How functionally diverse are fish in the deep? A comparison of fish communities in deep and shallow‐water systems

Aim: Functional diversity metrics inform how species' traits relate to ecosystem functions, useful for quantifying how exploitation and disturbance impact ecosystems. We compare the functional diversity of entire fish communities in a shallow-water region with a deep-sea region for further insight into the differences between these ecosystem types.

Location: The regions compared in this study were selected to represent a shallow-water coastal region, Tasman and Golden Bays (TBGB), and a deep-sea region, Chatham Rise (CR), in New Zealand.

Methods: Functional diversity was assessed using four metrics: functional richness, evenness, divergence, and dispersion. We compared these metrics across four key functions: habitat use, feeding, locomotion and life history.

Results: Our results showed that overall, the shallow-water and deep-sea ecosystems had equal diversity. When focusing on the four ecological functions, the two ecosystems exhibited equal diversity metrics across most analyses. Of the significantly different results, the deep-sea had higher functional richness for habitat use and locomotion traits, lower functional dispersion for feeding, and lower functional evenness for life history.

Main Conclusions: Differences across the functions highlight higher diversity of habitat utilisation by deep-sea fish, while lower diversity in feeding suggests deep-sea fish tend towards generalist diets, likely driven by low food availability. Deep-sea fish displayed an increased range of locomotive traits in our analyses, but this conflicts with existing evidence and warrants further study. Life history results suggests deep-sea fish exhibit higher clustering of traits, indicating potential under-utilisation of life history strategies in the deep-sea. Our results demonstrate that although deep-sea fish communities have similar levels of diversity to shallow-water communities, the traits that structure this diversity differ, and therefore, the systems may respond to exploitation differently.