Research Aims to Develop Fishery-independent Methods for Assessing Coral Reef Fish Populations in the Main Hawaiian Islands

September 1, 2012
Survey divers record information on the abundance of all reef fish species encountered, like these penantfish 
                 (Heniochus diphreutes).
Survey divers record information on the abundance of all reef fish species encountered, like these penantfish (Heniochus diphreutes).
Small boats launched from the Sette are used to shuttle divers between survey sites.
Small boats launched from the Sette are used to shuttle divers between survey sites.

During September 1-13, 2012, the NOAA Ship Oscar Elton Sette is conducting a research cruise to improve assessments of reef fish populations in the main Hawaiian Islands. The focus is on reef habitats around Oahu and the Maui-nui complex of Maui, Lanai and Molokai. A primary cruise objective is to compare fishery-independent methods for assessing fish populations on coral reefs. The data gathered will also supplement accumulated information on reef fishes gathered by the NOAA Pacific Islands Fisheries Science Center (PIFSC) as part of NOAA's Pacific Reef Assessment and Monitoring Program.

The 13-day research expedition is led by PIFSC associates Dr. Jill Zamzow and Kevin Lino of the Joint Institute for Marine and Atmospheric Research of the University of Hawaii (JIMAR). The research team includes scientists from PIFSC, JIMAR, and collaborators from Hawaii Division of Aquatic Resources.

During the expedition, 2 methods will be used to assess the abundance of reef fish. At about 200 selected survey sites in hardbottom habitats in waters < 30 m deep, SCUBA divers will conduct a stationary point count survey. The sampling locations are selected using a stratified-random survey design. At a subset of the selected sites, the survey team will place stationary stereo-video cameras (BRUVS) on the seafloor. Data collected in the shallow-water habitats where both methods are used will be compared to help in the development of a diver-independent capability for reef fish assessment.

The BRUVS will also be deployed at additional survey sites in 30-100 m deep habitats, waters too deep for safe SCUBA operations. Together, the survey operations will enable documentation of reef fish assemblages across the spectrum of reef fish habitats. These and other related survey efforts are designed to improve the ability of PIFSC and partners to generate an accurate picture of the status and trends of coral reef fishes around the Hawaiian Islands.

Stationary Point Counts

Stationary point count
In the stationary point count method, divers situated on the seafloor record the number of fish of each species visible within a cylinder 15 meters in diameter centered at the sampling location.

The stationary point count (SPC) method involves a pair of SCUBA divers conducting simultaneous counts in adjacent visually-estimated 15 m-diameter plots extending from the substrate to the limits of vertical visibility. Prior to beginning each SPC survey operation, the divers lay out a 30 m line across the seafloor. Markings on the line at 7.5 m, 15 m and 22.5 m enable the divers to locate the mid-point (7.5 m or 22.5 m) and two edges (0 m and 15 m; or 15 m and 30 m) of the survey plots. Each diver conducts the SPC survey in 2 steps. The first is a 5-minute species enumeration period in which the diver records all species observed within their cylinder. Following that is a tallying step, in which the diver systematically works through their list of species, and for each species counts and records the number of fish in the survey plot, along with an estimated size (total length, TL, to nearest cm) of each fish seen. The tallying step is conducted as a series of rapid visual sweeps of the plot, with one species-grouping counted per sweep. To the extent possible, divers remain at the center of their cylinder throughout the counts. In cases where a species is observed during the species enumeration step but not seen during the tally sweeps, the diver records their best estimates of the size and number of fish observed during the species enumeration step and marks the data record as 'non-instantaneous'.

During the 2-step SPC procedure, small and cryptic species will tend to be underrepresented in the counts made by an observer remaining in the center of the cylinder. So to estimate the abundance of these species, after the tallies are done the diver leaves the center observation position and swims through the plot, carefully searching for small and cryptic species and counting them.

BRUVS

BRUVS
A BRUVS system is used in both shallow and deep coral reef habitats to record stereo video of reef fish populations.

BRUVS are baited, remote, underwater video stations. The non-destructive stereo video samplers can provide scientifically rigorous estimates of fish abundance and size structure. BRUVS were originally developed in the laboratory of Dr. Euan Harvey at the University of Western Australia. The use of stereo cameras allows scientists to obtain accurate estimates of the number and size of fish attracted to the station and enables an understanding of each species' local length frequency distribution and biomass density. Each of a group of up to 8 BRUVS units is deployed for approximately 15 minutes, then recovered and placed at a different sampling location; in this manner, the units are deployed in a "leap frog" fashion throughout the day. This allows for considerable sampling replication in space and time throughout the cruise.

BRUVS are termed 'remote' because the systems are deployed on the seafloor and function independently of an operator or observer. Each BRUVS system uses 2 off-the-shelf high-definition video cameras mounted 0.7 m apart on a base bar that is inwardly converged at 8 degrees to gain an optimized field of view (with a forward-viewing range of ~10 m). The cameras are placed within PVC pipe housings with acrylic front and rear ports and mounted within a galvanized roll-bar frame. Stabilizing arms and bait arms (20 mm plastic conduit) are attached and then detached during and after deployment.

Each BRUVS can be left unbaited or can accommodate up to 1 kg of bait. The bait is placed in a plastic-coated wire basket suspended on a bait arm 1.2 m in front of the unit. Various types of bait may be used, depending on supply/local availability. At predefined GPS locations, each BRUVS is deployed from the vessel by hand (each unit weighs ~ 50kg) with a rope and floats attached. The BRUVS is left in place for 15 to 60 minutes (depending on survey design), after which vessel crew members can retrieve them by grappling surface floats and hauling the attached lines aboard with the aid of a hand-powered or electric winch or pot-hauler. Video footage collected by the BRUVS can be reviewed as soon as the camera is retrieved to the vessel and can be archived for later analysis.