Destruction of a Holothuria scabra population by overfishing at Abu Rhamada Island in the Red Sea

• Published in: Marine environmental research 2005-10, Vol.60 (4), p.489-511
• Main Author: Hasan, Mohamed Hamza
• Format: Article
• Language: English
• Subjects: Abu Rhamada Island; Animal and plant ecology; Animal, plant and microbial ecology; Animals; Applied ecology; Biological and medical sciences; Biomass; Conservation of Natural Resources; Ecology; Exploitation and management of natural biological resources (hunting, fishing and exploited populations survey, etc.); Fisheries; Fundamental and applied biological sciences. Psychology; Holothuria; Holothuria scabra; Indian Ocean; Marine; Overfishing; Population Density; Population Dynamics; Red Sea; Reproductive biology; Sea water ecosystems; Seasons; Synecology
• ISSN: 0141-1136; 1879-0291
• DOI: 10.1016/j.marenvres.2004.12.007

Populations of Holothuria scabra at Abu Rhamada Island were investigated during 52 months, from July 1999 to October 2003. During the first 23 months (July, 1999–May, 2001) the Island had a robust population with a tri-modal size frequency distribution curve, very high densities (85.7–95.1 ind./100 m 2 at the sandy habitat), high abundance (3362–3110 individuals) and biomass (46.7–34.3 kg/100 m 2). Also, during this period most individuals were at depths between 4 and 6 m and no individuals were recorded deeper than 15 m. The population declined after harvesting began (June, 2001) and by March, 2002 the size frequency distribution showed a bimodal pattern with an obvious decrease in abundance of large individuals. There was also a slight reduction in densities (73.2–60.1 ind./100 m 2 at the sandy habitat), abundance (2292–1682 individuals) and biomass (21.6–11.3 kg/100 m 2), and a marked shift towards deeper waters. Overfishing reached its maximum during the final 19 months of the study, and by October, 2003, density (30.7–0.4 ind./100 m 2 at the sandy habitat), abundance (802–10 individuals) and biomass (6.9–0.1 kg/100 m 2) were all greatly reduced. The size frequency distribution of the population became unimodal, large animals disappeared and no recruits were seen. During this period, individuals were found at very deep depths (30 to >40 m). The study also showed that sandy substrate was the preferred habitat for H. scabra, accommodating the largest number of individuals. The population of H. scabra at Abu Rhamada Island was found to spawn biannually from 1999 to 2001, then only once during 2002 when high fishing pressure occurred, and ceased completely in 2003. The sex ratio was not significantly different from 1:1 before fishing begun, but shifted to an increasing male bias reaching 93% males by January 2003. None of the small animals remaining after January, 2003 could be sexed. Size at sexual maturity decreased from prefishing (185 mm for females and 160 mm for males) to 155 mm for females and 125 mm for males in January 2003. There was a positive relationship between fecundity and size. And oocyte/female was highest in 1999 (0.73–1.7 million) and 2000 (0.75–1.72 million), decreased during 2001 (0.2–0.85 million) to reach its minimum at 2002 (0.28–0.29 million).