And # of species observed doesn’t necessarily speak to the quantity of those fish seen does it?
Correct. So lets delve into a little bit.
There are two kinds of surveys that you can submit to REEF. Species Only(SO), and Species & Abundance(SA). Species Only surveys while being easier to conduct are not necessarily encouraged, precisely because they do not provide abundance data. Out of 13159 surveys submitted for Cozumel thus far 12739 are SA and 420 are SO. So there is plenty of data with regards to species abundance for Cozumel.
The following is copied from the REEF website you can see all it at this link.
Interpreting REEF Data
Density Index (Den), Sighting Frequency (%SF), Abundance Score (Abund)
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:
(nS * 1) + (nF * 2) + (nM * 3) + (nA * 4)
Den = -------------------------------------------------
(Total number of surveys in which species was reported)
Where "nS" is the number of surveys that reported the species as Single, "nF" is the number of surveys that reported the species as Few, "nM" is the number of surveys that reported the species as Many, and "nA" is the number of surveys that reported the species as Abundant.
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 for a given species is calculated as:
%SF = number of surveys that the species was reported / total number of surveys conducted
Abundance Score (Abund)
By simultaneously examining the sighting frequency (%SF) and density index (Den), data summaries can be interpreted for different species. The Den and %SF scores can be multiplied to provide a measure of species abundance which includes zero observations.
Abund = %SF x Den
------ Break from the copy and paste of REEF
The Geographic Area Reports that were used to make the table in my last post do not include the abundance score. In order to get an abundance score I did Geographic Area Reports for total of Cozumel from 1993, and the years from 2011 until 2022. I loaded them into excel added a column to get an abundance score for each species sighted for each year. Than I took an average of all the abundance scores for each separate year.
If the abundance of fish in Cozumel has decreased over the last twelve years than the average abundances should be decreasing with time.
When I did the Abund calculation, I use the decimal equivalent for percent not the actual percent. For example, in 2017 Bluehead Wrasse had and SF% of 91.72 and Den of 3.59, so Abund is .9172 x 3.59= 3.29.
That is a very high Abund number. In 2022 Round Scad had a SF% of .21 and Den of 4 Abund is .0021 x 4=.0084, so despite the high density it is a low score, because Round Scad was observed once but in high numbers. Species Only surveys count density as 0, even though the density one of had to be at least one. So Species Only Surveys will drag down the total Abund avg, even though they increase the # of species observed for a given sight. This has a negligible effect given SO only account for 3% of total surveys.
The calculated Abund average for 13159 surveys in Cozumel is, .27. That seems like a pretty low number, but it makes sense. Of the 446 species observed in Cozumel 172 of them have only been seen ten times in or less in thirty years, an SF .1%. This drags the total Abund Avg way down. On a year by year basis the average is usually double that or .54, as seen the graph below. The Abund Avg has not fluctuated greatly for the past 11 years. There certainly is not a discernable trend in an increasing or decreasing fashion. Conclusion:
The # species and abundance of same has remained stable in Cozumel for the last 11 years.
