Reduced fish diversity despite increased fish biomass in a Gulf of California Marine Protected Area

Spatial and temporal patterns in pelagic fish egg assemblages in spring and late autumn-winter in eastern Beibu Gulf

Semi-enclosed gulfs play important roles in global marine ecosystems, but they are vulnerable to anthropogenic disturbance and the effects of climate change. Beibu Gulf, the largest semi-enclosed gulf in China, is characterized by complex oceanographical conditions and high fish diversity, and it is an important fishing and spawning ground for many fish species. However, little is known about where these fishes spawn. We examine spatial and temporal distributions of fish eggs and their assemblages in the eastern Beibu Gulf in spring and late autumn-winter of 2020. A total of 75 taxa of fish eggs were identified, belonging to 9 orders, 33 families and 52 genera. In spring, the taxa are dominated by species in the families Clupeidae, Leiognathidae, and Carangidae (43.62%, 19.84%, and 12.51% of the total catch, respectively); from late autumn-winter, dominant families are the Engraulidae (27.52%), Sparidae (15.08%), and Clupeidae (13.32%). Five egg assemblages are recognized in spring, and four in late autumn-winter. Of available environmental variables, the sea surface temperature anomaly, water depth, and chlorophyll-a concentrations most affect fish egg assemblages. These results provide information to inform protection of fish spawning grounds and to aid fisheries management in Beibu Gulf.

Conservation benefits of a large marine protected area network that spans multiple ecosystems

Marine protected areas (MPAs) are widely implemented tools for long-term ocean conservation and resource management. Assessments of MPA performance have largely focused on specific ecosystems individually and have rarely evaluated performance across multiple ecosystems either in an individual MPA or across an MPA network. We evaluated the conservation performance of 59 MPAs in California's large MPA network, which encompasses 4 primary ecosystems (surf zone, kelp forest, shallow reef, deep reef) and 4 bioregions, and identified MPA attributes that best explain performance. Using a meta-analytic framework, we evaluated the ability of MPAs to conserve fish biomass, richness, and diversity. At the scale of the network and for 3 of 4 regions, the biomass of species targeted by fishing was positively associated with the level of regulatory protection and was greater inside no-take MPAs, whereas species not targeted by fishing had similar biomass in MPAs and areas open to fishing. In contrast, species richness and diversity were not as strongly enhanced by MPA protection. The key features of conservation effectiveness included MPA age, preimplementation fisheries pressure, and habitat diversity. Important drivers of MPA effectiveness for single MPAs were consistent across MPAs in the network, spanning regions and ecosystems. With international targets aimed at protecting 30% of the world's oceans by 2030, MPA design and assessment frameworks should consider conservation performance at multiple ecologically relevant scales, from individual MPAs to MPA networks.

Decadal shifts in traits of reef fish communities in marine reserves

Marine reserves are known to impact the biomass, biodiversity, and functions of coral reef fish communities, but the effect of protective management on fish traits is less explored. We used a time-series modelling approach to simultaneously evaluate the abundance, biomass, and traits of eight fish families over a chronosequence spanning 44 years of protection. We constructed a multivariate functional space based on six traits known to respond to management or disturbance and affect ecosystem processes: size, diet, position in the water column, gregariousness, reef association, and length at maturity. We show that biomass increased with a log-linear trend over the time-series, but abundance only increased after 20 years of closure, and with more variation among reserves. This difference is attributed to recovery rates being dependent on body sizes. Abundance-weighted traits and the associated multivariate space of the community change is driven by increased proportions over time of the trait categories: 7-15 cm body size; planktivorous; species low in the water column; medium-large schools; and species with high levels of reef association. These findings suggest that the trait compositions emerging after the cessation of fishing are novel and dynamic.

How fish traits and functional diversity respond to environmental changes and species invasion in the largest river in Southeastern China

Background

Freshwater fish populations are facing multiple stressors, including climate change, species invasion, and anthropogenic interference. Temporal studies of fish functional diversity and community assembly rules based on trait-environment relationships provide insights into fish community structure in riverine ecosystems.

Methods

Fish samples were collected in 2015 in the Min River, the largest freshwater riverine system in Southeastern China. Fish functional diversity was compared with the background investigation in 1979. Changes in functional richness, functional evenness, functional divergence, and functional beta diversity were analyzed. Relationships between functional diversity and environmental factors were modeled by random forest regression. Correlations between fish functional traits and environmental factors were detected by fourth-corner combined with RLQ analysis.

Results

Functional richness was significantly reduced in 2015 compared with 1979. Functional beta diversity in 2015 was significantly higher than that in 1979, with functional nestedness being the driving component. Reduction of functional richness and domination of functional nestedness is associated with species loss. Trait convergence was the dominant mechanism driving the temporal changes of functional diversity. Precipitation, temperature, species invasion, and human population were the most significant factors driving fish functional diversity. Higher precipitation, higher temperature, and presence of invasive species were significantly associated with higher swimming factor and higher relative eye diameter, while the opposite environmental conditions were significantly associated with higher pectoral fin length and eurytopic water flow preference.

Conclusions

Environmental filtering is the dominant temporal assembly mechanism shaping fish community structure. This work contributes to the understanding of temporal freshwater fish community assembly and the associations between fish functional structure and local environmental conditions, which will be informative for future freshwater fish conservation.