Principal Investigator: Dr. Richard Grigg
Co-Principal Investigator: Dr. Christopher Kelley
Progress summary:
Significant progress has been made toward meeting all of the goals of the project. A new bathymetric map (Figure 1) of the entire black coral bed off Maui has been created and will be used as a basis to refine estimates of Maximum Sustained Yield (MSY). Completed fieldwork has clarified the timing of the annual spawning cycle. Six submersible dives in the Pisces V submersible have been completed. Two papers have been published in the refereed scientific literature and a third has been submitted.
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| Figure 1. Map of ‘Au‘au Channel showing drowned basins, ridges and pinnacles and location of prominent locations (Stonewall, Circus, Most Radicals and Shark Ridge) and previous dives in Pisces V (PV-N), ROV's (RCV-N), Deepworker 2000 (DW), and with SCUBA dives (S). The habitat of A. dichotoma and A. grandis is restricted to clean swept hard-ground topography at depths between 30-110 m in the Channel. |
The objectives of the study included, 1) Update stock assessment of black coral beds using new multibeam sonar bathymetric maps and bottom surveys and recalculate MSY for the beds, 2) Determine lower depth limit of black coral populations and determine reproductive condition of deep refugial populations of black coral, 3) Evaluate present and potential increased rate of harvest of black coral on the sustainability of the bed, and 4) Quantify the essential fish habitat (EFH) for bottom fish associated with black coral beds and the impacts of coral harvesting on bottom fish. The deep (75-110 m) segment of the population has never been quantified and maybe important in terms of contributing to recruitment within the overall bed.
Efforts:Over 25 hours of bottom time devoted to characterizing black coral beds were logged during the 6 Pisces V dives and 4 ROV deployments in the black coral bed off Maui in November, 2001. Dive and ROV sites were selected in order to survey the known geographic limits of the bed. Methods used to estimate patterns of abundance and size of both species of black coral were visual counts and estimates of colony height along 30 minute transects conducted on all six Pisces V dives, as well as, continuous video records on all Pisces V and ROV dives. Black coral colonies were also collected for a reproductive study. An additional Pisces V dive, and portions of the other dives, focused on stock assessment of bottom fish. Two transects and 4 bait stations were conducted to achieve objective 4. Each transect was 30 minutes long during which all fish encountered were identified and counted. Particular attention was paid to species that appeared to be associating with black coral. Bait stations were conducted to attract and record commercially valuable species of bottomfish in or near black coral beds. At each station, approximately 5 lbs of ground fish and squid (50/50 mixture) was placed on the bottom near the submersible, which remained stationary with all external lights turned off.
Customers:This research is designed to improve the current Fishery Management Plans (FMP's) for black corals. The results will be published in the scientific literature and made available to regulatory agencies. Agencies at both the State (Division of Aquatic Resources, Department of Land and Natural Resources) and Federal (Western Pacific Regional Fisheries Management Council) level that have jurisdiction over the resource will gain information which will help them to effectively manage the black coral fishery. The precious coral industry benefits over the long-term by virtue of a sustainable supply of black coral. The scientific community receives valuable data on the habitat conditions of the ‘Au‘au Channel between Maui and Lana‘i.
Significance:The black coral fishery in Hawaii has been sustainable for over forty years (Grigg 2001). As such, it is one of the few fisheries in the world that has enjoyed this kind of history. Management by the State of Hawaii and past compliance with regulations by the divers and the industry (buyers of raw material) have contributed to maintaining sustainability. Two species of black coral, Antipathes dichotoma and A. grandis, make up the bulk of the yield. Their dried skeletons constitute the raw material that is used to manufacture black coral jewelry. Three impending changes now threaten continued sustainability of the resource; 1) Increasing demand for the resource, 2) Gradual reduction in biomass produced by long-term harvesting, and 3) Recent accidental introduction of an alien species, Carijoa riisei. The significance of this research and transfer of information to the private sector and government agencies, is the benefit of a well managed fishery versus one in which the resource could be easily over-harvested.
Success:Seven submersible dives in the Pisces V submersible and four ROV deployments with have been completed. These dives allowed quantification of the size structure and abundance of black coral at depths below the limits of SCUBA. The dives were also used to evaluate bottom fish abundance patterns. No direct correlation between the abundance of bottom fish and black coral populations was found.
Observations and video records obtained during the above bottom surveys confirm previous estimates of depth range for both species, 30-110m for A. dichotoma and 45-110 m for A. grandis (Grigg, 1976). The average density of both species on all transects at depths between 40-110 m was 0.10 ± 0.05 colonies/m2. On suitable substrata, below 75 m, the density of both species of black coral was found to gradually decrease eventually dropping to zero at their lower depth limit at 110m. As black coral gradually decreased in abundance between these depths, Carijoa riisei, an invasive alien species, was found to gradually increase in abundance. On ROV transects 122, 123, and 125, at the northern edge of the bed, Carijoa was found to dominate substrata at depths of 80-110 m and in the process was overgrowing most (>75%) colonies of black coral (Figure 2). C. riisei was also observed at shallower depths between 50-70 m, but in much lower abundance and confined to undercut or well shaded surfaces. The deep-reef segment (75-110 m) of the population is too deep for harvest with conventional SCUBA, but presumably it is important in contributing to larval recruitment for the overall bed. If Carijoa continues to proliferate, the quantitative importance of this impact may become increasingly more significant.
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| Figure 2. Carijoa riisei (white polyps), introduced to Hawai‘i in 1972, shown overgrowing A. dichotoma (brown polyps) at 85 m (top) and overgrowing much of the substrata at 100 m (bottom) along RCV transects 123 and 125, in the ‘Au‘au Channel, Maui. | |
Observations of the habitat of both species of black coral at depths between 40 and 75 m, showed it to be relatively restrictive confined to vertical or near vertical substrata and/or under-cut terrace morphology. Many of these features are remnant products of former sea-level low-stands during Pleistocene interstadial events (Grigg et al, 2001, Fletcher and Sherman, 1995). Virtually all of the hard ground topography in the ‘Au‘au channel consists of limestone outcrops. This morphology was produced by karst erosion during lower sea-level stands. Today it is accentuated by a thin veneer of Holocene reef growth that occurred as sea-level gradually rose and flooded the channel during the last transgression.
A revised estimate for the standing crop of both species of black coral over the depth range of harvest (40-75 m) was produced by extrapolating mean density (0.01 col/m2) along all suitable steep-sided topography identified on the new USGS bathymetric map (Figure 1). Most of these topographic features represent solution ridges remnant of karst erosion as described above. The total linear extent of these ridges at depths of 40-75 m is 39 kilometers, producing a standing crop of 117,000 colonies (39,000 m x 30 m x 0.01 col/m2). This value is actually 46% greater than a similar estimate produced in 1976 (Grigg, 1976). However, since 1976, mean colony age and weight has changed from 14 years and 2.2 kg respectively then, versus 10 years and 1.1 kg, in 2001. The weight of a colony of mean height today is half of what it was in 1976!
Thus, in spite of a higher estimate of standing crop obtained in the 2001 survey compared to 1976, the biomass of the 2001 standing crop has decreased by 23%. This result is undoubtedly a result of harvesting which has progressively and selectively removed more and more of the largest colonies from the population. This shift toward smaller colonies is readily apparent in the 2001 age-frequency distribution (Figure 3). Not only are there fewer large colonies, but there is also an indication of reduced recruitment. The low values of recruitment in the first and second year classes as seen in 1975 and 1998 surveys of the bed are considered artifacts owing to the difficulty in seeing small colonies (Figure 3). However, in 2001 the reduction in young year classes extends out to year five. This decrease appears to be real and suggests the beginning of a slow-down in recruitment.
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| Figure 3. Age-frequency distributions of A. dichotoma and A. grandis (pooled) along transects taken at Stonewall and Most Radicals. The precision of year-class measurec by SCUBA using a meter rule in 1975 and 1998 was ± 1 year. The same measure estimated visually from the Pisces V submersible was less precise (±2.5 years). |
Fifty-four different fish species were observed during the transects, 45 of which were identified to species, 4 identified to genus, and 5 identified to family. Of these, five species were observed associating with black coral. However, these five, as well as the other 49 species, are commonly observed in areas and depths where black corals do not occur. Oxycirrhites typus, a species of hawkfish (Cirrhitidae) typically found on black coral trees, was not recorded on either transect. Due to its cryptic nature, however, it's likely that this species was present but could not be seen from the submersible.
With the exception of the second one, these bait stations were unproductive in comparison to those typically deployed for deeper studies on bottomfish. Only two snapper species were recorded, Lutjanus kasmira and Aprion virescens, the former being an introduced species with little commercial value. Three jacks (Carangidae) were recorded, two of which were identified to species. Seriola dumerili has no commercial value in Hawai‘i due to it being ciguatoxic. Carangoides orthogrammus is not caught in sufficient numbers to be considered a commercial species. Other incidental species recorded at the bait stations but not on the transects included Chaetodon multicinctus, Oxycheilinus unifasciatus, Myripristis kuntee, Neoniphon sammara, and Acanthurus blochii.
None of the species recorded during the transects or bait stations are known to be closely associated with black coral. In fact, all of these species are commonly observed in habitats where black coral is absent. The five species observed near black coral seemed to be using the trees for temporary shelter, possibly from the submersible. While our dataset is quite small, we tentatively conclude that harvesting black coral in these areas is unlikely to have a significant impact on bottomfish species.
To date, two papers describing the results of the project have been published peer-reviewed Journals and a third has been submitted:
Complete the study with regard to generating revised guidelines, which include a reduced estimate of the MSY and a larger size limit (48 inches vs. 36 inches in height for legal colonies of black coral), and recommend a monitoring program to the State. Two more Pisces V dives are planned for November 2002 that should yield more information.
ReferencesGrigg, R.W. 1976. Fishery management of precious and stony corals in Hawaii. UNIHI-SEAGRANT-TR77-03. 48 pp.
Grigg, R.W., E.E. Grossman, S.A. Earle, S.R. Gittings, D. Lott and J. McDonough. 2002. Drowned reefs and antecedent karst topography, ‘Au‘au Channel, S.E. Hawaiian Islands. Coral Reefs, 21:73-82.