red mangrove habitats as previously believed.
The results suggest that the species is most likely to be found in areas where the water is at least 80cm deep at high tide, and prefers to occupy areas with a canopy of mangrove trees and overhangs for the fish to shelter beneath.
It particularly prefers areas with a soft-bottom of mud or sand and has a penchant for a shady spot, and areas where there are lots of mangrove roots present in the water to hide among.
Frias-Torres says that predators, such as Bull sharks, Carcharinus leucas, Blacktip reef sharks, C. limbatus, Lemon sharks, Negaprion brevirostris, as well as Barracuda and snappers, all occur in the habitat and may be the reason why the Goliath grouper lives among such shelter while small.
The fish is found almost exclusively in the red mangrove swamps of tropical and subtropical waters around the Atlantic from Florida down to the Gulf of Mexico and Brazil, as well as in the Pacific coastal mangroves of Peru.
For more details see the paper: Frias-Torres, S (2006) - Habitat use of juvenile Goliath grouper, Epinephelus itajara in the Florida Keys, USA. Endangered Species Research, 1:1-6, 2006.