Satellite-Based Analysis of Night-Time Radiance Dynamics in Near-Shore and Offshore Aquaculture Systems in Çandarlı Bay (Northern Aegean Sea, Türkiye)

Artificial Light at Night (ALAN) has emerged as a growing environmental concern in coastal regions, where marine aquaculture operations coexist with expanding urbanization and tourism infrastructure. This study presents a decadal satellite-based assessment of night-time radiance dynamics associated with marine aquaculture systems in Çandarlı Bay, a semi-enclosed coastal environment in the Northern Aegean Sea, Türkiye. Monthly composites from the Visible Infrared Imaging Radiometer Suite Day/Night Band (VIIRS DNB) were analyzed for the period 2014–2024 to examine long-term trends in relative radiance at offshore and near-shore aquaculture sites, alongside marine and terrestrial control zones. Aquaculture installations were classified into two spatial typologies: a near-shore island-associated cluster and an offshore solitary system. To evaluate aquaculture-related radiance independently from broader coastal illumination, an Aquaculture Light Index (ALI) was developed to quantify radiance contrast relative to a dark-water marine reference. Temporal trends were assessed using STL decomposition, the Mann–Kendall trend test, and Theil–Sen slope estimation. The offshore aquaculture system exhibited no significant long-term trend in relative radiance, indicating a stable nocturnal light signature over the study period. In contrast, near-shore aquaculture sites showed a statistically significant decline in relative radiance contrast, coinciding with increasing night-time brightness in the adjacent terrestrial control area. These findings indicate that observed changes in aquaculture-related radiance are primarily associated with intensifying coastal background illumination rather than progressive increases in farm-level lighting. The study demonstrates the importance of spatial context and background radiance when interpreting satellite-derived ALAN signals in coastal aquaculture environments.