Group(er) sex in the central Pacific
Group(er) sex in the central Pacific
by Kevin L. Rhodes
The Marbled Grouper Epinephelus polyphekadion (Bleeker) is one of the most widely-distributed and heavily targeted of all Indo-Pacific groupers. It inhabits tropical and subtropical reef and lagoonal habitats from Africa to Japan, extending into the eastern Pacific. Large aggregations (100s to 1000s of individuals) gather for a few days at specific locations and during certain lunar phases to spawn, usually restricted to two or three months during the year. This type of reproductive behaviour makes these fish especially vulnerable to fishermen who know the timing and location of such events. Groupers are particularly susceptible to 'aggregation fishing', which is used and promoted by Hong Kong-based live reef fishing operations. This form of fishing removes large numbers of spawning individuals, often prior to egg release, reducing reproductive output and subsequent recruitment potential.
In the Caribbean and western Atlantic, prolonged aggregation fishing has frequently led to a complete disappearance of aggregations. Within the central Pacific, grouper have recently disappeared from areas of the Marshall Islands after just three years of intensive fishing pressure. In Palau, unrestricted aggregation fishing by a Hong Kong-based live reef fish trader decimated one of two aggregations in only three years. Other negative effects include changes in aggregation sex ratios, decreased effective population size and a rapid reduction in genetic diversity, which could profoundly effect local and regional population genetic structure.
The objectives of my research are to characterize for Marbled Grouper two interrelated aspects of fish biology: (1) population genetic structure and (2) reproduction. My findings will not only expand present knowledge of grouper genetics and reproduction, but will provide useful information and practical guidelines for management and conservation of Marbled Grouper stocks. Currently I am using microsatellite techniques to analyze population genetic structure on various spatial scales: local (inter-island ranges of only a few kms), regional (l00s of kms, e.g., Micronesia and the Marshall Islands), and inter-regional (several 1000 kms, e.g., the Maldives in the Indian Ocean to the Marshall Islands in the central Pacific). To date, tissues have been collected on >600 individuals from a variety of locations: Micronesia, the Marshall Islands, Palau, New Caledonia and the Great Barrier Reef, Australia. Samples are now being collected from t Maldives to further investigate inter-regional variation.
Reproductive assessment combines histological technique with field-based observation of spawning aggregations examine (a) mode of reproduction, (b) size at sexual maturity, (c) timing and periodicity of spawning (annual lunar, and diurnal), (d) fecundity, (e) aggregation size a dynamics, and if possible (f) spawning. To meet the objectives of the reproductive aspect of the research, fish gonads have been collected and are currently being examined from Yap and Pohnpei State, Micronesia, the Marshall Islands, the Republic of Palau, and New Caledonia. These materials are being used in combination with dive observations to detail reproductive patter during the spawning period.
Observations from Pohnpei in 1998 have already produced some exciting results An aggregation of several hundred >1,000 Marbled Grouper was observed over several days around full moon during both March and April. The aggregation "core" covered 100-150 m of reef wall al depth of 25-55 m. Aggregations formed and persisted over less than one week and reached a peak density of 20-25 fish / 10 m2. Over 300 individuals were collected for histological examination and fecundity analysis. Preliminary analysis suggests that spawning occurred in the evening two days prior to full moon. The observations and collections from Pohnpei have already provided insights into variation in reproductive ecology between locales. Full moon spawning in March and April, contrasts sharply with new moon spawning observed in Palau in June-August and in the Marshall Islands from December-February. Additionally, aggregations in Pohnpei formed not at the channel aprons, as in Palau, but along the outer reef wall far away from the channel. These results pose a number of vital questions: What implications do the temporal variations have for the reproductive ecology and genetic separation of the species? What are the factors causing variations in annual and lunar cycles for Marbled Grouper? Do different stocks of Marbled Grouper spawn within the same month but at different lunar cycles at the same locale? Does this pattern repeat itself in other locales? Stay tuned for answers to these and other exciting questions.
P.20
What causes variation in limpet population structure between sites? A result of food availability and primary production?
by Richard Huang
Today I am going to tell you a story about a limpet, Cellana grata, which is common on Hong Kong exposed shores. C. grata is a dominant intertidal herbivore. During low tide, it rests in cracks or crevices often associated with the barnacle Tetraclita squamosa. As the tide comes in C. grata moves upshore and feeds whilst awash, retreating downshore with the ebbing tide.
I am studying the feeding ecology of Cellana grata as no one else has investigated this subject, and my supervisor suggested it! At the beginning of my study, during summer, I visited many sites around Hong Kong to make myself familiar with this animal and to look for good survey sites. I observed that the population structure of C. grata varied greatly from site to site; some sites have low-densities of large limpets whilst some have high-densities of small limpets. The cover of algae also varies between sites and shore levels; some sites have high cover of cyanobacterial films (e.g., Kyrtuthrix maculans) in the high shore and some have encrustmg algae (e.g., Pseudulvella applanata) in mid shore whilst other shores have no visible algal growth. Sites with Kyrtuthrix or Pseudulvella have higher chlorophyll levels than sites without visible algal growth. Regression analysis reveals a strong positive relationship between chlorophyll levels and limpet size, and a negative relationship between density and limpet size. I thus proposed that there was competition for food at 'high-density-small-limpet' sites which may cause stow growth rates and eventually result in small limpets. In contrast, at 'low-density-big-limpet' sites, food is unlikely to be a limiting factor due to the high algal growth, which allows limpets to grow at a fast rate and eventually, results in bigger limpet size.
This proposition, however, assumes that the limpets show no food selection and feed on Kyrtuthrix or Pseudulvella in proportion to their availability on the shores. In order to justify these assumptions, I examined limpet gut contents in tandem with life history patterns at two sites, Heng Fa Chuen (HFC) and Shek O (SO). In brief, HFC has a low density of big limpets and a high cover of Pseudulvella; whereas SO has a high density of small limpets and no visible algal growth. Gut content analysis reveals that limpets feed on Pseudulvella and cyanobacteria at HFC, and mainly on cyanobacteria at SO. The population at HFC is stable; mortality is low and both adult and juveniles grow fast. In contrast, the population at SO is unstable; adults grow more slowly than juveniles and die after the reproductive season. From these results, it appears that life history parameters such as growth rate and mortality could be controlled by food abundance and competition does act on high-density populations.
I cannot understand, however, why low densities of limpets are found at sites with a good food supply and high densities of limpets are found at seemingly food limited sites. I have some clues, though, from examining recruitment at the sites. I found that HFC has relatively poor recruitment when compared to SO. Thus it seems that different population densities could be due to variation in recruitment. But why does recruitment vary between sites? I am not satisfied with the simple argument of "natural variation", however, an answer may be provided by my investigations of primary production rates around Hong Kong. I found that SO has higher primary production during the recruitment period than HFC. I think, therefore, that there maybe a relationship between recruitment and primary production (benthic and planktonic food supply). Cellana grata is an external fertilizer with planktonic larvae and may be stimulated to settle where production rates are high.
Production rates do vary greatly between sites, but so do recruitment rates, for example at Cape d'Aguilar (CDA), primary production was as high as SO but recruitment was comparatively less; but greater than at HFC. What is causing this? I suggest that the large limpets at CDA are likely to have an effect on recruitment due to their bulldozing effect (scraping and eating new settlers), therefore reducing recruitment.
From a variety of observations (gut content analysis, life history patterns and primary production rates), it seems that I may have uncovered the reason for variation in Cellana grata populations on Hong Kong rocky shores. My logical proposition is, so far, only supported by observations and measurements, not manipulative experiments. In order to draw more firm conclusions I am going to manipulate food abundance as well as limpet density to test my ideas. I have a question, however: Are my results from mensurative experiments enough to justify my conclusions?
I hope you enjoyed this story......
P.21
A new site for the seagrass Halophila beccarii in Hong Kong, and some notes on the ecology of Starling Inlet
by Captain Wong
In May 1997, a 1m2 patch of seagrass was spotted by accident, when I was using a telescope to observe the feeding behaviour of Little Egrets at Nam Chung, southern Starling Inlet. A sample was brought to the laboratory at Hong Kong University and identified by Ng Sai Chit as Halophila beccarii. However, the seagrass on the Nam Chung mudflat died out subsequently. From telescope observations, I suspect that a larger patch of seagrass (probably also H. beccarii) is present on the mudflat between A Chau and Luk Keng Road.
In 1998, a new seagrass site was discovered in southern Starling Inlet. Four patches, from 1.0 x 1.0 to 1.5 x 2.0 m2, are present on the mudflat off Yim Tso Ha mangrove. Unfortunately, judging by the colour of the leaves, the seagrass is not very healthy. I think that the seagrasses have been present there since the beginning of this year.
Halophila beccarii was reported from Deep Bay in 1978 (Morton & Morton, 1983), and recently at Black Point (Prof. William Xing, pers. comm.), as well as at Tai Ho (Ng and Ecosystems Ltd., pers. comm.). It apparently occurs seasonally in these areas. H. beccarii is relatively rare in Hong Kong in comparison with the three other seagrass species - Zostera japonica, Ruppia maritima and Halophila ovata - which occur locally (Lee, 1997). All four species have fairly wide distributions along the coast of China.
Ecology of Starling Inlet, northeast New Territories
Apart from seagrass, waterbirds are another important ecological component at Starling Inlet. Egrets and herons forage in the shallow waters of the inlet, especially between A Chau and Nam Chung, where most of the nesting Great Egrets and Little Egrets feed during low tide. The waterbirds also utilize feeding area outside of the inlet, such as fishponds at Lai Chi Wo (some 8 km away).
The egretry on A Chau is the biggest colony in Hong Kong, in terms of number of breeding pairs and period. Night Herons are the dominant breeders but Great Egrets, Little Egrets, Cattle Egrets and Chinese Pond Herons also nest there. In 1997, Night Herons started to breed in mid-January and finished by the end of October. This long breeding period probably indicates that food supply is adequate in the nearby Yim Tso Ha mangrove and the Nam Chung fish ponds.
Being terrestrial feeders, nesting Cattle Egrets visit the Luk Keng freshwater marsh (where I also frequently saw two juvenile Wild Boar foraging during daytime in April and May 1998). Cattle Egrets also feed in abandoned fields at Luk Keng and Lai Chi Wo, where feral cattle are common. Agricultural fields at Dameisha, Shenzhen (15 km away), the northeast New Territories landfill (7 km away) and lawns of military camps near Fanling (8 km away) are also visited by nesting Cattle Egrets, so the A Chau egretry is located near the centre of their feeding sites.
My survey of waterbirds at Starling Inlet reveals that two coastal flats at Sha Tau Kok, which were saltpans in the 1930s, are attractive to waders such as Grey Plovers and Sandplovers. Up to 40 individuals and 14 species have been regularly seen there during low tide.
Wetland habitats at Starling Inlet are locally important. Mangroves and reedbeds at Yim Tso Ha are the second largest in Hong Kong, the reedbed at Kuk Po is probably the third largest in Hong Kong, and the freshwater marsh at Luk Keng attracts not only waterbirds but also macroinvertebrates such as dragonflies. Estuarine fish at the Inlet support more than 500 wading birds in winter and the biggest breeding wading bird population in Hong Kong. Of these habitats, the Luk Keng freshwater marsh is regarded by the Freshwater Wetland Survey (Dudgeon & Chan, 1996) as having the highest conservation value among freshwater wetlands in Hong Kong. Although these wetlands are ecologically important to local wildlife, they generally receive very little attention from local naturalists and conservationists, probably because this area is not immediately threatened by development. It is hoped that more visits and research can be carried out at Starling Inlet, so that its importance can be better understood.
Lee, S Y (1997). Annual cycle of biomass of a threatened population of the intertidal seagrass Zostera japonica in Hong Kong Marine Biology 129:183-193.
Morton, B.S & Morton, J. (1983). The Seashore Ecology of Hong Kong. Hong Kong University Press, Hong Kong.
Dudgeon, D. & Chan, E.W.C. (1996). Ecological Study of Freshwater Wet/and Habitats in Hong Kong. Report for the Agriculture & Fisheries Department, Hong Kong Government.
P.22
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