At least half of the world's population resides in the coastal zone and the livelihoods of billions of people are affected either directly or indirectly by the production and sustainability of nearshore fisheries. Landscape change, specifically development of tree plantations, is accelerating worldwide as developing countries integrate into global markets to sell goods, offer climate-mitigation services (carbon), and/or provide renewable energy. These changes can release excess nutrients into adjacent coastal waters causing eutrophication that alters the structure and function of coastal ecosystems. This study examined the relationship between coastal drainage basin land use/land cover change (LCLUC), specifically development of tree plantations, patterns of chlorophyll-a in nearshore coastal waters, and the biological condition of commercially important shellfish, Concholepas concholepas (loco) in southern Chile. Locos (N = 1374) were sampled across 13 watersheds (35 853 km2) and 42 fisheries management areas (spanning 250 km of coastline). Locos harvested from management areas influenced by tree plantations had approximately 30% more endobiont (shell-boring) phoronids, almost twice as many endobiont polychaetes and twice as many epibiont (shell-attaching) barnacles than locos from areas in close proximity to watersheds dominated by native forests (15–20% of the watershed). Phoronid infested locos from coastal waters adjacent to watersheds with tree plantations were of relatively poor biological condition (smaller and narrower in width) and of reduced market value. Our study suggests that tree plantations result in indirect ecological impacts to coastal fisheries (more nutrients and higher phytoplankton biomass, resulting in smaller, low quality locos), and costs are born by coastal fishers (lower prices for locos). Increases in tree plantations could thus potentially significantly impact coastal fisheries worldwide and such problems should be managed as an interconnected network of land use change, oceanic ecosystems, and economic systems that are considered an integrated socio-ecological system.