Uthicke et al., 2004: 

"Commercially fished holothurians have important functions in nutrient recycling, which increases the benthic productivity of coral reef ecosystems. Thus, removal of these animals through fishing may reduce the overall productivity of affected coral reefs. To investigate the potential for recovery of over fished holothurian (Holothuria nobilis) stocks on the Great Barrier Reef (GBR), we (1) conducted field surveys on 23 reefs after fishery closure, (2) modeled total virgin biomass and compared it with the total amount fished, and (3) estimated individual growth rates with a DNA fingerprinting technique. Two years after fishery closure, no recovery of H. nobilis stocks on reefs previously open to fishing was observed. Densities on reefs protected from fishing since the onset of the fishery in the mid 1980s remained about four times higher than on fished reefs. Based on density estimates and geographic information system data on the habitat area of each reef, we calculated that the virgin biomass (in the main fished area between 12^o and 19^oS) was about 5500t and is now about 2500 t. The reduction is on the same order of magnitude as the total amount fished until 1999 (approximately 2500 t). The DNA analysis of repeated samples on three locations indicated high recapture rates of fingerprinted and released individuals of H. nobilis. Fitting growth curves with Francis’s growth function indicated that medium-sized individuals (1 kg) grew 35–533 g /year, whereas large animals (2.5 kg) consistently shrank. Small animals (<500 g) were rarely observed. In combination, these data indicate that production of H. nobilis stocks is very low, presumably with low mortality, low recruitment, and slow individual growth rates. Consistent with anecdotal evidence, recovery of H. nobilis stocks on the GBR may take several decades, and we suggest a highly conservative management plan to protect both the stocks and the ecosystem.

The problem of overfishing of holothurians also occurs in developed countries such as Australia. Despite being located in a World Heritage Site, the fishery on H. nobilis in the Great Barrier Reef (GBR) had to be closed in 1999. At that time, densities of this species on fished reefs were reduced to about 25% of those observed on reefs protected from fishing, and these species had become uneconomic to fish (Uthicke & Benzie 2000a).

The ecological consequences of removing (fishing) these animals is unknown, even though they have been shown to have important functions in their environment (Massin 1982; Birkeland 1988). On coral reefs, for example, holothurian populations can bioturbate the upper 5 mm of sediment (equivalent to 4.6 t/ha) once a year (Uthicke 1999). Because these animals digest bacteria, diatoms, and detritus (Yingst 1976; Moriarty 1982), they remineralize large quantities of organic nutrients (Uthicke 2001a). This provides an important loop in nutrient recycling, which increases the benthic productivity of oligotrophic systems such as coral reefs (Uthicke & Klumpp 1998; Uthicke 2001b). Therefore, it is possible that removal of these animals reduces the overall productivity of affected coral reefs.

Population genetic techniques show that populations of H. nobilis on individual reefs in the GBR are highly connected and that even populations from West Australia and on reefs in the Coral Sea are potential sources of recruits (Uthicke & Benzie 2000b, 2003). The genetic data indicate that on evolutionary time scales stocks can be replenished from a large variety of sources, but this may not be relevant on the ecological time scales required for fishery management. To investigate connectivity on these time scales, direct field measurements of recruitment are necessary but have usually not been undertaken because of the relatively small size of individual fisheries and because of limitations in funding sources. Population surveys are usually discontinued after overfishing has occurred, so there is little information on population recovery."

"Uthicke and Benzie (2000a) showed in the initial survey in 1999 that protected areas are effective in maintaining larger densities in populations only if the whole reef is protected. There were no indications of recovery 2 years after closure of the fishery for H. nobilis. Densities on the green reefs (protected from fishing even before the closure of the fishery) remained relatively stable, although hardly any new recruits were observed. This may be taken as evidence that animals are long-lived because densities should decrease if mortality is high and recruitment low. Densities of holothurians in the green and blue zones did not change significantly in those areas within the reefs divided into two zones. On two of the Ribbon reefs, densities were higher in the green zone, indicating that fishing had taken place in the blue zone. Because the density differences remained constant, it appears that no major migration occurred between the two zones. On two smaller reefs, the difference between the blue and green zone was not distinct. Densities on the blue reefs fluctuated but showed no sign of recovery overall.

Low or no recovery for several species 4 years after fishery closure has also been suggested in marine protected areas in the Solomon Islands (Lincoln-Smith et al. 2000). Anecdotal evidence suggests that recovery in some locations in the Indo-Pacific may take between 40 and 50 years. This agrees with historical fishing cycles for these species. For example, the fishery on the GBR commenced in the mid-1980s, after no fishing had taken place for over 50 years (Harriott 1985). Although this may be due partially to political reasons (e.g., the Second World War, integration of China into the world market), another reason may be that stocks needed this time to recover. Data obtained here cannot be used to predict recovery rates without further monitoring, but the absence of any sign of recovery suggests that the rebuilding of the stocks is not rapid and is likely to be on the order of decades. During
our surveys and many additional search surveys, we also looked intensively for the presence of small (<500 g) H. nobilis (potentially juveniles). On all reefs during both resurveys, however, we only found five such individuals. These observations provide further evidence that recruitment to H. nobilis populations is very low."

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Citation and/or URL

Uthicke, S., Welch, D. & Benzie, J.A.H. 2004, Slow growth and lack of recovery in overfished holothurians on the Great Barrier Reef: evidence from DNA fingerprints and repeated large-scale surveys, Conservation Biology, 18: (5) 1395-1404 

Spatial Coverage

 Northern and central GBR

Temporal Coverage

 1999-2001

Update Frequency

 Not applicable 

Other Information

 None