Stable Isotopic Niche Variability and Overlap across Four Fish Guilds in the North‐Central Gulf of Mexico

The quantification of niche diversity and niche overlap is useful for understanding the impacts of fisheries regulation and management. In this work, we evaluate isotopic niche size and overlap at the guild and species levels. Specifically, we analyzed guild- and species-specific isotopic niche space and niche space overlap using the SIBER and nicheROVER packages, and we evaluated length-specific and geographic contrasts in isotopes for fishes in the north-central Gulf of Mexico. Stable carbon and nitrogen isotope values (delta C-13 and delta N-15) of fish species were compared among four ecotype-based fish guilds: estuarine, coastal migratory pelagic (CMP), reef, and large offshore pelagic (LOP). Significant differences in the mean stable isotope values were found among fish guilds. Estuarine guild species exhibited the highest delta C-13 variability, and Red Drum Sciaenops ocellatus were the most isotopically diverse. Variability of delta C-13 for CMP fish was comparable to that of some estuarine species, whereas reef and LOP fish were less variable. Fishes within the LOP guild had the largest delta N-15 range. Reef guild fishes had the smallest isotopic niche space and LOP guild species consistently had the largest, although no species in any guild occupied as large of an isotopic niche space as Red Drum. Distinct and well-separated isotopic niches were also observed between most estuarine species and CMP species; however, high niche overlap was observed for species within the reef and LOP guilds. We found a positive relationship of delta N-15 and fish TL for Red Drum, Blackfin Tuna Thunnus atlanticus, and Yellowfin Tuna T. albacares, and we found that delta N-15 values for Red Drum and Red Snapper Lutjanus campechanus showed distinct isotope differences between geographic areas. This study provides data not only on how species within and without ecotypes interact but also on the variability of their interactions, all of which can inform ecosystem-based fisheries management models.