Development of a metabolic bio-indicator of nursery habitat quality for juvenile snapper (Chrysophrys auratus, Sparidae)

Abstract

Estuarine nursery habitats for marine fish contribute disproportionately to the adult stock relative to other areas juveniles inhabit, yet are amongst the most anthropogenically degraded ecosystems on Earth. Thus, early life stages are critical to adult recruitment, yet are the most vulnerable to direct and indirect effects of anthropogenic disturbance, which may work synergistically to exacerbate deleterious effects. It is therefore necessary to advance methodology which allows researchers to assess overall physiological response of juvenile fish to the mosaic of stressors present in nursery habitats. This study aimed to utilise the relatively new field of metabolomics to pinpoint metabolites or metabolic pathways which may prove valuable as bio-indicator of habitat quality for juvenile snapper, a culturally and commercially important species. Juveniles were collected from harbours which represent a gradient of nursery habitat quality based on water clarity and sediment type. Differences in growth using otolith increments were not reflective of the gradient, however metabolomic profiling was highly sensitive and reproducible, with a multivariate Q-DFA approach discriminating juveniles with 100% accuracy both between and within harbour sites. The differences in wild-caught juveniles was largely driven by metabolites and pathways involved in energy production, growth processes, or immune response. The amino acid 4-aminobutyric acid (GABA) is linked to growth and development, and was elevated in juveniles from more pristine harbours; it is a promising candidate for use as a biomarker of habitat quality. Concurrently, an experimental trial was conducted on juveniles exposed to different levels of turbidity which included a known threshold for optimal health. While the analysis was successful in discriminating juveniles from different treatment groups in the experimental trial, results were not reflective of a physiological response to turbidity alone. This work provides a novel baseline for the use of metabolomic profiling on wild-caught snapper to inform management and conservation decisions, with the potential to identify biomarkers reflective of habitat quality which physical measures alone cannot detect. Future studies should aim to identify the underlying cause of the stress response exhibited in metabolomes of juveniles from harbours of diminished quality by isolating the effect of individual stressors, and by examining metabolomic plasticity over temporal scales.