Contrasting environmental and demographic influences on growth variability in two tropical snappers spanning the equator in the Indo-West Pacific

Climate change potentially poses a major threat to tropical marine fish populations, many of which are nearing their thermal tolerance limits. Despite their importance to global food security, our understanding of how changing ocean conditions will affect these fish populations, particularly from the equator, remains limited. Annual growth increments accreted in fish otoliths, when correlated to environmental parameters, provide valuable insights into how species might respond to climate variation. This study aimed to assess the relative importance of demographic and environmental drivers on the annual otolith growth expression of two tropical snappers, Lutjanus johnii and L. malabaricus, from the Indo-West Pacific region. We analysed 5594 growth increments from the otoliths of 1042 fish, reconstructing growth chronologies over four decades, spanning more than 25 degrees of latitude across the equator. For L. johnii from Southeast Asia, growth was explained by mean sea surface temperatures over the past two years, consistent with the Temperature-Size Rule. Growth of L. johnii from northwestern (NW) Australia was linked to the intensity of the Indian Ocean Dipole during the austral summer and autumn, likely associated with changes in ecosystem productivity. For juvenile L. johnii from NW Australia, growth was negatively impacted by rainfall in February and March. In comparison, L. malabaricus showed a more pronounced influence of demographic variables across regions and life stages, with only weak effects from environmental drivers. These findings highlight the complex growth responses of these species to demographic and environmental factors from an important yet understudied region of the world.