Award: OCE-2348548

Disease outbreaks in wild populations of reef fishes are poorly documented, much less understood. With NSF funding, we were able to document a widespread outbreak of ulcerative skin disease linked to a second consecutive major El Niño event, providing strong evidence of the causal nature of this climate cycle. We documented seven species of fish characteristic ulcerative skin lesions in 2023. The last two major El Niño events (2016, 2023) resulted in outbreaks, with an absence of ulcerative skin disease in the interim confirmed by our bi-annual long-term monitoring program. It appears that ulcerative skin disease outbreaks in Galapagos reef fishes may be a consistent outcome of future extreme El Niño events. Bacterial species identified more frequently and most commonly correlated with lesions in fish, especially Vibrio harveyi and Photobacterium damselae, are both known to be opportunistic pathogens. We hypothesize that these two known opportunistic pathogens will become increasingly relevant during higher temperature events in the wild in other species of fishes and in other parts of the globe.

The isolated location of the Galapagos on the eastern side of the Pacific and at the confluence of multiple warm and cold-water currents has resulted in overlapping fish faunas with the highest reef fish functional diversity of any known marine region globally. The rich variety of combinations of functional traits represented by Galapagos reef fishes provides a unique opportunity to understand how each characteristic contributes to population sensitivity to ENSO variation. Our findings suggest that indirect mechanisms of oceanographic change, namely food requirements and susceptibility to disease outbreaks, may be more important in determining the fate of marine populations than temperature change alone. Different fish species varied in their dietary shifts with oceanographic conditions, with some species showing increased trophic level with temperature and others showing an inverse relationship. Thus, supposedly ecologically similar species have different mechanisms for coping with environmental change, providing a new testable hypothesis regarding the determinism of population responses of suites of species that share functional traits.

This research has already produced one publication on the re-discovery of Rhizopsammia wellingtoni. This study was impactful since many endemic marine species in the Galapagos suffered catastrophic population declines after extreme El Niño events in 1982 and 1997. The rediscovery of Rhizopsammia found after a 20-year absence and living at 55 m or deeper, below the thermocline where temperatures are relatively cool and constant, provides strong evidence of a “deep reef refuge” for this species. Such refugia have long been hypothesized as an escape or rescue for shallow populations exposed to excessive ocean warming, but few examples exist in the literature. This publication was selected as the cover for the issue.

The accelerating pace of environmental change due to human activities is rapidly altering the conditions for life on Earth. The resulting shifts in population sizes, species distributions, and wildlife disease outbreaks are having widespread negative impacts on global biodiversity as well as on human health and wellbeing. Management of marine natural resources for long-term enjoyment thus requires understanding how environmental change will affect the usefulness of tools such as no-take areas and catch limits. Our project leveraged natural temporal variation in ENSO and geographic gradients in Galapagos oceanography to provide insight into the mechanisms through which warmer oceans affect marine ecosystems. Our findings of trophic and disease-mediated impacts of El Niño on fish populations can be used to design adaptive management plans to adjust fisheries targets according to ENSO phase. Bacterial pathogens known to cause disease in aquaculture during stressful periods, including high temperatures, will most likely be of increasing and similar importance in wild fisheries as environmental conditions continue to change and subject wild fish to similar stressors. Our discovery of extant populations of Rhizopsammia provides hope for the survival of this species and suggests the possibility of the survival of other endemic marine species not observed in the Galapagos since their disappearance during the extreme El Niño events of the 1980s and 1990s, such as the Galapagos damselfish Azurina eupalama and the Galapagos sun star Heliaster solaris. Exploring the possibility of a deep reef refuge for these and other species sensitive to warming of surface oceans holds great promise for sustaining unique biodiversity for long-term benefit to humans.

Last Modified: 04/23/2026
Modified by: Robert W Lamb