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Abundance, distribution and some biological aspects of Holothuria edulis in the North Western coastal waters of Sri Lanka
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The large and conspicuous holothurians on the tops of 46 reefs in Torres Strait were sampled in February 1995, November 1995 and February 1996 as part of a project to map the reef habitats and survey the marine resources of the reefs. A total of 1,209 sites were sampled on the tops of the 46 reefs; and 310 sites were
sampled along the edge of 44 of these reefs. Holothurians were collected along 20 x 2 m transects, counted and weighed at all sites sampled on the tops of the reefs. Holothurians were counted along the 40 x 4 m transects sampled at the edge of the reefs. A total of 1,637 holothurians from 34 species were identified and surveyed.
A Bermudan population of the fissiparous holothurian Holothuria parvula (Selenka) was sampled over a 13-month period (1984–1985). Fission was most frequent in the summer when water temperatures were > 25 °C. During fission, the holothurian split into roughly equal parts, and there was little difference in survival of the oral and anal ends. Regeneration of a new gut is a priority and feeding was possible within 2 months of fission. The majority of growth following fission occurred between April and July, just prior to the peak occurrence of fission. Many individuals were fully regenerated within a year, so fission is possibly an annual event. Individuals showing evidence of multiple fission were found. The capacity for sexual reproduction was limited and it appeared to occur mainly during the summer, which was also the peak period for asexual reproduction. No small (< 18 mm) individuals were ever found suggesting that larval recruitment to this population had not recently been successful. The population has probably been maintained recently by fission.
This paper provides preliminary results on the present status of the sea cucumber fishery in Sri Lanka. At present, the fishery is restricted to the northwestern and eastern parts of the country. Sea cucumber fish- ing activities are greatly influenced by the monsoon. About 4,000-5,000 families are dependant on sea cucumber fishing activities. The major sea cucumber processing procedures include grading and cleaning, evisceration, boiling, store in salt or burying, boiling (second time) and drying. The entire annual produc- tion is currently exported to Singapore, Hong Kong and China. The fishery is open access, and there are no regulations or precautionary approaches used, except for issuing licenses for diving and transportation. After realizing the needs of implementing suitable management plans for the sustainable use of sea cucum- ber resources in Sri Lanka, the National Aquatic Resources Research Development Agency began a project under the technical assistance of the Food and Agriculture Organization of the United Nations.
The sea cucumber stocks off the east and northwest coasts of Sri Lanka were estimated by
surveying 1307 km2 and 1779 km2 by an underwater visual census (UVC)
in June and October 2008 respectively. The presence of twenty-five sea cucumber species
belonging to seven genera; Actinopyga, Bohadschia, Holothuria, Pearsonothuria,
Stichopus, Thelenota and Acaudina was identified in five
hundred sampling sites randomly selected in each area. The overall average density (± SD) of sea cucumbers was higher in the northwest (350 ± 648 ind ha-1) than in
the east (90 ± 130 ind ha-1) and low-value species were predominant in both
survey areas (79 ± 125 ind ha-1 in the east, 244 ± 488 in the northwest) when
compared with medium (10 ± 34 ind ha-1, 105 ± 175 ind ha-1,
respectively) and high-value category (<2 ind ha-1).
The estimated total biomass in the northwest (13 024 t) was roughly four times greater
than the east (3027 t). Holothuria edulis was the most abundant species
in numbers while Holothuria atra had the highest stock biomass in both
areas. Sea cucumber densities were significantly different among the habitat
(p < 0.001) and depth categories. The highest
density was reported in rocky habitat associated with seagrass and / or macroalgae. The
density of low-value species was significantly high in 1–10 m depth category
(p < 0.001) in the northwest while other
commercial groups were dominant in 20–30 m depth range in both areas. Temporal banning of
the fishery for high and medium-value species would be a better management option to avoid
further stock depletion.
Some aspects of the biology of the sea cucumber Holothuria scabra Jaeger, 1935 were studied along the coast of Dar es Salaam. Samples of H. scabra were collected from Kunduchi and Buyuni between January and December 1999. The abundance of Holothuria scabra was higher in Buyuni (3951) than in Kunduchi (1176). However, the level of exploitation per year from far reefs was higher in Kunduchi than in Buyuni. Sea cucumber harvesting by villagers is done monthly on intertidal areas and from reef flats near to the villages. Higher catches from Buyuni were recorded during August and November (perhaps due to light winds such that trips could be made to far-off reefs), while the catch from Kunduchi was relatively low throughout a year. The length-frequency distribution of H. scabra was found to be unimodal, with most individuals ranging between 8.5 cm and 26.5 cm. The species showed continuous breeding with peaks between May and September and December. The average size at first maturity in H. scabra was 16.8 cm.
Surveying the distribution and abundance of Holothuria scabra (sandfish) in Moreton Bay
To determine the effect of asexual reproduction by transverse fission on the
population structure of holothurians, ensities and individual weights of
Holothuria atra (JÄÄger,1833)and
Stichopus chloronotus (Brandt, 1835)were measured over a
26-month period on near-shore reefs of the Great Barrier Reef. Densities of
H.atra (0.01—0.69 individuals m
–2 )and S. chloronotus
(0.11—1.67) did not increase during periods of intense asexual
reproduction, and the density on each reef remained relatively stable.The
effect of fission was clearly visible in the weight —frequency
distributions of both species. The average individual wet weight in each
population declined (H. atra from 105/134 g to
64/94 g in two respective populations;S. chloronotus
from 128/302 g to 82/190 g in two respective populations) with the
onset of fission in early winter, and more individuals in the weight range of
fission products were found. The biomass followed the seasonal trend of the
average weights for both species, with minimum values in winter. A conceptual
model based on these and previous findings identified five possible factors
(mortality, habitat stability, optimum individual size, food availability,
larval supply) involved in promoting or repressing transverse fission, and the
model indicated the consequences for the population.
— Juveniles of four species of tropical sea cucumbers of moderate to high commercial value were studied to determine their potential for being propagated asexually by evaluating their survival and regeneration times after being forced to undergo transverse fission. The four species were Holothuria fuscogilva.H. nobilis, Actinopyga mauritiana, and Stichopus variegatus. Rubber bands placed midbody on the sea cucumbers provided an effective yet simple technique to induce fission. Posterior parts of animals had similar or higher survivorship and shorter regeneration times relative to the anterior parts. Combining this information with that collected earlier on adults indicated that smaller animals (both anterior and posterior parts) had higher survivorship (up to 100%) and shorter regeneration times (as fast as 41 d) relative to adult animals of the same species. Using percent weight retained after processing into beche-de-mer, a rough measure of body wall thickness, and size of the animal relative to the adult weight, a multiple regression equation was calculated to predict percent survival and regeneration times for both anterior and posterior parts. The equation illustrated that small, thin-walled species would have the highest survivorship and shortest regeneration times. As the percent weight retained increased, as would be the case with more thick-walled species, and as the animal approached the adult weight, survivorship declined and the regeneration time increased.
Asexual reproduction by fission was monitored for 18 mo in populations of Holothuria (Halodeima) atra, H. (H.) edulis and Stichopus chloronotus on three nearshore fringing reefs and one midshelf reef in the Great Barrier Reef. Fission in S. chloronotus occurred exclusively between March and October, with a peak value of 31% recently divided individuals in one population in
July. H. atra showed a similar pattern, with maxima of between 16 and 26% from May to July. In H. edulis, asexual reproduction occurred only between March and July, with a maximum of 17% recently divided individuals in March.
Fission rates of H. atra and S. chloronotus in winter were significantly higher than in all other seasons. For H. atra, at least 76% of all individuals at Fantome Island were estimated to undergo fission per year, whereas only 9% undergo fission
on the midshelf reef. Highest annual fission rates (43%) for S. chloronotus were found in a dense population on Great Palm Island. The lower-density midshelf reef population exhibited comparatively
lower annual fission rates (19%). About 24% of H. edulis undergo fission each year. Annual fission rate and population density were positively correlated in the four populations
of S. chloronotus and H. atra studied.
This study investigated the daily activities of juvenile sea cucumbers Holothuria scabra Jaeger. All individuals exhibited daily burrowing and feeding rhythms in response to environmental factors. The cycle of the smallest juvenile, >10–40 mm, was linked to light; they began to burrow around sunrise and emerged close to sunset. Their burrowing activity was inhibited by continuous darkness. Juveniles >40–140 mm responded to changes in temperature; they burrowed earlier around 03:30 h, seeking shelter in the sediment as temperature declined, and emerged around mid-day. The maintenance of a constant warm temperature prevented them from burrowing. For all juveniles, time spent on the surface corresponded with feeding and periods of locomotion; while burrowed they remained stationary, did not feed and had a low intestinal transit. Organic matter content in the intestine was also found to vary with the daily cycle. Decreases in salinity from 35 to 30, 25 and 20‰ induced the burrowing of all juveniles within minutes, but they began to re-emerge after a few hours. Acclimation occurred most rapidly at salinity 30‰ and was slowest at salinity 20‰. Conversely, nearly 40% of the juveniles were unable to cope with a decrease to salinity 15‰. Juveniles of all sizes demonstrated a strong selectivity for sediment characteristics. In two substrate selection experiments, their preference for sand with a grain size around 0.4 mm and for organically rich material was firmly expressed within an hour. The general behavior of juvenile H. scabra reflects their ability to inhabit shallow sandy areas with high terrigenous inputs and variable environmental conditions.
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