SE16-02: American Samoa Reef Fish Survey Summary

by Adel Heenan and Marc Nadon

For the past three weeks, the NOAA Ship Oscar Elton Sette has been the support platform for the Pacific Islands Fisheries Science Center’s reef fish survey project. This research project was led by the NOAA Coral Reef Ecosystem Program (CREP), with partner agency representatives from the American Samoa Department of Marine and Wildlife Resources (DMWR) and the Bigelow Laboratory of Ocean Sciences. The mission was similar to the Pacific RAMP work, but with a particular focus on surveying reef fish assemblages.

Divers collected length observations for all reef fishes recorded during their underwater surveys. To do so accurately, trained divers regularly practice fish sizing using wooden cut-outs in-between research cruises. Length measurements for each reef fish surveyed allows an estimation of biomass by using pre-determined length-weight relationships. Furthermore, it is also used to estimate the size composition of fish populations and obtain a key indicator of population status: average length of exploited size classes. The reason we use this indicator is intuitive: as the exploitation rate of a fish population increases, fewer individual fish have a chance to reach older ages, and therefore, fewer individuals reach larger sizes. Mathematical expressions developed in the 1950s by fisheries scientists can actually relate average length to current fishing mortality rates, and these can be used in computer population simulations to investigate current stock status and generate management advice.

4.Survey_P.Ayotte

Reef fish survey divers regularly train in estimating fish size by using wooden cut-outs of known sizes (NOAA Photo by Paula Ayotte).

Outlined below is a summary of our recently completed survey efforts. More detailed survey results will be available in a forthcoming survey report.

Sampling effort

  •  Ecological monitoring took place in American Samoa from April 15 2016- May 5 2016.
  • Data were collected at 202 sites. Surveys were conducted at Ofu and Olosega (n=11), Rose (n=47), Tau (n=50) and Tutuila (n=94).
  • At each site, the fish assemblage was survey by underwater visual census and the benthic community rapidly assessed.
  • At a subset of sites (n=51), paired comparisons of fish surveys performed using closed circuit re-breathers versus open circuit SCUBA were conducted. Those data will be analyzed and presented in a separate publication.
TCW_rebreather

Diver conducts reef fish survey with a closed circuit re-breather (NOAA photo by Tate Wester).

Overview of the data collected

Primary consumers include herbivores (which eat plants) and detritivores (which bottom feed on detritus), and secondary consumers are largely omnivores (which mostly eat a variety of fishes and invertebrates) and invertivores (which eat invertebrates).

Spatial sampling design

Survey site locations are randomly selected using a depth-stratified design. During project planning and the project itself, logistic and weather conditions factor into the allocation of monitoring effort around sectors of each island or atoll. The geographic coordinates of sample sites are then randomly drawn from a map of the area of target habitat per study area. The target habitat is hard-bottom reef, the study area is typically an island or atoll, or in the case of larger islands, sectors per island, and the depth strata are shallow (0-6 m), mid (6-18 m), and deep (18-30 m).

Sampling methods

A pair of divers surveys the fish assemblage at each site using a stationary-point-count method (Figure 5). Each diver identifies, enumerates, and estimates the total length of fishes within a visually estimated 15-m-diameter cylinder with the diver stationed in the center. These data are used to calculate fish biomass per unit area (g m-2) for each species. Mean biomass estimates per island are calculated by weighting averages by the area per strata. Island-scale estimates presented here represent only the areas surveyed during this project. For gaps or areas not surveyed during this project, data from this and other survey efforts will generally be pooled to improve island-scale estimates.

Fig5.REA_method

Figure 5. Method used to monitor fish assemblages and benthic communities at the Rapid Ecological Assessment (REA) sites.

Each diver also conducts a rapid visual assessment of reef composition, by estimating the percentage cover of major benthic functional groups (encrusting algae, fleshy macroalgae, hard corals, turf algae and soft corals) in each cylinder. Divers also estimate the complexity of the surface of the reef structure, and they take photos along a transect at each site that are archived to allow for future analysis.

About the monitoring program

Pacific RAMP forms a key part of the National Coral Reef Monitoring Program of NOAA’s Coral Reef Conservation Program (CRCP), providing integrated, consistent, and comparable data across U.S. Pacific islands and atolls. CRCP monitoring efforts aim to:

  • Document the status of reef species of ecological and economic importance.
  • Track and assess changes in reef communities in response to environmental stressors or human activities.
  • Evaluate the effectiveness of specific management strategies and identify actions for future and adaptive responses.

In addition to the fish community surveys outlined here, Pacific RAMP efforts include interdisciplinary monitoring of oceanographic conditions, coral reef habitat assessments and mapping. Most data are available upon request.

For more information:

CREP publications

CREP monitoring reports

CREP fish team

Fish team lead and fish survey data requests: ivor.williams@noaa.gov, adel.heenan@noaa.gov

 
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