Marine protected areas do not prevent marine heatwave-induced fish community structure changes in a temperate transition zone

Marine heatwaves, periods of abnormally high ocean temperatures lasting at least five days, are increasing in frequency and intensity due to climate change. These events can drastically alter marine ecosystems, potentially leading to shifts in community structure, biodiversity loss, and changes in ecosystem services. Understanding how marine communities respond to these heatwaves and identifying effective management strategies are critical for ocean resource management. While species are predicted to shift their ranges in response to warming, individual communities react uniquely based on their sensitivity to local environmental conditions. Some species might thrive during heatwaves, while others suffer negative impacts. Marine Protected Areas (MPAs) are often promoted as a tool to buffer communities from climate change, offering potential resistance and resilience to various pressures, including heatwaves. However, the extent to which MPAs can mitigate large-scale community structural changes caused by marine heatwaves remains largely unknown. This study aimed to quantify the magnitude and consistency of changes in kelp forest fish communities in response to a marine heatwave and assess the effectiveness of MPAs in mitigating these shifts in a biogeographic transition zone characterized by a strong thermal gradient.

The literature highlights the significant impacts of marine heatwaves on marine ecosystems, with studies documenting rapid shifts in community structure within months. While some species benefit from warmer waters, others are negatively affected, leading to alterations in ecosystem function and services. MPAs have been proposed as a potential mitigation tool, with some evidence suggesting benefits in resisting and enhancing resilience to climate-related pressures. However, whether MPAs can effectively mitigate large-scale community changes during marine heatwaves has been an open question. Studies have shown that MPAs may benefit single species' recovery from heatwaves, but their effect on entire community structures during such events is less clear.

The study utilized a 17-year (2001-2017) dataset of kelp forest fish community data from the Channel Islands National Marine Sanctuary (CINMS) in Southern California, a region with a strong east-west thermal gradient. The CINMS contains a well-established network of MPAs. Fish species were classified as warm-water or cool-water affinity based on their biogeographic distribution and abundance patterns. Data were collected via SCUBA surveys, which involved divers counting and measuring fish along transects at different water column depths. Biomass was estimated using length-weight relationships. Recruitment was assessed using artificial larval fish collectors (SMURFs). Data were analyzed before, during, and after a major marine heatwave (October 2014-June 2016). Bayesian Highest Density Interval (HDI) and Region of Practical Equivalence (ROPE) tests were used to assess annual similarity among warm-water and cool-water species groups. Linear mixed models were employed to test the effects of MPAs and targeted fishing status on fish density during the heatwave. The analysis considered abundance, biomass, diversity (effective species number), and recruitment for each thermal group, both inside and outside MPAs, and further divided species into 'targeted' (fished) and 'non-targeted' categories.

The marine heatwave significantly impacted the kelp forest fish community. Warm-water species exhibited a rapid and significant increase in density and recruitment during and immediately after the heatwave. Cool-water species showed less dramatic, non-significant responses. Biomass changes were less pronounced than density changes. MPAs did not significantly mitigate the community shifts caused by the heatwave. Both warm-water and cool-water species densities changed similarly inside and outside MPAs. The analysis of targeted versus non-targeted species revealed that non-targeted species drove the observed community changes, with targeted species showing muted responses. This suggests that the cessation of fishing within MPAs did not protect the species most affected by the heatwave, as those species were not targeted by fishing.

The findings highlight the significant impact of marine heatwaves on kelp forest fish communities and challenge the assumption that MPAs alone can mitigate these impacts. The lack of MPA effectiveness in this case is likely due to the fact that the species most affected by the heatwave were not targeted by fisheries. The results underscore the need for flexible and dynamic management strategies that consider the specific responses of different species to climate change. While MPAs remain important conservation tools, additional measures such as habitat restoration or stock enhancements may be necessary to complement MPA protection in mitigating climate change effects on marine ecosystems. The study's focus on an acute event might not fully capture long-term impacts, suggesting a need for continued monitoring to understand recovery trajectories.

This study demonstrates that marine heatwaves significantly alter kelp forest fish community structure, and that existing MPA networks may not adequately protect these communities from the impacts of such events. The results emphasize the importance of considering species-specific responses to climate change and implementing management strategies beyond MPAs to effectively mitigate climate change impacts on marine ecosystems. Future research should focus on examining long-term recovery patterns after heatwaves and exploring other management strategies in conjunction with MPAs, such as habitat restoration or stock enhancements. Understanding the interplay between fishing pressure and climate change impacts is also critical for effective resource management.

The study's focus on a single marine heatwave event limits the generalizability of the findings to other events or regions. The specific characteristics of the heatwave (intensity, duration) might influence the observed responses. The study primarily focused on density as a measure of response, while other aspects of community structure might be affected differently. Finally, the study primarily focuses on fish communities, leaving out other important components of the ecosystem that might exhibit different responses.