Anisakid nematodes in thread herrings from the Gulf of California: food safety considerations and spatial patterns in...

Thread herrings (Opisthonema spp.) are ecologically and economically important small pelagic fishes in the Gulf of California, yet their parasitic fauna remains poorly documented. Given their role in regional fisheries and marine food webs, clarifying the identity of anisakid nematodes infecting these fishes is relevant for food safety surveillance. In this study, we examined 306 individuals of Opisthonema libertate, 25 of O. medirastre, and 89 of O. bulleri collected from multiple fishing grounds between 2022 and 2023. A total of 140 third-stage anisakid larvae were recovered from O. libertate, whereas infections were rare in the other two host species. Molecular analyses of the ITS rDNA region confirmed the presence of Contracaecum bioccai and revealed an additional Contracaecum species that could not be assigned to species level. Although infection intensities were low, the occurrence of anisakid nematodes belonging to a genus that includes species associated with anisakidosis highlights the importance of continued monitoring of small pelagic fishes in the region. In addition to anisakids, the parasite assemblage of O. libertate was dominated by digeneans, and multivariate analyses revealed significant spatial variation in infracommunity structure across fishing grounds. Similar spatial patterns reported previously were also detected in the present survey, indicating reproducible interannual differences within the Gulf of California. These findings provide the first molecular confirmation of anisakid species infecting O. libertate in the region and underscore the complementary value of parasitological research for food safety and fisheries science.