Fishing drives declines in fish parasite diversity and has variable effects on parasite abundance: Evidence from fished and unfished coral atolls of the Northern Line Islands

Background/Question/Methods

A primary goal of ecology is to understand how anthropogenic environmental change affects populations, communities, and ecosystems. But despite the overwhelming ubiquity of parasites, relatively few studies have comprehensively assessed the response of parasitic species to human impacts like biodiversity loss. Substantial disagreement exists as to whether parasite abundance will increase or decrease in response to disturbance, or whether such generalizations are even possible. To address this knowledge gap, we assessed the abundance of fish parasites of seven coral reef fishes collected from three fished and three unfished atolls of the Northern Line Islands archipelago in the central equatorial Pacific.

Results/Conclusions

We found that fishing substantially depressed jackknife estimates of parasite species richness (t₃₃ = 2.225, p = 0.0331), with mean ± SE of 7.4 ± 1.5 parasite taxa on fished atolls and 11.4 ± 1.8 on unfished atolls. We also found that the response of parasite abundance (# of parasites per host) varied substantially among parasite taxa, with 13% of parasite taxa showing significantly higher abundance on unfished than fished atolls, 24% with higher abundance on fished than unfished atolls, and 60% with no significant response to fishing. However, predictable patterns emerged at higher levels of taxonomic organization, with meta-analysis demonstrating that directly transmitted parasites were significantly more abundant on fished than on unfished atolls (effect size ± SE = 1.1907 ± 0.1654, df = 12, p < 0.0001), while indirectly transmitted parasites were significantly more abundant on unfished than fished atolls (–0.1803 ± 0.0879, df = 26, p = 0.0404). This might arise because increasing life cycle complexity increases the probability that some obligately required host will be negatively affected by fishing, and because parasites with a single parasitic life stage can take advantage of fishing-driven, compensatory increases in abundance of some hosts within the host range. Parasites of unfished hosts were significantly more abundant on fished than on unfished atolls (effect size ± SE = 0.6931 ± 0.2087, df = 15, p = 0.0009), possibly because these unfished hosts experience a relaxation in predation pressure and a consequent increase in density on fished atolls, where their predators are removed by fishing. Together, these results suggest that fishing disturbance erodes parasite richness, but has variable impacts on parasite abundance. As for free-living species, there are likely to be winners and losers among parasites in a changing environment, with a given parasite’s response to environmental change determined by its traits.