Coral Sea sea cucumber survey, 2017
Timothy Skewes, Sanna Persson. June 2017. Parks Australia.
Australia’s Coral Sea Commonwealth Marine Reserve (CSCMR) is an almost 1M km2 region off the Australian north-east coast.
It contains 18 emergent coral reef systems and is recognised as a large, diverse, and globally significant coral reef domain.
The CSMR currently has 2 National Nature Reserves (NNR) which were declared 1982 covering Lihou Reef, Coringa Bank and Herald Reefs.
At least 12 commercially valuable species of tropical sea cucumber have been fished on these reefs of the CSCMR for many years, by the Australian Coral Sea Sea Cucumber Fishery (CSSCF), and recently, illegally by foreign owned and crewed fishing vessels (FFV).
In April 2017, we surveyed sea cucumber populations on 8 reefs of the CSCMR, including the reefs of the NNR, with several reefs being surveyed for the first time.
We visited 109 sites at 15 different locations and collected data in 28 of 105 possible reef-habitat strata combinations (excluding deep terrace and drowned bank habitats; and Cato and Mellish Reefs); which contained approximately 51% of available shallow reef habitat area within the CSCMR.
The sample coverage was somewhat restricted due to depth restrictions and adverse weather and sea conditions.
Important reef habitats that remained under-sampled on surveyed reefs included:
- forereef habitat on most reefs
- reef pinnacles on Lihou Reef and Coringa Bank
- most deep lagoon habitats.
The survey was able to estimate the density and population biomass of 10 commercial species with sufficient precision to make some inferences about their biogeography and population status.
Species composition was extremely variable among reefs sampled, most likely due to a combination of differences in sea cucumber biogeography and fishing pressure.
The density of most sea cucumber species, and especially high value sea cucumbers was relatively low throughout the Coral Sea survey area, but especially those reefs that have been subject to most fishing.
This low density probably reflects a naturally low carrying capacity for Coral Sea reefs, but is also due to unsustainable fishing pressure by FFV in particular for high value species at least.
This is especially true for the southern reefs of the Coral Sea and also for the reef pass and sheltered deeper back reef habitats that were depleted of high value species.
Holothuria atra (Lollyfish) was the most abundant species in the study area, and were super abundant at several sites on Coringa Bank, usually on the shallow reef or backreef habitats in close proximity to coral cays.
Stichopus chloronotus (Greenfish) was the second most abundant species, and (similarly to H. atra) was found in a few high-density locations on the shallow reef flat in close association with reef cays, primarily on Coringa Bank and Kenn Reef.
The third highest density species was the high value species Thelenota ananas (Prickly redfish), which was found in the majority of habitats and on most reefs, but had its highest density in the reef passes and lagoon pinnacles; and on Lihou Reef.
The population biomass estimate of T. ananas was 1,903 t live weight (± 624 t; 90% CI) and made up over 30% of all sea cucumbers biomass.
Its fishery (landed) biomass estimate of 1,109 t (± 364 t; 90% CI) is encouraging given the modest estimate of catches for the Australian CSSCF (18 t) and observed illegal FFV catch (240 kg for one vessel).
The highly targeted high value species, Holothuria fuscogilva (White teatfish), was found in very low densities throughout the study area, but was particularly scarce on the southern reefs, and was not seen at all on Saumarez Reef or Wreck Reef.
H. fuscogilva has made up the largest component of the Australian CSSCF catch and has also been the focus of recent illegal fishing by FFV, indicating a strong causal link to unsustainable fishing pressure.
However, the estimated population size for H. fuscogilva from this survey will be underestimated due to limitations on survey depth (generally <20 m) and sea cucumber visibility, especially of juvenile H. fuscogilva.
It is likely that some stocks of this deep-water species still occur in deeper habitat on the southern reefs due to the predominance of H. fuscogilva in the recent catch of a FFV on those reefs.
Most larger reef systems in the Coral Sea have extensive areas of deep lagoon habitat in the 20 to 40 m depth range that may provide suitable habitat for a deep-water species such as H. fuscogilva.
Very little is known about these deep lagoon areas, making it difficult to assess the status of H. fuscogilva populations. Even so, it is likely that the shallower habitats at least have been depleted, either by the Australian CSSCF, FFV, or a combination of the two.
The density of the other high value species, Holothuria whitmaei (Black teatfish), was relatively low overall (1.64 per ha). Its density in the Lihou Reef NNR was only 2.6 per hectare in the present survey compared to 4.5 per ha in 2008; and 9.4 per ha in Torres Strait and 20.9 per ha on closed reefs on the GBR.
The low population density and downward trend of H. whitmaei would appear to indicate some fishery related depletion of this species.
Even so, the estimated biomass for H. whitmaei, 215 t (± 155 t, 90% CI) landed weight, was large in relation to the known annual catch of the Australian CSSCF of less than 2 t per year (Woodhams et al., 2015); and the estimated catch for one apprehended FFV of 0.6 t (Skewes et al., 2017).
It may be that undocumented FFV catch of this specie has occurred in recent times.
The deep lagoon reef pinnacles still have high densities of high value species (38.5 per ha for H. whitmaei and 17.2 per ha for H. fuscogilva) indicating that this habitat may not have been fished to a large extent, and may provide a significant source of recruitment for these high value species.
The protection afforded by the established NNR would also appear to have afforded some protection to the higher value species on these reefs, probably due to high levels of compliance by Australian CSSCF.
Low and medium value species (e.g. H. atra and S. chloronotus) have been only lightly exploited and most are likely to be at near virgin biomass levels.
There is also the possibility of large populations of diurnally burying species occurring in sandy reef habitats.
While this survey provided estimates of sea cucumber population density and biomass for several important reefs in the Coral Sea territory, the sampling was not representative of all reef habitats and the number of sample sites per reef was small.
Regular resurveys would provide more certainty to these estimates and provide information suitable for determining trends in sea cucumber density.
Every opportunity should be taken to gather additional sea cucumber density data for the CSCMR region, particularly of un-sampled reef-habitats (forereef habitat; reef pinnacles on Lihou Reef and Coringa Bank; and deep lagoon habitats); and un-sampled reefs (e.g. Flinders Reef, Osprey Reef, Willis Islets, Diane Bank, Tregrosse Reefs).
If revisiting already sampled reefs, the periodicity should be at least once every 3 years to maintain population trend connectivity regards year class strength and stock requitement considerations.
Sea cucumber fisheries globally have been overexploited, highlighting the need for careful and responsive management. Australian fisheries are among the few tropical shallow water sea cucumber fisheries globally to have continued viability, especially for high value species.
However, controlling illegal FFV activity is a high priority if the Coral Sea populations are to remain viable. The collection and analysis of fishery dependent data from the CSSCF, together with periodic surveys, should provide the basis for informed management decisions to promote sustainable utilisation and protect the values of the Coral Sea Commonwealth Marine Reserve.
1.1 Sea cucumbers as ecological indicators
Sea cucumbers are an appropriate ecological indicator species due to their important role in ecosystem functioning, and their risk from a range of threats, including fishing and climate change.
However, the extremely patchy distribution of some high-density species (e.g. H. atra and S. chloronotus) means there could be large changes in density and distribution due to natural environmental and recruitment related factors and the drivers of their distribution is poorly understood.
Additional surveys of sea cucumber density will be required in un-sampled reefs and some reef-habitat combinations before a there is a sufficient understanding of sea cucumber biogeography to make totally informed decisions about future monitoring strategies, especially if a Coral Sea wide assessment is the goal.
However, if the existing NNRs of Lihou Reef, Coringa Bank and Herald Cays is the focus, then the 3 known surveys carried out so far in those locations should provide enough information to formulate a survey approach suitable for monitoring sea cucumbers in those locations.
We make a series of recommendations on the design and implementation of monitoring strategies for sea cucumbers in the Coral Sea.
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