Ontogenetic spatial distributions of red grouper (Epinephelus morio) and gag grouper (Mycteroperca microlepis) in the U.S. Gulf of Mexico

Abstract

Mapping the spatial distributions of fish populations is an integral component of ecosystem-based fisheries management (EBFM). Particularly for red grouper (Epinephelus morio) and gag grouper (“gag”; Mycteroperca microlepis), two economically important species, the lack of mapping due to data limitations (i.e., inconsistent capture in research surveys) has left a critical gap in the science needed to assess how ecosystem processes and EBFM measures in the Gulf of Mexico (GOM) impact their population dynamics. We combined multiple fisheries-dependent and fisheries-independent data sources to map the long-term spatial distributions of older juveniles and adults of red and gag groupers in the U.S. GOM, using spatio-temporal binomial generalized linear mixed models (GLMMs). Spatio-temporal binomial GLMMs rely on the idea that probability of encounter at a given site is more similar to probability of encounter at nearby sites than to probability of encounter at geographically remote locations; this tenet allows one to estimate a smoothed surface depicting how probability of encounter varies spatially. Our spatio-temporal binomial GLMMs do not integrate environmental covariates, yet they account for the effects of year and research survey. The distribution maps produced from the predictions of the spatio-temporal binomial GLMMs aligned with the current understanding of the long-term ontogenetic spatial distributions of red and gag groupers in the U.S. GOM. Red grouper was predicted to be encountered throughout the West Florida Shelf (WFS), primarily at depths ranging from 20 to 60m. Both older juvenile and adult female gags were predicted to be encountered from Apalachicola, Florida, to the region northwest of Tampa, Florida, along the 20m depth contour, especially in Apalachee Bay. The probability of encounter of adult female gag was also high in the Florida Middle Grounds and in deeper (>40m) areas of the WFS. The probability of encounter of adult male gag was highest along the edge of the WFS, both inside recognized spawning grounds (including the Madison-Swanson marine protected area) and outside, i.e., below 27°N (including Pulley Ridge). The distribution maps produced are valuable for understanding the ecology of grouper species and can be used as a basis for further analyses. Our spatio-temporal binomial GLMM framework will serve many important EBFM projects, including the construction of reliable distribution maps in bulk for spatially explicit ecosystem models of the GOM, which will improve spatial distributions and species spatial overlaps in spatially explicit ecosystem models and, therefore, the trophic interactions predicted by these models.