Grouper spawning aggregations: Be careful what you measure and how you measure it: A rebuttal of Golbuu and Friedlander (2011)

Golbuu and Friedlander (2011, Estuar. Coast. Shelf Sci. 92:223) used faulty methods, improper interpretation of historical data, and assumptions of little or no validity to conclude that despite protected status spawning aggregations of three groupers in Palau, western Caroline Islands declined between 1995–96 and 2005–06. Alternate survey methods indicated no drastic declines in these aggregations over the same period. Golbuu and Friedlander (2011) failed to document the overall distribution of fishes in their aggregations, used poorly-located inadequate transects to sample the overall aggregation area, and did not identify and sample peak aggregations days. The use of visual length estimates as the basis for biomass values may introduce errors. Comparison of aggregation persistence between reference (“fished out”) and protected sites is not possible because equivalent protected and exploited sites are not available. Different species at multi-species spawning aggregation sites commonly occupy somewhat discrete locations, with the densest concentrations (core areas) of one often being separated from those for another. There is usually a single peak day each month of a lunar-based aggregation, but it requires multiple days data collection to determine that peak. The shortcomings identified provide important lessons for the study of fish spawning aggregations and signal caution about the incomplete documentation of sampling methodology. Overly simplistic aggregation sampling methodologies may be superficially credible, but are not reflective of a complicated reality. Monitoring needs to produce a definitive repeatable baseline against which data can be gathered authoritatively in the future.