Assessment of Nursery Habitats for Fishery Stock Enhancement

Marine Science Research

@ U.V.I.

Quantitative Assessment of Nursery Habitats for Fishery Stock Enhancement.

It is commonly suggested that mangrove lagoons and sea grass beds function as juvenile reef fish nursery habitats. This assumption has been largely based on the observation that juveniles of certain reef fish species are commonly found in these habitats whereas older juveniles and adults primarily occupy coral reefs. It is believed that mangrove lagoons and sea grass beds enhance growth and survivorship of newly settled reef fishes and that these nursery habitats contribute significantly to the reef-based fish population through ontogenetic habitat shifts. Several researchers have argued that loss of these essential fish habitats may accelerate stock declines in commercially important reef fishes. To demonstrate the importance of sea grass and mangrove habitats to reef fisheries production, studies need to quantitatively determine the proportion of a population that pass though a nursery habitat before reaching a reef and the actual rates of survival and growth experienced by fish in these different habitats. The analysis of chemical and structural elements within fish otoliths can provide a precise quantitative method for addressing this problem.

Fish otoliths are small, calcified bones in the inner ear of the fish (Figure 1) that can provide a wealth of information on age, growth rates and place of origin. Fish age can be determined by counting the number of daily rings or increments (Figure 2) within the otolith whereas growth can be determined by measuring the width between these bands. These bands are similar to the annual rings found in trees. We viewed the polished otoliths from juvenile yellowtail snapper under a compound microscope and used sophisticated image analysis software to count and measure the otolith increments (Figure 3). To estimate the contribution of nursery habitats on fisheries production we collaborated with Dr. Simon Thorrold of Woods Hole Oceanographic Institute and Dr. Zhongxing Chen at Old Dominion University to examine the unique chemical signatures in the otoliths of fish. The periodic deposition of increments within otoliths, and the inert nature of the materials once deposited, allows the isotopic and trace element chemistry to be resolved at temporal scales approaching the daily level using a specialized laser that vaporizes a section of the otolith. The vapor is then analyzed for its chemical composition using a very sensitive mass spectrophotometer.This study was conducted at three nursery habitat sites on St. Thomas US Virgin Islands (Figure 4). We expect to find differences in the otolith chemical signatures of juvenile yellowtail snapper (Ocyurus chrysurus) (Figure 5) from the nursery habitats and thus estimate the proportion of fish on coral reefs containing these same unique chemical signatures. The chemical and structural differences of fish otoliths from different bays will allow us to determine the relative importance of nursery and non-nursery habitats to reef fish production. These unique natural tags will provide some of the first information on source/sink dynamics of mangrove, seagrass and coral reef habitats. In addition UVI students assisted in all aspects of this study including field work to determine movement, migration and mortality rates using elastomer tags (Figure 6). The results of this study will greatly benefit fishery managers by providing a basis for establishing essential fish habitat status and for evaluating the importance of including certain habitats in marine protected areas. This project was funded by the University of Puerto Rico Sea Grant College Program.
	Figure 1. Typical shape of the three pairs of otoliths in marine fishes.

	Figure 2. Daily otolith are emphasized using colored marks to show pre-settlement or larval pelagic phase (blue), the transition or metamorphosis (red) and post-settlement or benthic juvenile stage (yellow). Click to enlarge


	Figure 3. UVI student Temesha Buckley examining yellowtail otolith using microscope and Image Pro software to automate ring counts and measurements.

	Figure 4. Map of St. Thomas, USVI showing three potential fish nursery habitats: Brewer's Bay (Syringodium sea grass and drift algae), Sprat Bay (Thalassia grass bed) and Mangrove Lagoon (Mangrove habitat). Click map to enlarge.

Figure 5. Juvenile yellowtail snapper (Ocyurus chrysurus) from nursery habitat.		Figure 6. Tagging juvenile yellow tail snapper with elastomer pigment.