Changes in fish assemblages after marine heatwave events in West Hawai‘i Island

Abstract

Marine heatwaves are prolonged events of anomalously warm water that affect diverse marine habitats and their associated biota. Evidence shows that anthropogenic climate change is increasing the frequency and duration of marine heatwaves and that coral reef systems are sensitive to the thermal stress imposed by these heatwaves. In this study, we examined fish community response to consecutive marine heatwaves (2014-2015) by analyzing changes in fish assemblages in Hawai‘i over 11 yr (2009-2019). Subtidal video survey data were collected in 3 areas on the west side of the Big Island of Hawai‘i. Fish were counted and identified to species or genus, then assigned to one of 7 functional groups: predators, secondary consumers, planktivores, corallivores, scrapers, grazers or browsers. Our study revealed 4 key findings. We show that all fish assemblages changed significantly in each area after the marine heatwaves. Across all 3 areas, the 3 most abundant functional groups (planktivores, grazers and secondary consumers) drove the observed changes in the community. Following the marine heatwaves, fish abundance increased in 2 areas with fewer fishing regulations. In the most protected area, fish abundance remained high and diversity indices were significantly higher post-marine heatwaves. Our results support the hypothesis that marine heatwaves can cause shifts in fish assemblages and that the precise nature of these shifts can vary over relatively short spatial scales that may coincide with scales of management.

A marine protected area network does not confer community structure resilience to a marine heatwave across coastal ecosystems

“…Climate change can rapidly reshape the distribution of species and the composition of ecological communities (Pörtner et al, 2023;Smale et al, 2019), imperiling nature's contributions to people. In particular, episodic periods of anomalous ocean warming, hereafter "marine heatwaves," are driving pronounced shifts in species distributions across marine ecosystems (Azzurro & D'Amen, 2022;Olsen et al, 2022), with direct implications for ecological processes and associated human benefits (Cheung et al, 2021;Cinner et al, 2022;Payne et al, 2021;Smale et al, 2019;Smith et al, 2023). While the urgency to plan for adaptation to climate change is clear, as marine heatwaves increase in frequency and severity (Holbrook et al, 2019), pathways to enhance ecosystem resilience are mixed.…”

Anomalous sea temperatures can impair coral reef fish recruitment

“…Climate warming, however, can benefit some species of temperate ectotherms by increasing their summer growing season [24]. Warmer winter temperatures have also been linked to increases in tropical fish diversity and abundance [25] with similar results seen on West Hawai'i reefs [26].…”

“…It appears that recruiting in early summer, before July, is not beneficial for recruits as they are exposed to SST below the thermal optimum. While the long-term mean SST of November is similar to July, and could provide recruits the optimal temperature to develop in, recruiting after July is more likely to expose the recruits to SST that are warmer than optimal, where heatwaves tend to occur during September onwards in Hawai'i [26,48].…”

Temporal patterns in coral reef fish assemblages with varying disturbance levels: A tale of two National Parks in Hawai´i

“…This observation confirms that the gradual shifts captured by d b t values in KAHO were likely trends occurring within individual regions rather than turnover of various species that happened to occur over consecutive years. Mechanisms of the shifts are unclear but may be partly related to the 2015 mass coral bleaching (e.g., Olsen et al, 2022). For example, the 2015 bleaching event resulted in complete loss of the branching coral Pocillopora meandrina along the 15 fixed transects (McCutcheon and McKenna, 2021), and P. meandrina was relatively abundant in the North (Site 01; Table 1) and South (Site 15; Table 1) regions.…”