Thresholds in seascape connectivity: the spatial arrangement of nursery habitats structure fish communities on nearby reefs

Abstract

Ecosystems are linked by the movement of organisms across habitat boundaries and the arrangement of habitat patches can affect species abundance and composition. In tropical seascapes many coral reef fishes settle in adjacent habitats and undergo ontogenetic habitat shifts to coral reefs as they grow. Few studies have attempted to measure at what distances from nursery habitats these fish migrations (connectivity) cease to exist and how the abundance, biomass and proportion of nursery species change on coral reefs along distance gradients away from nursery areas. The present study examines seascape spatial arrangement, including distances between habitats, and its consequences on connectivity within a tropical seascape in Mozambique using a seascape ecology approach. Fish and habitat surveys were undertaken in 2016/2017 and a thematic habitat map was created in ArcGIS, where cover and distances between habitat patches were calculated. Distance to mangroves and seagrasses were significant predictors for abundance and biomass of most nursery species. The proportions of nursery species were highest in the south of the archipelago, where mangroves were present and decreased with distance to nurseries (mangroves and seagrasses). Some nursery species were absent on reef sites farthest from nursery habitats, at 80 km from mangroves and at 12 km from seagrass habitats. The proportion of nursery/non-nursery snapper and parrotfish species, as well as abundance and biomass of seagrass nursery species abruptly declined at 8 km from seagrass habitats, indicating a threshold distance at which migrations may cease. Additionally, reefs isolated by large stretches of sand and deep water had very low abundances of several nursery species despite being within moderate distances from nursery habitats. This highlights the importance of considering the matrix (sand and deep water) as barriers for fish migration.

Drivers for genetic structure at different geographic scales for Pacific red snapper (Lutjanus peru) and yellow snapper (Lutjanus argentiventris) in the tropical eastern Pacific

“…Besides the gaps in habitat, other studies have found that at scales <1000 km the larval dispersal and recruitment were the main processes affecting genetic structure (Berkström et al, 2020;Dalongeville et al, 2018;Pascual et al, 2017;Schunter et al, 2019).…”

“…These include habitat discontinuities (Blanco‐Gonzalez et al ., 2016; Sandoval‐Huerta et al ., 2019; Torquato et al ., 2019) and strong environmental gradients (García‐De León et al ., 2018; Sandoval‐Huerta et al ., 2019). In addition, the life cycle of each species, including the breeding season, dispersal ability (Frisk et al ., 2012; Treml et al ., 2012) and the location of the habitats for juveniles and adults, plays an important role in population demography, connectivity and the location of those genetic breaks (Berkström et al ., 2020; Blanco‐Gonzalez et al ., 2016; Pascual et al ., 2017).…”

“…The planktonic larval phase (Peña et al ., 2017; Zapata and Herrón, 2002) and ontogenetic habitat shifts (Berkström et al ., 2020), together with physical factors such as geographic distances, marine currents, habitat discontinuities and environmental differences (temperature and salinity) (Berkström et al ., 2012, 2020; Blanco‐Gonzalez et al ., 2016; García‐De León et al ., 2018; Reguera‐Rouzaud et al ., 2020), are the main processes determining genetic structure or homogenization of the populations (Reis‐Santos et al ., 2018; Schunter et al ., 2019).…”

“…Genetic structure because of isolation by distance has been found at scales ≥1000 km (Dalongeville et al ., 2018; Kennington et al ., 2017), whereas on smaller scales the genetic structure is caused mainly by larval dispersal driven by ocean currents (Dalongeville et al ., 2018; Perez‐Enriquez and Taniguchi, 1999). The availability of nursery habitats near rocky reefs within a radius of 150 km from mangroves also increases the abundance and connectivity of snappers (Berkström et al ., 2020). In this regard, habitat discontinuities, strong environmental gradients and permanent oceanographic fronts have been reported to be physical barriers to gene flow (Blanco‐Gonzalez et al ., 2016; García‐De León et al ., 2018).…”

“…and the location of the habitats for juveniles and adults, plays an important role in population demography, connectivity and the location of those genetic breaks (Berkström et al, 2020;Blanco-Gonzalez et al, 2016;Pascual et al, 2017).…”