Abstract: Island ecosystems, as critical marine ecological units with high biodiversity and productivity, are the core support for offshore fisheries and the “seed bank” of fishery resources, underpinning the livelihoods of approximately 500 million people globally. Under the dual pressures of global climate change and intensive human activities, island ecosystems are undergoing severe structural transformation and functional degradation, with coral bleaching, habitat loss, overfishing, and land-sea pollution jointly impairing their fishery service functions and threatening the sustainable development of coastal regions. This study aims to systematically elaborate the fishery functions, degradation mechanisms, and stress factors of island ecosystems, and propose targeted governance strategies to provide theoretical support and practical solutions for ecological protection and sustainable fishery utilization of island ecosystems. Based on literature review, inductive analysis, and comprehensive judgment, this paper systematically analyzes the core functions, stress mechanisms, and international research progress of island ecosystems, and explores the internal mechanism and governance dilemmas of fishery function degradation. The results show that island ecosystems have four key fishery functions: habitat provision, spawning and nursery, energy conversion, and nutrient supply. Fish biomass in coral reef areas reaches 50-300 t/km², and 30-150 t/km² in rocky reef areas, both significantly higher than that in muddy waters; the larval spillover rate of coral reefs is 20%-40%, and juvenile fish density in natural oyster reefs is 6 times that in adjacent muddy sediments. The annual total system flux of natural rocky reefs in Yantai coastal waters is 9490.48 t/km², with an average energy transfer efficiency of 10.56%, close to the theoretical optimal Lindeman efficiency. However, island ecosystems face severe stresses: marine heatwaves reduce coral calcification rate by 45%-62% and reef structural complexity by 38%-55%; overfishing with top predator removal rate exceeding 60% triggers trophic cascades, leading to 74.64% of energy flowing to detrital pathways in Hainan oyster reef ecosystems; total mercury content in surface sediments of nearshore coral reefs in the South China Sea is up to 60-80 ng/g; combined stresses increase the annual decline rate of coral cover from 8.2% to 18.9% and reduce ecosystem resilience index by 67%. In addition, imperfect management systems, unclear carrying capacity, imbalance between protection and development, and fragmented governance measures exacerbate the continuous degradation of fishery functions. To resolve the degradation dilemma of island ecosystems, this study concludes that we should adhere to the concepts of ecosystem integrity and land-sea coordination, strengthen space-air-sea integrated monitoring and assessment, implement ecosystem-based fisheries management, carry out classified habitat restoration by degradation degrees, and build a government-led, science-supported, and community-participated land-sea collaborative governance mechanism. These strategies will realize the coordinated development of ecological protection and sustainable fishery utilization of island ecosystems, and provide a scientific basis for enhancing the resilience and sustainable service capacity of island ecosystems worldwide.