The Aquatic Exodus: an Overview of Migratory Species in Jamaican Freshwater Systems

The Aquatic Exodus: An Overview of Migratory Species in Jamaican Freshwater Systems Prepared by Ms. Kimberly John Freshwater Conservation Specialist The Nature Conservancy & Ms. Aisha Bailey Volunteer - Freshwater Conservation The Nature Conservancy Introduction Two mass migration events were recently reported in the Jamaican media (Daily Observer, January 9 th , 2006 and Jamaica Gleaner November 25 th , 2005). Both involved the natural migration of freshwater species from the sea to upstream areas in rivers. Active migrations within river systems are not usually noticed or recorded by scientists and locals in Jamaica (although they used to be a regular part of our rural culture). Consequently, the migrations of freshwater gobies between November 2005 and January 2006 sparked some temporary curiosity in the Jamaican public. What was not publicly emphasised at the time was that these were not rare, unnatural or ominous events. On the contrary the migration of many of Jamaica’s freshwater species from upstream areas to the sea and vice versa is a regular natural occurrence and an important facet of healthy, functional, aquatic ecosystems. This article aims to provide general ecological information on migration in Jamaica’s rivers -a virtual aquatic exodus- repeated over millennia across the island. This overview will hopefully provide scientists and non-scientists alike with some insight into this fascinating aspect of Jamaica’s natural history and set us thinking more critically about how our activities impact freshwater ecosystems. Figure 1: Front page of the Daily Observer, January 9th, 2005 - Spectators watch the upstream migration of thousands of juvenile gobies ( Sicidium Plumieri ) at the Yallahs fording. Ecology of migration Baker (1978), defined migration as the act of moving from one spatial unit to another. In ecological studies many kinds of migrations are described; from accidental to non - accidental, home range to navigational and latitudinal to altitudinal migrations. Se asonal migration -migration that corresponds with changes in seasons - is the most common interpretation of the term “migration”. Reproductive migration is a type of seasonal migration in which animals move to other places or habitats in order to bear and/o r raise young. Relocation occurs generally in order to protect young from predators or other environmental stresses or because the animal needs a different kind of habitat for that particular life cycle stage. This article deals specifically with the reproductive migratory patterns of Jamaican freshwater fauna. Freshwater reproductive migration is classified based on the capacities of freshwater species to cope with waters of varying salinities during specific stages in their life cycles (McDowell 1988). Species that complete their entire life cycle within freshwater systems (feeding and breeding grounds often being several kilometres apart) are known as potadromous species. Of even greater significance to freshwater and evolutionary ecology are those species whose life cycle takes place partly in freshwater and partly in sea water. These are called diadromous species. Two groups are distinguished among diadromous species. Anadromous species, for instance the well-known Atlantic salmon (Salmo salar ), reproduce in freshwater while the growing phase occurs in the sea. Catadromous species, the American Eel ( Anguilla rostrata ) for example, migrates to the sea for breeding and then migrate to freshwaters for growth and trophic purposes. Jamaica’s Freshwater Migratory Fauna This article is by no means an exhaustive survey of migratory species in Jamaican rivers. However we will highlight some of the more prominent travellers in Jamaican streams which have adapted to incorporate both freshwater and marine habitats into their life cycles. Among these are fish such as mountain mullet, gobies and the American Eel, and invertebrates for instance shrimp (janga) and the freshwater snail locally called bussu . Mullets Figure 2: Young mountain mullet caught with hand Agonostomus monticola (mountain Mugilidae (mullets). It is a pelagic fish that occurs in high neo-tropical regions such as Mexico, Central America and the West Indies. Mullets have been recorded in th e Swift, Buff Bay and Yallahs Rivers in Jamaica, although their distribution is probably widespread. Aiken (1998) reports that a relatively familiar species in Jamaican streams, but highlights the limited knowledge regarding its bi ology and ecology. Mountain mullets appear to be a catadromous species migrating to the sea from freshwater to spawn. Several reports of mountain mullets in other Caribbean rivers note the presence of spent individuals and the absence of gravid females in the rivers coupled with the presence of larval forms in sea with a simultaneous migration of spent individuals back to the rivers which suggest that these mullets breed in sea water. It is postulated that the spawning period of the mountain mullet coincides closely, but not always, with the rainy season. Gobies The upstream migration of Sicydium plumieri (Sirajo goby) (locally known as ‘suckstone’) in Great and Yallahs Rivers was recently captured in Jamaica’s print and electronic media (figure 1) . Sicydium plumieri is a dermersal anadromous fish belonging to the family Gobiidae (gobies), and subfamily Sicydiinae. They are distributed mainly in the subtropical freshwaters of Central America and the West Indies. S. plumieri post-larval migrations have been noted across the Caribbean. The post-larvae are a local delicacy in Puerto Rico, Guadeloupe, Dominica and Martinique where they are called “ceti” and in Jamaica where they are known as “fryers”. Sicydium adults spawn in headwaters where females deposit their eggs on the substratum under supporting structures such as boulders, stones and crevices or even among vegetation. The eggs are attached to these structures with adhesive filaments. These eggs are then fertilised by the male gobies which also protect them from predation. After about two days the eggs hatch and the larvae drift downstream. Spawning occurs in every month of the year but peaks during the rainy seasons when the increased volumes of the river water carry the larvae downstream to the river mouth. It is in these brackish or marine environments that the larval and post-larval stages remain for one lunar month. At about the last quarter of the moon phase, post-larvae appear along the sea shore as they approach the river mouth. The post-larvae then metamorphose into juveniles. Upon completing the juvenile stage the fish aggregate in large numbers and set out on the journey back to the headwater regions of their ancestors. They normally move inconspicuously below the water surface. However in areas where the channel is constricted, - such as the Great River Dam or Yallahs Fording- massive shoals of these young fish become visible as they congregate along the banks, walls and boulders. These goby juveniles have specialized pelvic discs and tails that are used to climb physical barriers, even outside of the water, in order to migrate upstream. American Eel The American Eel ( Anguilla rostrata ) is a catadromous fish, outstanding because of the scale of its migration and the morphological and physiological changes that occur during its life. A. rostrata is distributed across coastal areas and streams in the West Atlantic, from Canada in the north to Trinidad in the south. It occurs in Jamaica, however its specific distribution across the island is unknown (We have observed a child catching an eel in upper Rio Grande with a simple hook and line). The eel’s life cycle begins with spawning Sargasso Sea in the south-west Atlantic. Sexually mature eels migrate from streams across their range for this annual mass spawning event between February and April. It is assumed that adult eels die after spawning. The first stage in the life cycle, called a leptocephalus, drifts and swims towards the continental shelf where it metamorphoses into a “glass eel” because of its transparency. Figure 3: Diagram of A. rostrata 's life cycle. (taken from Communications D irectorate Fisheries and Oceans Canada, 2000) Upon reaching coastal areas, glass eels become pigmented and are known as “elvers ”. Many elvers swim upstream into rivers and streams in search of less crowded or better quality habitat and replace eels that have already matured and left the river. However, a proportion of the elvers remain in estuaries. Elvers mature into adults which are called “yellow eels”. A. rostrata may remain in freshwater ecosystems for up to 15 years before sexual maturity and embar king on the spawning migration. Freshwater shrimp Invertebrate freshwater species are also common migrants in Jamaican rivers. There are several species of migratory shrimp in Jamaica (Hunte, 1978) usually belonging to the Palaemonidae family. Macrobrach ium , commonly called Janga by locals, is one migratory palaemonid genus which is relatively abundant in Jamaican rivers. These organisms are generally subtropical and are very important for nutrition, recreation and commerce throughout the Caribbean. Figure 4: Macrobrachium spp in Rio Grande tributary. R. Love. Macrobrachium spp. are anadromous spending most of their life cycle in freshwater streams while larval stages reside temporarily in estuarine waters. Gravid females remain in stream headwaters and release planktonic larvae which drift downstream to estuarine regions. The larval stages remain planktonic for approximately 12 weeks after which they

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