Ecological Assessment of Fish
Quantitative assessment and monitoring of shallow reef fish assemblages is conducted around the US Pacific Islands as an integral part of the NOAA/NMFS/PIFSC Coral Reef Ecosystem Division's mission to improve our scientific understanding of these fish resources and related ecosystems. As fish are the primary sustainable living resource on Pacific coral reefs, results contribute to the scientific basis essential for sound management. Related objectives include: creating a fish baseline to measure marine protected area (MPA) effectiveness; monitoring size-frequency of fish assemblages; assessing the status of target, indicator or keystone species; assessing responses by fish communities to possible ecosystem impacts (e.g., fishing, ecotourism, pollution, habitat damage, sedimentation, hurricanes); and assessing species composition and diversity, by area, and the effectiveness of long-term monitoring of managed areas.
An initial species inventory was a prerequisite for comprehensive assessment of reef fish assemblages in each area. Subsequent biennial monitoring surveys are planned for each geographic sub-region to document temporal variability in reef fish assemblages. Habitat types surveyed include mainly outer reef slopes around most islands, but also include lagoon patch reefs, bays, back-reefs, and shallow oceanic banks, where present.
Inventories and assessments of shallow reef fishes have been completed by CRED at all US Pacific Islands (except for two islands restricted by the military), and monitoring has been initiated. Ongoing analysis of this growing database will enable species-specific numerical and biomass densities to be estimated, fish assemblage structure to be described at various spatial and temporal scales, and statistical correlations to be tested. Further analysis of CRED's oceanographic and biological data will aid in understanding patterns of fish distribution and abundance as well as ecosystem associations.
Fish Survey Methods
Several complementary, noninvasive underwater-surveys are used to enumerate the diverse components of diurnally active shallow-water reef fish assemblages. Survey types include: 1) Rapid Ecological Assessments (REA)- to document simple species presence at a station or reef/bank; 2) Belt Transects (BLT)- mainly to quantify relatively small-bodied and abundant fishes; 3) Stationary Point Counts (SPC)- to quantify relatively larger (>25 cm Total Length [TL]) and more vagile fish species; and 4) Towed-Diver Video Surveys (TDVS)- to quantify relatively large-bodied (>50 cm TL), wide-ranging fishes over a broad-spatial scale. Each method is replicated at sites within and/or among the various habitat types present around each island or bank. Fish length-class is estimated for all quantified fish to provide an estimate of numerical size structure and biomass densities by taxa.
A pair of diver-observers conducts a random swim throughout the selected station area, recording the presence of fishes visually encountered by species or lowest recognizable taxon. This method is typically used at deeper, time-limited sites or where current is too strong to conduct transects. The REA protocol is also used following completion of a belt-transect or SPC, dive time permitting. The REA data complement the other visual protocols to assemble more complete reef-and archipelago-specific fish species inventories at each island or bank.
A pair of scuba diver-observers conducts parallel swims along three 25 m long transect lines, recording size-class specific counts of all fishes encountered, to species-level where possible, within visually estimated but defined belt widths: 4 m wide for fishes > 20 cm TL (100 m2 area) on the initial swim-out, and 2 m wide for fishes < 20 cm TL (50 m2 area) on the subsequent swim back. Transects lines are typically set at depths of 10-15 m. Reef ledges and holes are visually searched. Stations are completed on all sides of the island/atoll, weather and sea conditions permitting.
One SPC diver-observer conducts surveys in conjunction with, but at least 10 m away from, the two belt-transect divers. All fishes >25 cm TL are recorded to species-level that enter a 20 m diameter cylinder (area ~314 m2) during a timed 5 minute count. Individuals or groups are estimated to the nearest 5 cm TL size-class bin. Four replicate, 5 minute cylinder counts are conducted at each station. Care is taken to avoid over-counting large transient or schooling species.
A pair of scuba divers is towed about 1 m above the reef ~60 m behind a small boat at a constant speed of about 1.5 kt. Each diver maneuvers one of two towboards that are connected to the boat by a bridle and towline, and outfitted with various survey equipment, including digital still and video cameras. The fish towboard diver is responsible for recording large (>50 cm TL) fishes to species in 25-50 cm TL size-class bins, while the benthic towboard diver quantifies habitat composition by type. Observations and species tallies are recorded on pre-printed datasheets attached to the towboard. The digital video camera on the fish towboard faces forward and records a permanent record of fishes encountered in a standard field of view. The towed survey is typically 50 min long and covers about 2 km of habitat, depending on the depth profile of the divers. The survey is divided into 5 min segments, which include a 1 min circle count and a 4 min transect. During the 1 min circle survey, a 360o scan is completed and all large fish species within the range of visibility are recorded (this is done to enumerate species such as sharks and jacks that tend to follow behind the divers without entering the survey swath). During the remaining 4 min of each segment the fish towboarder records large fishes within a survey swath 5 m to either side of the diver and 10 m in front. Throughout the survey the latitude and longitude coordinates of the survey track are recorded from the small boat using a Global Positioning System (GPS), allowing data to later be geo-referenced. Laboratory analyses of the digital videos acquired during towed diver surveys are ongoing.
Data from the various fish survey methodologies are analyzed using standard statistical software (e.g., Statistical Analysis System, SAS). Aspects are investigated such as mean numerical and biomass fish densities by site and reef, trophic composition, relative abundance and occurrence of fish species and families, size-frequency distributions among islands experiencing various fishing pressure, and length-weight conversions (for biomass estimation). Analysis of Variance (ANOVA) tests are used for comparisons such as fish at specific sites of human disturbance or natural disturbance to nearby undisturbed islands or sites.
Species Inventories: Results of the CRED fish surveys provide for more complete species inventories from populated and remote US Pacific Islands. The total number of species per island or reef, as well as the percent of new species records and, possibly, new species discovered are recorded. The degree of endemism of reef fishes in specific locales will be identified. Species found to have restrictive distribution might be important for assessing reef ecosystem health. Species composition may be related to local geographic position and oceanography, concurrently investigated by collaborators. Unique forms or geographic-hybrids may be discovered that will contribute to further study to determine genetic differences and possibly support taxonomic revisions. By developing an inventory of species composition and species-specific biomass, the project contributes toward the understanding of biodiversity of these unique geographic regions and management of fishery resources within them. Reef habitats characterized by unique fauna may warrant special value for conservation.
Habitat Associations: Results on the spatial distribution of fishes, in conjunction with photographic and video documentation of their respective habitats, allow CRED researchers to better understand habitat utilization patterns of individual components of the fish assemblage. Fish results can then be related to oceanographic conditions (e.g., upwellings) or other habitat factors, as recorded by project collaborators. Such results may have implications for the design and siting of marine reserves or no-take MPAs, (e.g., based on areas or habitat types found to be preferred by newly recruited fishes, or habitats discovered to function as important spawning sites or juvenile nursery grounds, necessary for the sustained replenishment of fish populations at these remote coral reef ecosystems.
Ecological Functions: Comprehensive fish surveys also help to understand the important role that fishes play in the coral reef ecosystem. For example, numerical and biomass densities are more precisely estimable by pooling taxa into functional groups such as trophic levels (e.g., primary and secondary consumers, apex predators) and foraging guilds (e.g., detritivores, herbivores, benthic invertebrate-feeders). Direct diver counts of sharks, intercepted on ~2-km long towed-diver/video surveys, allow densities of these large apex predators to be estimated and compared between reefs and islands. Fish size-distribution data are used to corroborate the importance of apex predators in shaping reef fish populations and community structure. Comparisons of the densities of large and small fish between remote islands and populated islands can be used as an indicator of fishing pressure (e.g., Guam and Saipan, compared to the largely uninhabited northern end of the Marianas Archipelago).
Indicator Species: Fish species can be identified for use as indicators of reef health or condition. The presence and relative abundance of species such as the Chevron butterflyfish (Chaetodon trifasciatus), which feed on live corals, may reveal the relative health of coral reefs. Keystone species such as humphead wrasse (Cheilinus undulatus), bumphead parrotfish (Bolbometopon muricatum), and large jacks and sharks may be indicative of the degree of fishing pressure around remote islands. Relative abundance of groupers (e.g., Epinephelus spp., Plectropomus spp.) provides insight on the level of undetected poaching by the international live reef fish trade. Comparing the densities of planktivorous fishes (e.g., Anthias spp., Chromis spp.) around the islands may help reveal if and how spatial patterns exist for nutrient-enhancing upwelling currents (e.g., high densities of planktivores as found by CRED at the equatorial islands of Jarvis, Howland and Baker). An unusual abundance of herbivores at a site may be caused by a recent or persistent disturbance (e.g., corroding iron from a decade-old ship-grounding, that was enhancing algal production, as found by CRED researchers at Rose Atoll, American Samoa).