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INTERPRETING REEF DATA

After REEF field surveys have been completed, the forms are electronically scanned. A computer program is then used to process the resulting datafile to generate summary reports. On the top of every report the number of surveys and total bottom time (hours) are given. Two parameters are presented in standard summary reports. These are Den and %SF. The density index (Den) and percent sighting frequency (%SF) parameters provide a measure of the relative density of species and the frequency with which these species were observed. In addition, there are two categories of observers for which data may be reported. These experience categories are Novice and Expert. Data recorded by all observers (Total) is listed as well as the number of species recorded by Expert and Novice observers.

Survey Type (SO and SA)

Most surveys completed are Species and Abundance (SA) surveys. These surveys record the species seen and an abundance category for each species. The categories are order of magnitude estimates of the number of individuals signed during the survey: Single=1, Few=2-10, Many=11-100, and Abundant=over 100.

Species Only (SO) surveys record only the presence of the species with no abundance information. On the survey form, the Species Only category in the Survey Type section is filled in and the Single category is used to indicate the presence of the species. A single SO survey may span multiple dives (or even an entire dive trip). Its primary use is to list species that were sighted outside of dives where SA surveys were taken. These surveys help to fill in the distribution data for species but contribute nothing to the survey bottom time or to the Sighting Frequency or Density calculations. For species that are included on a report only because they appeared in a SO survey, the %SF and Den columns are filled in with the text "----SO----".

Density Index (Den) and Sighting Frequency (%SF)

Density Index (Den) - This is a measure of how many individuals of a species are observed based on a scale of 1-4. It is representative of the abundance category (1-4) which was most frequently recorded for the species when it was observed. Abundance category weights are Single=1, Few=2, Many=3, and Abundant=4.

 

This weighted density average is calculated as:

            (S * 1) + (F * 2) + (M * 3) + (A * 4)
Den = -------------------------------------------------
      (Number of surveys in which species was observed)

This number indicates which abundance category the species was most often recorded in when it was recorded. For example, Den=2.2 would be reflective of a species that was most often recorded in category 2 (Few) but because the density index is greater than 2, there were some abundances recorded for this species in the other, larger abundance categories (either category 3 or 4). The density index should be used as an abundance guide because area is not rigorously controlled in the RDT method. It should also be kept in mind that the density (Den) parameter is reflective of sighting distributions in the four different abundance categories (S, F, M, and A) and different distributions of sightings in each abundance category could potentially give similar values of Den (in other words, it does not account for non-sightings).

Sighting Frequency (%SF) - This is a measure of how often the species was observed. It indicates the percentage of times out of all surveys that the species was recorded.

The %SF parameter is calculated as:

 

            S + F + M + A (for each species)
%SF = 100 * --------------------------------
                  (Number of surveys) 

By simultaneously examining the sighting frequency (%SF) and density index (Den), data summaries can be interpreted for fish species. The Den and %SF scores could be multiplied to provide a measure of species abundance which includes zero observations.

The following table shows an examples of how the summary information can be interpreted at the species level.

Example of Data Interpretation

Caribbean Example

Den %SF Explanation
HIGH
Den >3.0
HIGH
%SF >50
Species is often observed and observed at high densities. Species is seen > 50% of the time and when it is seen the abundance category most often recorded is M or A.

Species examples: bicolor damselfish, blue chromis, brown chromis

HIGH
Den >3.0
LOW
%SF <50
Species is not often seen, but when it is seen, it is observed at high densities. Species is seen < 50% of the time and when it is seen the abundance category most often recorded is M or A.

Species examples: silversides/herrings, garden eel

LOW
Den <3.0
HIGH
%SF >50
Species is often observed, but always at low densities. Species is seen > 50% of the time and when it is seen the abundance category most often recorded is F or S.

Species examples: trumpetfish, rock beauty, foureye butterflyfish

LOW
Den <3.0
LOW
%SF <50
Species is not often observed and when it is observed, it is at very low densities. Species is seen < 50% of the time and when it is seen the abundance category most often recorded is F or S.

Species examples: green moray, saucereye porgy, spotted scorpionfish

 


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Date Last Modified: 07/26/00

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