Bermuda's coastal seagrass beds as habitat for fish

Ward, Jack Andrew David (1999) Bermuda's coastal seagrass beds as habitat for fish. MSc(R) thesis, University of Glasgow.

Abstract

Tropical seagrass communities throughout the world are known to be highly productive centres of biodiversity. Along with coral reefs and mangrove forests, seagrasses are recognised as forming critical habitats for a wide variety of marine organisms. Not only are these habitats important as discrete entities, they act synergistically to promote the stability and productivity of tropical coral reef ecosystems. In particular, seagrass meadows are noted for stabilising sediments, thereby reducing turbidity, and serving as important nurseries and foraging grounds for a variety of economically important fish and crustaceans. Located at 32°N 65°W, the remote oceanic islands of Bermuda support the northernmost coral reef ecosystem in the world. Populated by humans since 1609, and with over 3,500 scientific publications describing its natural history, Bermuda is arguably one of the most carefully studied of all oceanic islands. Despite this wealth of knowledge, the extent of the island's seagrass meadows, the vigour of this important resource and the composition of the local seagrass-associated communities remain poorly documented. In 1990 the use of seine nets over seagrass beds was banned in certain coastal areas to protect the juvenile fish living there. This action was taken in absence of any documented information on the distribution and seasonality of fish inhabiting Bermuda's coastal seagrasses. However, this measure was taken in response to anecdotal information that large numbers of juvenile fish were being destroyed as bycatch in the local bait fishery and reflects the management agency's sensitivity to the protection of inshore nurseries. To address this lack of information the objectives of the current study were; 1) to document the area covered by Bermuda's coastal seagrass beds and to examine recent trends in the spatial extent of these habitats through the examination of photographic aerial surveys spanning the period 1962-1997, 2) to use samples taken by means of a standard Bermuda bait net to describe the composition and seasonality of the fish communities inhabiting three inshore seagrass habitats, and to compare these three communities, 3) to describe the planktonic, epifaunal and infaunal components of the local seagrass-associated micro-invertebrate community, and 4) to document the feeding patterns and food preferences of the dominant members of the fish community found in these seagrass beds. The coastal seagrass meadows were found to occupy approximately 500Ha in 1981. Since 1962 substantial changes, both expansions and contractions, in the seagrass beds have occurred. The largest declines happened in seagrass beds well removed from any apparent anthropogenic input. The causes and implications of these changes remain unclear. Forty-two species of fish were identified from the three sampling sites. Large numbers of a few common species often dominated the samples. Both site and season were found to have significant effects on the species diversity of the seagrass-associated fish communities captured by bait net. Species that were common at one site were sometimes rare or absent at others. Observations of seasonal recruitment pulses and modal progression analysis revealed patterns of residence and growth of fish within this habitat. The seagrass-associated invertebrate community was found to be both abundant and diverse. A significant difference was found between the extremely abundant microfauna dwelling upon and between the seagrass blades and the less numerous organisms of the adjacent water column. Samples of the infauna revealed far fewer organisms than did epifaunal samples. Direct diver observations of the feeding behaviour of fish over and within seagrass beds largely confirmed the feeding strategies inferred from analysis of gut contents. The epibiota coating the grass blades formed the primary food source for the majority of seagrass-associated fish. Ontogenetic changes in feeding strategies were observed in a number of fish species. The results of these investigations indicate that Bermuda's inshore seagrasses are important to local fish production and that the magnitude of this resource is in a state of flux. While indicating that site-specific information is required to assess the importance of particular seagrass meadows, the available information supports management efforts aimed at protecting these habitats.

• Item Type: Thesis (MSc(R))
• Qualification Level: Masters
• Keywords: Biological oceanography.
• Colleges/Schools: College of Science and Engineering
• Supervisor's Name: Davenport, Professor John
• Date of Award: 1999
• Depositing User: Enlighten Team
• Unique ID: glathesis:1999-71862
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 17 May 2019 09:31
• Last Modified: 28 Oct 2022 08:54
• Thesis DOI: 10.5525/gla.thesis.71862
• URI: https://theses.gla.ac.uk/id/eprint/71862