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DIVERSITY AND THERMAL PHYSIOLOGY OF FISHES INHABITING MARGINAL HABITATS

Introduction and Background:

Shallow sand/seagrass flats, intertidal pools, and mangrove ponds are commonly found in close association with coral reef communities (Erftemeijer and Allen 1993).  The term marginal habitat may be applied to these areas in that it accurately describes both their geographical location relative to the reef, as well as the harsh abiotic conditions each exhibits.  The term might also be applicable to our overall understanding of the ecology of fishes living in these systems.  Whereas a large body of literature documents ecology and distribution of fishes inhabiting coral reefs, the use of marginal habitats by reef fishes has been virtually overlooked (Ley et al. 1999; Kimani et al. 1996).  A few studies have suggested that these areas may provide important nursery habitat (Bennett 1987; Chong et al. 1990; Vidy 2000; Laegdsgaard and Johnson 1995, 2001) or genetic (Bernardi et al. 2001; Horn et al. 1999) and nutrient reservoirs (Vega and Arreguin 2001; Laegdsgaard and Johnson 2001) for reef fishes but none quantify this relationship directly.  Typically, these habitats differ markedly in water quality, spatial architecture, and community structure from the adjacent reef making it difficult to understand their ecology from previous reef research.  As a result, no consensus exists as to the basic ecological function of marginal habitats or their relationship to coral reef fish communities (Nybakken 2001).

We conducted preliminary investigations into the abiotic and biotic structure of mangrove ponds, intertidal pools and shallow seagrass/sand flats surrounding Hoga Island coral reefs.  Specific objectives of this research agenda were to 1) identify and catalogue the distribution and diversity of marginal habitats and fishes inhabiting them, 2) quantify physiological thermal tolerance limits of fishes and, 3) determine and quantify to what degree abiotic habitat conditions limit coral reef fishes living in marginal habits.  Data from these studies provide a better understanding of coral reef fish biology and ecological-physiology of fishes living in harsh environments.

Methods:

Physical characterization of marginal study sites

Students trained in the use non-destructive techniques observed and collected fishes from mangroves, sand/seagrass flats, and rockpools on the north end of Hoga Island. General descriptions were made of each study site including overall dimensions, depth profiles, bottom morphology, and water quality (pH, temperature, oxygen, salinity and pH) using standard methods described by Nielsen and Johnson (1989). Onset temperature loggers were used to monitor diel rates of water temperature change (±0.1°C).  Temperature (±0.1°C), dissolved oxygen (±0.1 mg/l), salinity (±1.0‰), and pH (±0.1 unit) at each site were monitored at low tide during both day and night.

Biodiversity and tolerance experiments of study site fishes

Biodiversity Measures – Fishes were collected from study sites (away from the behavior experiments) using small dip nets, and seines.  Captured fishes were identified to the lowest taxonomic level possible, measured to the nearest 1mm and weighed to the nearest 0.1 g.  Up to 10 fish captured from each species were held in floating baskets for later transport to the laboratory for use in temperature tolerance trials.  Fishes in excess of 10 were immediately released at the site of capture.  Species richness estimates were made from the collection data. 

Determination of Thermal Tolerance – High-temperature tolerance of fishes collected from each sample site (up to 10 fish) were estimated using the critical thermal methodology (CTM) (Cox 1974; Becker and Genoway 1979; Palidino et al. 1980).  Briefly, a critical thermal maxima (CTMax) was determined by exposing groups of 20 field-acclimatized fish to a constant rate of temperature increase (~0.5°C/min) until loss of equilibrium (LOE) is observed.  The arithmetic mean of the collective endpoints was taken as the CTMax for the population (Cox 1974).

Preliminary Results: 

Abiotic conditions: 

• Seagrass temperature, salinity, oxygen, and pH ranged from 24.4-33.3°C, 32-41ppt, 0.05-7.31 ppm, and 7.6-8.5, respectively. 
• Rockpool temperature, salinity, oxygen, and pH ranged 24.4-35.0°C, 31-43ppt, 1.2-10.78 ppm, and 7.9-8.7, respectively. 
• Overall, the combined study sites yielded temperature, salinity, oxygen, and pH ranges from 24.4-35.0°C, 31-43ppt, 0.05-10.78 ppm, and 7.6-8.7, respectively. 

Biodiversity: 

• A total of 59 fish species from 28 families and two classes were collected over a 20 day period.
• Of the fish species collected, only a single elasmobranch representative was collected (Blue-spotted ribbontail ray, Taeniura lymma).
• Biodiversity was greater in seagrass habitats (88 % of species richness) than in rockpool habitats (28 % of species richness).
• Seagrass habitats adjacent to the intertidal reef flat were more diverse than mangrove seagrass areas (species richness = 57 and 28 species respectively).
• Rockpool habitats in the mangroves were more diverse than rockpools adjacent to reef flat intertidal areas due to the diverse rockskipper and mudskipper fauna specific to mangroves habitats.

Reports:

A paper entitled The thermal, saline and oxygen tolerances of fish living in intertidal rock pool habitats in SE Sulawesi, will be prepared by Dr Wayne Bennett, Nann Fangue and Jodie Rummer, University of West Florida by May 2003   Data collected on thermal tolerance of bluespotted ribbontail rays and regal mudskippers will be analyzed in more detail at the laboratory in Pensacola and used to generate 2 manuscripts on thermal ecology of these species.

References:  

Becker, C. D. and R. G. Genoway. 1979. Evaluation of the critical thermal maximum for determining thermal tolerance of freshwater fish. Environmental Biology of Fishes. 4(3): 245─256. 

Bennett, B. A. 1987. The rockpool fish community of Koppie Alleen South Africa and an assessment of the importance of Cape Rock-pool as nurseries for juvenile fish. South African Journal of Zoology 22(1):25-32. 

Bernardi, G., S. J. Holbrook, and J. R. Schmitt. 2001. Gene flow at three spatial scales in a coral reef fish, the three-spot dascyllus, Dascyllus trimaculatus. Marine Biology 138(3):457-465. 

Chong, V. C., A. Sasekumar, M. U. C. Leh, and R. D. Cruz. 1990. The fish and prawn communities of a Malaysian coastal mangrove system with comparisons to adjacent mud flats and inshore waters. Estuarine, Coastal, and Shelf Science 31(5):703-722. 

Cox, D. K.  1974.  Effects of three heating rates on the critical thermal maximum of bluegill.  Pages 158-163 in J. W. Gibbons and R. R. Sharitz, editors.  Thermal Ecology.  National Technical Information Service, CONF –730505, Springfield, Virginia. 

Erftemeijer, P. L. A. and R. G. Allen. 1993. Fish fauna of seagrass beds in south Sulawesi, Indonesia. Records of the Western Australian Museum 16(2):269-277. 

Fangue, N. A. and eight co-authors.  2001.  Temperature and hypoxia tolerance of selected fishes from a hyperthermal rockpool in the Dry Tortugas, with notes on diversity and behavior. Caribbean Journal of Science. 37(2):In Press. 

Horn, M. H., K. L. M. Martin, and M. A. Chotkowski, eds. 1999. Intertidal fishes: Life in two worlds. San Diego: Academic Press. 

Kimani, E. N., G. K. Mwatha, E. O. Wakwabi, J. M. Ntiba, and B. K. Okoth. 1996. Fishes of a shallow tropical mangrove estuary, Gazi, Kenya.  Marine and Freshwater Research 47(7):857-868. 

Kramer, D. L.  1983.  Aquatic surface respiration in the fishes of Panama: distribution in relation to risk of hypoxia.  Environmental Biology of Fishes 8:49-54. 

Kramer, D. L. and M. McClure. 1982. Aquatic surface respiration, a widespread adaptation to hypoxia in tropical freshwater fishes. Environmental Biology of Fishes 7: 47-55. 

Laegdsgaard, P. and C. Johnson. 2001. Why do juvenile fish utilize mangrove habitats? Journal of Experimental Marine Biology and Ecology 257(2):229-253. 

Laegdsgaard, P. and C. R. Johnson. 1995. Mangrove habitats as nurseries: Unique assemblages of juvenile fish in subtropical mangroves in eastern Australia. Marine Ecology Progress Series 126(1-3):67-81. 

Ley, J. A., C. C. McIvor, and C. L. Montague. 1999. Fishes in mangrove prop-root habitats of northeastern Florida Bay: Distinct assemblages across an estuarine gradient. Estuarine, Coastal, and Shelf Science 48(6):701-723. 

Nielsen, L. A., and D. L. Johnson, eds.  1989.  Fisheries Techniques.  American Fisheries Society, Bethesda, Maryland. 

Nybakken, J. W., ed. 2001. Marine biology: An ecological approach. 5^th edition. San Francisco: Benjamin Cummings. 

Paladino, F. V., J. R. Spotila, J. P. Schubauer, and K. T. Kowalski.  1980.  The critical thermal maximum; a technique used to elucidate physiological stress and adaptation in fishes.  Review of Canadian Biology 39:115-122. 

Prins, H. H. T. and J. Wind. 1992. Research for nature conservation in South-east Asia. Biological Conservation 63(1):43-46. 

Smale, M. A. and C. F. Rabeni. 1995. Hypoxia and hyperthermia tolerances of headwater stream fishes. Transactions of the American Fisheries Society 124(5): 698-710. 

Vega, C. M. E and S. F. Sanchez. 2001. Energy flux in a mangrove ecosystem from a coastal lagoon in Yucatan Peninsula, Mexico. Ecological Modeling 137(2-3):119-133. 

Vidy, G. 2000. Estuarine and mangrove systems and the nursery concept: Which is which?  The case of the Sine Saloum system (Senegal). Wetlands Ecology and Management 8(1):37-51. 

Appendix:

Table 1: Species richness by site for shallow water habitats on the north end of Hoga Island,      

Indonesia.