Characterisation of the Commercial Mangrove Land Crab Fishery in Trinidad
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CERMES Technical Report No 76 Characterisation of the commercial mangrove land crab fishery in Trinidad M.E. MAYNARD AND H.A. OXENFORD Centre for Resource Management and Environmental Studies Faculty of Science and Technology The University of the West Indies Cave Hill Campus, Barbados 2014 ABSTRACT Mangroves are typically found along coastlines within the tropics. The natural resources encompassed within these mangrove ecosystems provide goods and services which can be of great commercial value to humans. Mangrove land crabs are one such resource. Within the Caribbean region, mangrove land crab harvesting is common though small scale in nature, often not included in countries‟ commercial fisheries reports and with little to no management. As such, these „fisheries‟ are plagued by uncontrolled exploitation and a lack of formal information, as it relates to the status of the resource. The Caribbean island of Trinidad is no exception. Trinidad is home to two species of mangrove land crab; the blue land crab, Cardisoma guanhumi and the hairy land crab, Ucides cordatus, with both species being harvested and sold commercially. The country of Trinidad and Tobago is either a contracting member or a signatory party to a number of conventions including the Convention on Wetlands of International Importance Especially as Waterfowl Habitat (Ramsar Convention), the Convention on Biodiversity (CBD) and the Protocol concerning Specially Protected Areas and Wildlife to the Cartagena Convention (the SPAW Protocol), with each promoting the sustainable use, monitoring, regulation and or protection of fauna due to their possible value and importance. In an effort to provide baseline information which can help Trinidad better attain the goals and responsibilities stipulated under these conventions, this study seeks to characterise the commercial mangrove land crab fishery in Trinidad for the first time. Data were collected from 29 crab harvesters and 18 vendors via semi-structured interviews, general observations and informal conversations at their vending sites in public markets or along major roads. Crabs were captured in eight mangrove ecosystems throughout the country with the Caroni swamp, on the western coast of the island, being the largest and most commonly used harvesting site on the island. Bamboo and net traps were the main gear types used for catching blue and hairy crabs respectively, and a typical harvester can catch between 41 - 60 crabs per trip. Crabs were found to range between 56 -92 mm in carapace width and 75 - 350 g in weight. The exact number of crabs caught per trip was generally unknown by the harvesters. Once caught, the crabs are bound alive and either sold singly or in bunches at prices ranging between TT$10-$501, depending on the size of the crab. Crude estimates provided by harvesters and vendors suggest that the amount of crab sold by each harvester or vendor per week can range from anywhere between 0.5 to over 90 kg. The majority, 72% each of harvesters and vendors, rely on this fishery as their main source of income. Women comprise a major part of this fishery, representing a third of the harvesting population and two-thirds of the vendors. From the sample population of 110 crabs of each species, no statistically significant difference was detected between the weights and carapace widths of blue and hairy land crabs. However, within each species there was a significant difference between the weights of males and females for both blue and hairy crab, and a significant difference observed in the carapace widths of the males and females of blue crabs. This difference in carapace width was not observed between male and female hairy crabs. The weight-width relationships of males and females for each species indicated a pattern of negative allometric growth. The sex ratio was similar for both 1 Exchange rate: US $1= TT $6.42 i species, with male to female ratios being approximately 2:1 for both. Juveniles made up a very small percentage of the sample population, with only seven being found amongst the sampled blue crabs and none amongst the hairy crabs. Crab harvesting is not only a small scale commercial activity in Trinidad, but is also a popular recreational activity amongst locals during the spawning months of July to September, known as the “running” season. During this time, the crabs are abundant and easy to catch during their trek to and from the sea, where they deposit their eggs. These recreational harvesters, who may also sell a portion of their catch, present competition for regular harvesters and vendors. Despite the importance of crab harvesting, it remains unregulated and unmonitored, and more than half of the harvesters reported that they have observed a decline in the abundance of the mangrove land crabs in recent years. This observation, coupled with the fact that crabs are particularly targeted during their spawning migrations and that their mangrove habitats are being degraded, suggests that the current practices are unlikely to be sustainable over the long term. Keywords: blue land crab; Cardisoma guanhumi; hairy land crab; Ucides cordatus; small scale fishery; Trinidad ii ACKNOWLEDGEMENTS We gratefully acknowledge the participation and assistance of all the Trinidad crab harvesters, vendors and key informants who contributed their time and vast knowledge to this study. We also acknowledge Rosemarie Kishore of the Institute of Marine Affairs (IMA) for her inspiration and key role in providing the necessary tools needed to conduct this study. Special thanks are also extended to Hamish Asmath and Bheshem Ramlal of the IMA and the Department of Geomatics Engineering and Land Management at the UWI St. Augustine Campus respectively, for providing GIS data. In-kind contributions are acknowledged from the Institute of Marine Affairs, Trinidad and Tobago. iii TABLE OF CONTENTS ABSTRACT ...................................................................................................................................... i ACKNOWLEDGEMENTS ........................................................................................................... iii 1 INTRODUCTION .................................................................................................................... 1 2 RESEARCH AIMS AND OBJECTIVES ................................................................................ 3 3 BACKGROUND ...................................................................................................................... 3 3.1 Distribution ........................................................................................................................ 3 3.2 General biology ................................................................................................................. 4 3.2.1 Blue land crab (Cardisoma guanhumi) ...................................................................... 4 3.2.2 Hairy land crab (Ucides cordatus) ............................................................................. 6 3.3 Mangrove land crabs in Trinidad and Tobago ................................................................... 7 4 METHODOLOGY ................................................................................................................. 11 4.1 Field Interviews and Observations .................................................................................. 11 4.1.1 Crab vending ............................................................................................................ 11 4.1.2 Crab harvesting ........................................................................................................ 12 4.1.3 Geographic Information System (GIS) mapping ..................................................... 12 4.2 Biological measurements ................................................................................................. 13 4.3 Data analysis .................................................................................................................... 14 5 RESULTS ............................................................................................................................... 15 5.1 Harvest sector .................................................................................................................. 15 5.1.1 Profile of a harvester ................................................................................................ 15 5.1.2 Harvest sites ............................................................................................................. 21 5.1.3 Harvesting methods .................................................................................................. 21 5.1.4 Harvesting effort ...................................................................................................... 23 5.1.5 Catch per trip ............................................................................................................ 29 iv 5.1.6 Harvesting inputs ...................................................................................................... 30 5.1.7 Factors affecting crab harvesters .............................................................................. 32 5.2 Vendors ............................................................................................................................ 33 5.2.1 Profile of a Vendor ................................................................................................... 33 5.2.2 Sources of crab ........................................................................................................