[Coral-List] FW: Reliable coral reef stats

[Clive Wilkinson]

I hesitate to enter into this discussion about methodology, as throughout my years with the Global Coral Reef Monitoring Network, the most vigorous discussions were about 'methodology' rather than on the validity of the data obtained by that methodology. It appears that every reef scientist wanted to develop their own special methods with something resembling religious fervour! In an attempt to consolidate the methods (and reduce a proliferation of methods) we published a compendium of the current methods that was released at the Okinawa ICRS in 2004:
Hill, J. and Wilkinson, C. (2004). Methods for Ecological Monitoring of Coral Reefs: A Resource for Managers.  Australian Institute of Marine Science and Reef Check, Townsville, p. 118 Jos Hill was then the head of Reef Check in Australia. I presume that this is available on the GCRMN, NOAA, ReefBase, AIMS and other websites.

Clive Wilkinson

-----Original Message-----
From: coral-list-bounces at coral.aoml.noaa.gov [mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of John McManus
Sent: Friday, 20 September 2013 4:35 AM
To: coral-list at coral.aoml.noaa.gov
Subject: Re: [Coral-List] Reliable coral reef stats

A few years ago, we were asked to look into existing coral reef sampling protocols to determine what is most appropriate as use as a baseline for a future mitigation concern. We studied every standard we could find from the standpoint of its utility in identifying and quantifying what ecological 'functions' would be lost over time after the damage (as per federal regulations, where in these functions are not well-defined -- but we did what we could in that regard). The answer was: none.

The closest was a methodology in which coral colonies were identified and measured as part of the sampling, combined with remote sensing bottom classifications. From this one can obtain rough estimates of potential rates of recovery from previous rates of recruitment and mortality, and quantify the recovery spatially. However, aside from the obvious problem in defining a 'colony' in some circumstances, the approach involved measuring only the longest dimension. This was fine for the purposes of the original methodology designers, but for us, it added a systematic error -- comparable to that of putting transect lines on places with the highest local cover, as sometimes encountered in other protocols. Still, because the bias is known and systematic, prior data could still be used with a correction factor, based either on rationalization or, ultimately, on comparative sampling (something that would be good for those 'highest cover' surveys). Of course, we recommended that multiple colony measures be used for mitigation cases. 

One point of this is that there is no 'one size fits all' sampling approach..
If someone needs to identify change in a specific reef, one should match a sampling unit to the purposes of the study, select an appropriate statistical design, choose the appropriate number of sample units from which to obtain data based on an analysis of estimated or pre-sampled local variance, and try to minimize bias throughout the process. Although some people use experimental design procedures for this, it is much better to actually use sampling design principles (see books by Hayek and Buzas, Thompson, or the old Cochran book, or better, get a statistician to help
early on!).   

Usually, formal statistical design is not practical for sampling across many reefs (though given the appropriate prior funding commitments it would be feasible), so we must trade off intensiveness for extensiveness. Every methodology has pros and cons for this. Reef Check's approach is very 'extensive', gathering less data to lower cost and permit very wide, rapid sampling.. All existing methods have biases one way or another. I think the worst bias we see in regional and global surveys is that of reef selection
-- we sample primarily convenient, well-known reefs. We have seen specific efforts to gather information from less-known and distant reefs (including efforts from Reef Check, AGRRA, NOAA, Cordio, etc.), and so this bias can increasingly be adjusted for via appropriate data groupings and weightings
-- assuming that all this data are made publicly and easily available .    

I am not associated with Reef Check, but it clearly is the best thing we currently have in terms of global coverage using a uniform standard. 

However, as global concerns about reef issues increases, there may be new opportunities to improve sampling protocols. So, I encourage discussion on how assessments at national, regional and global scales can be improved. So, please do suggest better ways to obtain this vital information. Critique without suggestion is not terribly helpful.

Cheers!


John