Is parasitism in fish a good metric to assess ecological water quality in transitional waters? What can be learned from two estuarine resident species?

Fish-based indices are becoming important bioassessment tools for estuaries since the European Water Framework Directive included fish as a biological quality element to be monitored for the assessment of ecological status of those systems. In particular, data on species composition and abundance of the ichthyofauna must be used to evaluate the ecological status of European estuaries, but other factors like measures of fish health are also considered important metrics. These indicators of fish health include infections by parasites. The common goby, Pomastoschistus microps, and the Lusitanian toadfish, Halobatrachus didactylus, are fairly sedentary fish, very abundant in Portuguese estuaries, and were used to investigate if parasitism in fishes in transitional waters is a good metric to reflect anthropogenic impacts in the environment. No significant relationships were observed between the parasite levels in these two species and the intensity of human pressures, either at a large or small spatial scale. Results obtained also are contrary to the generalized idea that a higher proportion of parasitized fish necessarily reflects a decrease in their condition as a result of habitat degradation. A high degree of temporal and ontogenetic variability in the parasite levels of both P. microps and H. didactylus was detected, which make it difficult to establish the scores for this hypothetical metric even if significant relationships between parasite levels and human pressures are identified in the future. Therefore, the present work does not uphold the hypothesis that parasitism in fish is a good metric to assess ecological water quality in transitional waters.