National Undersea Research Laboratory - Quarter 1, 2001 Milestone Report

Project: Aggregation of deepwater fish communities to precious coral beds: implications of coral harvest to sub-photic seal foraging.

Principal Investigator: Frank Parrish

Overview

Submersible time awarded in 2000 to the National Marine Fisheries Service, Honolulu Laboratory was used to census known deepwater coral beds of the Hawaiian Archipelago. The survey collected data on the number and sizes of corals and fish within precious coral beds and in adjacent habitats. These surveys comprise a baseline data set with which to evaluate the impacts of future coral harvesting activities. Concern about potential impacts of coral harvesting on deepwater fish assemblages is relevant because it has been documented that foraging monk seals feed around deepwater coral habitats. In 1998, surveys with HURL submersibles found coral beds at sites where seals fitted with satellite tags and dive recorders had repeatedly foraged at sub-photic depths (Fig 1.). One hypothesis is that colonies of deepwater corals tend to aggregate fish, and foraging monk seals may exploit this by frequently revisiting coral beds to improve their access to prey.

Sampling

The project planned to visit all known coral beds throughout the Hawaiian Archipelago. However, two sites (Makapuu and Brooks Bank) were lost to mechanical difficulties and poor weather. Complete surveys using the submersible and ROV were successfully conducted at banks of WestPac, East French Frigate Shoals, Kaena Point, Cross Seamount, Keahole Point, and one station added opportunistically in South Kona.

Coral occurrence and size structure

Significant stands of precious coral were identified at all of the surveyed locations except at Kaena Point and the South Kona site. Chronic erosional movement of the substrate likely caused the low abundance of corals at those sites. Pink coral was found in abundance at all the remaining beds except East French Frigate Shoals. Gold coral was found in abundance at beds of East French Frigate Shoals, Keahole Point, and Cross Seamount. The size of gold coral colonies ranged from recently recruited to fully mature with the best size distributions at Keahole Point and Cross Seamount. At French Frigate Shoals some gold coral colonies were marked for a growth rate measurement when the site is revisited by HURL in 2001. At the Keahole bed three colonies of pink coral were found for the first time growing on fallen fragments of dead gold coral in varying states of decay (Fig. 2). The bathymetry at French Frigate Shoals and Cross Seamount beds was partially mapped using the Seabeamä multibeam bathymetric acquisition system (Fig. 3). The bathymetry and submarine surveys at the French Frigate Shoals beds indicate that the size of the bed is less than 0.5 km². Corals at Cross Seamount were found on some of the pinnacles on the northeast side of the summit.

Fish assemblages

Some deepwater fish species clearly associate with relief features including colonies of deepwater corals. However, the hypothesized aggregation of deep-water arrowtooth eels (e.g., Meadia Abyssla) to corals was not evident in these surveys, and at Cross Seamount eels were found in abundance outside the coral beds. At WestPac Bank armorhead (Psuedopentaceros wheeleri) were found hiding under ledges in areas adjacent to precious coral beds. Rigorous analyses of densities and the size structure of the fish assemblages for this baseline are in process and will be used to evaluate future impacts to the fish community at sites where corals are harvested.

Ecological Implications

The roughly equivalent numbers of young and old gold coral colonies found at Keahole Point and Cross Seamount suggest healthy recruitment to the population. The surveys at French Frigate Shoals and Brooks Banks indicate fewer recruiting colonies and perhaps more episodic recruitment. Determining the growth rate of gold coral remains an important focus of future study. The pink coral found growing on the dead gold may permit a generalized estimate of the decay rate of gold coral. If a decay rate can be estimated, the relative abundance of dead versus live gold coral can be used to infer a growth rate for gold coral. Based on this, speculations of the growth rate range between 5 and 13 cm a year. The re-measurement of marked colonies at French Frigate Shoals in 2001 should provide some check on these estimates. The species composition of fish assemblages differs between the coral beds. At some locations a fish species can be ubiquitous and at other locations it is totally unrepresented. Little more can be said about fish until a full analysis is completed.

Management Implications

Given the lack of corals at Kaena Point, this bed should be removed from the management plan. The coordinates of the Keahole Point bed in the precious coral fishery management plan should be changed to reflect the area identified in the 2000 survey. The corals found at Cross Seamount represent a significant resource and will require additional surveys to establish the extent of their distribution on the summit of the seamount. Fish aggregating to coral colonies was not obvious in these surveys, and the analysis of the data will have to focus some effort on separating the effects of abiotic structural relief (e.g., rock pinnacles) from whatever effect the corals have on fish distributions. Finding fish assemblages (e.g., armorhead and arrowtooth eels) in habitats adjacent to coral beds, but not in coral beds, should be considered as one possible explanation for the seals’ sub-photic activity in the vicinity of precious coral beds. This investigation has yet to inspect the coral beds of Makapuu or North Kona where the corals have been commercially harvested. Surveying beds that have been harvested is a priority for HURL dives in 2001.