Relative influence of environmental factors on the timing and occurrence of multi-species coral reef fish aggregations

A social-ecological engagement with reef passages in New Caledonia: Connectors between coastal and oceanic spaces and species

Healthy and protected coral reefs help island systems in the tropics thrive and survive. Reef passages link the open ocean to lagoon and coastal areas in these ecosystems and are home to an exceptionally diverse and abundant marine life, hosting emblematic species and fish spawning aggregations. Their multiple benefits for the islands and their peoples (e.g., for transport, fishing, socio-cultural aspects) remain yet understudied. Drawing from qualitative interviews with fishers, scuba divers, and surfers along the coast of Grande Terre in New Caledonia, this study highlights the multi-faceted importance of these keystone places. It shows that reef passages are locally deemed 'communication zones' between coastal and oceanic spaces and species, and have significant un(der)explored ecological and socio-cultural roles. Understanding and protecting these ecological and cultural keystone places will strengthen both the reef ecosystems and the people dependent on them.

Status of Napoleon wrasse in Laamu Atoll, Maldives, after three decades of protection

The Napoleon or humphead wrasse Cheilinus undulatus is a large, globally threatened coral reef fish. In recognition of suspected declines and considering its ecotourism importance, the Maldives was the first country to ban its export in 1995. The present study, a quarter-century after the ban, documents the species around Laamu Atoll, southern Maldives, using dive guides and researchers to conduct surveys during commercial dive tourism trips. Data were collected on abundance, body size, and potential spawning sites in 4 habitat types: channels (reef passes to open ocean), inner reefs (reef slopes inside the atoll), outer reefs (reef slopes outside the atoll), and faros (isolated submerged reefs inside the atoll). Abundance was recorded in number of fish per hectare, and fish length was classified as <100 cm, ≥100 cm, or unknown size. Possible spawning aggregation sites were identified based on multiple occasions of temporarily increased fish density associated with large male presence. Mean densities ranged from 6.02 fish ha-1 in channels, 2.65 and 2.29 fish ha-1 in outer and inner reefs, respectively, and 0.73 fish ha-1 in faros. These densities are consistent with those found in unexploited areas in similar habitats elsewhere and indicate successful protection of the species on Laamu Atoll. The data highlight the significance of reef channel habitat for this species as important megafauna and the need for management to conserve both species and channel habitats. The value of non-specialist observers for conducting labour-intensive surveys on large, wide-ranging species is demonstrated.

Coral reef carnivorous fish biomass relates to oceanographic features depending on habitat and prey preference

Carnivorous fish are a key part of the Indonesian human population sustenance, and it is important to design marine protected areas that include environmental features that allow these species to thrive. Many studies report the role of coral cover and habitat complexity in determining fish distribution on coral reefs but broader environmental factors such as current velocity and productivity are less studied. Southern Indonesia is characterised by upwellings and strong currents, stemming from the tidal cycle and the Indonesian Throughflow. In this study we investigate how current velocity, chlorophyll-a (chl-a), sea surface height and temperature relate to the biomass of carnivorous fish, considering the influence of habitat complexity and coral cover. Data were collected by surveying seven sites around Nusa Penida MPA for a total of 97 h of observation. Serranids and Lutjanids showed higher dependency on coral cover than fish from family Lethrinidae, Carangidae and Scombridae for which current, sea surface height, chl-a, and temperature were more influential predictors. Considering the similar trophic ecology of these species, the different relationship with oceanographic factors is likely related to different body shapes, living, and feeding habits between fish families. Changes in sea surface temperature and current velocity induced by vertical mixing are affecting coral reef fisheries-targeted species distribution in Nusa Penida and investigating these relationships on a broader scale will better inform marine spatial planning decisions.

Spatial subsidies drive sweet spots of tropical marine biomass production

Spatial subsidies increase local productivity and boost consumer abundance beyond the limits imposed by local resources. In marine ecosystems, deeper water and open ocean subsidies promote animal aggregations and enhance biomass that is critical for human harvesting. However, the scale of this phenomenon in tropical marine systems remains unknown. Here, we integrate a detailed assessment of biomass production in 3 key locations, spanning a major biodiversity and abundance gradient, with an ocean-scale dataset of fish counts to predict the extent and magnitude of plankton subsidies to fishes on coral reefs. We show that planktivorous fish-mediated spatial subsidies are widespread across the Indian and Pacific oceans and drive local spikes in biomass production that can lead to extreme productivity, up to 30 kg ha-1 day-1. Plankton subsidies form the basis of productivity "sweet spots" where planktivores provide more than 50% of the total fish production, more than all other trophic groups combined. These sweet spots operate at regional, site, and smaller local scales. By harvesting oceanic productivity, planktivores bypass spatial constraints imposed by local primary productivity, creating "oases" of tropical fish biomass that are accessible to humans.