Study of Fishery-Independent Sampling Methods Aimed at Assessment of Coral Reef Fish Populations

April 2, 2012
Fishery-independent sampling methods will be tested at selected study sites around Tutuila in shallow, moderate and deep waters.
Fishery-independent sampling methods will be tested at selected study sites around Tutuila in shallow, moderate and deep waters.

From April 2-19, 2012, the NOAA Ship Oscar Elton Sette is conducting research in waters of American Samoa to compare fishery-independent methods for assessing fish populations on coral reefs. Quantitative assessment and monitoring of shallow reef fish assemblages are conducted throughout the Pacific Islands Region (PIR) as an integral part of the NOAA Pacific Islands Fisheries Science Center (PIFSC) mission to improve scientific understanding of fish resources and related ecosystems and to improve assessments of regional fish stocks.

The 15-day research expedition is led by Dr. Benjamin Richards of PIFSC and Dr. John Rooney of the University of Hawaii Joint Institute for Marine and Atmospheric Research (JIMAR). The research team includes scientists from PIFSC, JIMAR, and the Northwest Fisheries Science Center and collaborators from the University of Miami Rosenstiel School for Marine and Atmospheric Science, Woods Hole Oceanographic Institution, and the University of Western Australia. The team will also include a participant from the NOAA Teacher-at-Sea Program.

During the expedition, which will also involve the NOAA Ship Hi'ialakai, several fishery-independent methods will be used to survey the composition and abundance of the coral reef fish community and collect data to enable a rigorous comparison of the methods. The fishery-independent methods of assessing fish abundance include SCUBA divers conducting stationary point count surveys, stationary stereo-video cameras that sit on the seafloor (BRUVS), and an autonomous underwater vehicle (AUV) outfitted with stereo-video cameras and a high resolution sonar imaging system. Successful development of fishery-independent methods would provide stock assessment scientists with an alternative to the traditional approach in which fish abundance is monitored using commercial fishery catch and effort statistics.

Over the course of the expedition, surveys will be carried out in water depths from 0 to 100 m, which include the majority of the coral reef ecosystem. In the shallower portion of this depth range, from 0 - 30 meters (m), the research team will conduct surveys using all three methods. In the 30 to 100 m depths, which are beyond the range of SCUBA divers, only the camera and AUV systems will be used. This will allow for a comparison among all methods in the shallow-water depth range as well as a comparison between the shallow and seldom-surveyed deeper-waters using the camera systems. Analysis of the data will enable PIFSC to assess the strengths and weaknesses of each method and the type of data that can be collected with each approach. Data will be compared quantitatively to determine the suitability of using these methods to generate a more accurate picture of the status and trends of coral reef fish stocks.

Stationary Point Counts

Stationary point count

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 pair, a 30 m line is laid across the seafloor. Markings at 7.5 m, 15 m and 22.5 m enabled survey 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 count consists of two components. 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 portion, in which the diver systematically works through their species list successively recording the number and size (total length, TL, to nearest cm) of all fishes on the species list. The tallying portions are 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 count. However, small and cryptic species, which will tend to be underrepresented in counts made by an observer remaining in the center of a 7.5 m radius cylinder, are left to the end of the tally period, at which time the observer swims through their plot area carefully searching for those species. In cases where a species is observed during the enumeration period but is not present in the cylinder during the tallying period, divers record their best estimates of size and number observed in the first encounter during the enumeration period and mark the data record as 'non-instantaneous'.

BRUVS

BRUVS

Baited remote underwater video stations (BRUVS) are non-destructive baited stereo-video samplers which can provide scientifically rigorous estimates of fish abundance and size structure. BRUVS were originally developed by the lab of Dr. Euan Harvey at the University of Western Australia. The use of stereo-cameras enables accurate size (and hence length-frequency and biomass) estimates to be obtained. Each of a group of up to 8 units is deployed for approximately 15 minutes and is recovered and redeployed in a "leap frog" fashion throughout the day. This allows for considerable replication in space and time throughout the cruise.

BRUVS are termed 'remote' because the systems are deployed on the seafloor independent from an operator or observer. Each BRUVS system uses two off-the-shelf high definition (HD) 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). These 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 detached during and after deployment.

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

AUV

AUV

The SeaBED-class AUV, unlike other more traditional AUV's, employs a twin-hull design that provides enhanced stability for low-speed photographic surveys. Designed and built by the lab of Dr. Hanaumant Singh at the Woods Hole Oceanographic Institute (WHOI), SeaBED is designed to autonomously follow the terrain approximately 3 to 4 m above the sea floor, collecting high resolution color and black-and-white imagery while maintaining a forward speed of .25 - .5 m/sec. For this mission, SeaBed will also be outfitted with a forward-looking stereo video camera system as well as a forward-looking imaging SONAR unit. The stereo-video system is similar to that use on the BRUVS and allows for accurate measures of fish abundance and size structure. The imaging SONAR unit is being tested as a means to assess fish assemblage outside the visual range of the cameras and in zero light situations including nocturnal or operations in depths to which light does not reach.

SeaBED is approximately two meters long and weighs nearly 200 kg. It has two main pressure housings, a top hull and a bottom hull. The CPU electronics are located in the top hull, and the batteries, cameras, and sensors are located in the bottom hull, and all are connected by wet cabling that is routed through vertical struts. With a maximum depth range of 2,000 m, and maximum single-dive time of 6 - 8 hours, SeaBED can be used to survey habitats ranging from shallow coral reefs to deep groundfish environments.

The AUV is programmed while still aboard the ship. Programming parameters include navigational waypoints, speed, altitude to maintain above the seafloor, and frequency of photographs. Once submerged, the AUV does not resurface until the end of its mission. An RD Instruments 1200 kHz Doppler Velocity Log, iXSea Octans Inertial Navigation Unit, and Paroscientific Depth Sensor provide the data necessary for the vehicle's autonomous navigation. The AUV does report its position to the ship periodically in telemetry messages via acoustic MODEM. Additionally USBL tracking shows range and bearing between the ship and AUV during the mission. If any of these telemetry messages indicate an unexpected change in the AUV's planned mission, the mission can be aborted via acoustic MODEM message, resulting in the AUV returning to the surface for recovery.

The SeaBED AUV carries a forward-facing ROS Navigator black-and-white, low-light stereo-video camera system, two 5 megapixel, 12 bit dynamic range Prosilica GigE strobe-lighted cameras, one perpendicularly downward-looking and one forward looking (~35°). Imagery from the downward-looking camera can be analyzed to characterize the benthic communities while the forward-looking cameras are used to collect species-specific abundance and length information. Combined, these 2 imagery data sets can be used to create spatial species-specific abundance, biomass, and length-frequency distributions, along with the benthic communities around which they associate. An onboard Seabird model 49 FastCat CTD records temperature and salinity data along the AUV track, providing further environmental insight.