Resampling 25 years later reveals fewer species but higher abundance of juvenile fishes in a Caribbean mangrove bay

The prop roots of mangroves play an important role as habitat for juvenile fishes. Revisit studies (>10 yrs apart) provide rare and valuable insights into the structure and stability of these systems in the face of escalating pressure from coastal development, climate change, and fisheries. We compared assemblages of juvenile fishes in the mangroves of a Caribbean estuary from 1991 to 1993 and 2018 to 2019 using trap collections to quantify changes and identify their potential drivers. Although the environmental and physical properties of the mangrove landscape were similar between the two periods, there were significant changes in many aspects of the fish assemblage. Compared to 25 yrs earlier, overall fish abundance (catch per unit effort, CPUE) was 3 to 7 times higher but overall biodiversity was 30% lower at the species level and 50% lower at the genus level. Taxa with dramatic changes in CPUE were mojarras (Gerreidae, 84% lower CPUE in 2018–2019), snappers (17 times higher CPUE), puffers (Tetraodontidae, 14 times higher CPUE), and moray eels (>200 times higher CPUE). Traps in 2019 also captured the first documented invasive Indo-Pacific swimming crab (Charybdis helerii) in the US Virgin Islands. The observed changes in the fish assemblage may be due to natural variations in recruitment, environmental influences, and/or hurricane disturbance, but for some species likely relate to declines in adult reef fish populations in the region.