Abou Ghosoun area showed a reduction in the number of sea cucumber species recorded along the period of study. In 2000, the total number of recorded species was 13 species, and the same number of species was recorded in 2006, while in 2016 only 7 species were recorded (Table 1). The species composition included four high-value species: Holothuria fuscogilva, Holothuria whitmaei, Holothuria scabra, and Thelenota ananas; six medium-value species: Actinopyga echinites, Actinopyga mauritiana, Bohadschia argus, Holothuria atra, Stichopus hermanni, and Stichopus horrens; three low-value species: Bohadschia vitiensis, Actinopyga miliaris, and Bohadschia similis.

The population density of sea cucumber species differed widely according to the period of survey due to the species behavior and level of exploitation. The surveyed site showed a uniform pattern of very low density in 2006 and 2016 after the high density recorded in 2000. Holothuria atra recorded the highest density among the recorded species; it recorded 128.1 ind./100 m2 in 2000, which decreased to 120.4 ind./100 m2 in 2006 and 87.4 ind./100 m2 in 2016. Holothuria fuscogilva recorded the second highest density among the recorded species; it recorded 84.2 ind./100 m2 in 2000, which decreased to 51.5 ind./100 m2 in 2006 and only 5.7 ind./100 m2 in 2016. The dramatic decrease in species densities was observed in the species recorded as in Thelenota ananas which decreased from 48.1 ind./100 m2 in 2000 to only 5.6 ind./100 m2 in 2006 and completely disappeared during 2016. The same situation was recorded for Stichopus horrens, which was 42.4 ind./100 m2 in 2000 to only 8.8 ind./100 m2 in 2006 and completely disappeared during 2016, and Holothuria scabra, which was 65.6 ind./100 m2 in 2000, decreased to only 10.3 ind./100 m2 in 2006, and completely disappeared during 2016 (Table 2).

The total abundance of all sea cucumber species recorded during 2000 was 13,880 individuals and decreased to 7700 individuals in 2006 and then to only 2420 individuals in 2016. The remaining individuals in 2016 comprise only 17.4% of the original population recorded in 2000, which means that 82.6% of the sea cucumber populations were lost during the 16 years.

The different sea cucumber populations showed differences in abundance during the period of study. Holothuria atra showed the highest abundance among all the sea cucumber species, with 2560 individuals recorded in 2000; it also showed minimum loss in the number of individuals during the years of study (2400 and 1740 individuals in 2006 and 2016, respectively) and high remaining percentage (67.9%), and the loss in the population was only 32.1% of the population. For Holothuria scabra, 1300 individuals were recorded in 2000, which dramatically decreased to only 200 individuals in 2006 and completely disappeared in 2016; with 0% of remaining individuals and 100% of loss, all the population vanished in 2016. The same situation was recorded for Actinopyga mauritiana that decreased from 520 individuals in 2000 to 180 individuals in 2006 and was completely destroyed in 2016 and Bohadschia similis that decreased from 700 individuals in 2000 to 360 individuals in 2006 and was completely destroyed in 2016. The populations of Bohadschia argus, Stichopus horrens, and Thelenota ananas exhibit the same recorded pattern of 680, 840, and 960 individuals, respectively, in 2000 which decreased to 440, 160, and 100 individuals in 2006 and disappeared in 2016 (Table 3).

The relative abundance of different sea cucumber species were calculated, and it was obvious that there were differences in relative abundance between different species and during the different years. In 2000, the relative abundance ranged between 3.03% for Actinopyga miliaris and 18.44 for Holothuria atra. In 2006, the relative abundance ranged between 2.08% for Stichopus horrens and 31.17% for Holothuria atra. While in 2016, the relative abundance ranged between 0% for several species and 71.9% for Holothuria atra (Table 4).

The total biomass recorded showed a high temporal variation. It decreased from 10,373.8 kg in 2000 to 5461 kg during 2006. The decrease of the total biomass continued during 2016, recording 1379.6 kg (Table 5). Holothuria atra was recorded with the majority of the biomass with a total of 1254.4 kg wet wt in 2000 which decreased to 1176 kg wet wt in 2006 and to 852.6 kg in 2016. Holothuria scabra was one of the high-value species found during the current study, and it was recorded that its maximum biomass was 1254.4 kg wet wt in 2000 and decreased to only 176 kg in 2006, and completely disappeared in 2016.

The percentage of the remaining total biomass from the original population in 2000 was recorded at 50.86% in 2006; this represented a reduction equal to 49.24% in biomass, while in 2016 the remaining populations’ total biomass was recorded at 12.85% with a reduction of 87.15% from the original populations in 2000 (Fig. 2). The reduction of Holothuria atra represented only 6.2% in 2006 and 27.5% in 2016. The remaining biomass of Holothuria scabra population was recorded at 15.4% in 2006 with an 84.6% reduction of the original population in 2000, completely disappearing in 2016.

The percentage of sea cucumber biomass remaining in 2006 and 2016 from the original populations in 2000

The distribution pattern of each sea cucumber species was determined using the coefficient of dispersion. The study revealed a difference in dispersion among the different years of investigation. In the year 2000, nine species out of the 13 species recorded exhibited regular distributions while the other four species exhibited regular or uniform dispersion (Table 6). The situation recorded little difference in 2006 where only four species recorded regular dispersion, six species recorded clumped dispersion, and for the first time three species showed random dispersion. In 2016, from seven species recorded, four species showed clumped dispersion and three species showed random dispersion. The species showed a clumped dispersion tendency in 2006 and random dispersion in 2006, indicating a change in species behavior during the years of study (2000–2016).

A one-way ANOVA statistics was carried out for the species density data during the current study; the analysis showed a significant difference in species density between the years of study (2000, 2006, and 2016) and for all species (Table 7).

Hence, the one-way ANOVA analysis revealed a significant difference in all species in the different periods of study; we carried out a post hoc test (Scheffe test) to determine the significant difference between the different periods. The test revealed that there was significant difference recorded in species density between 2000 and 2006, 2000 and 2016, and 2006 and 2016 for, Actinopyga echinites, Actinopyga mauritiana, Actinopyga miliaris, Bohadschia similis, Bohadschia argus, Bohadschia vitiensis, Holothuria scabra, Holothuria whitmaei, Holothuria fuscogilva, Stichopus hermanni, Stichopus horrens, and Thelenota ananas, whereas Holothuria atra showed insignificant difference between the years 2000 and 2006, while the same species recorded a significant difference in its density between the years 2000 and 2016, and 2006 and 2016. The analysis showed that all species decreased significantly earlier (2000–2006) whereas only Holothuria atra decreased in a second phase (2006–2016).