The increased efficiency of fish swimming in a school

TL;DR

This study uses a computer model to demonstrate that fish swimming in various school configurations generally achieve higher efficiency than solitary swimming, due to hydrodynamic interactions, even without intentional wake exploitation.

Contribution

The paper introduces a detailed hydrodynamic computer model that incorporates viscosity and wake interactions, revealing increased swimming efficiency in fish schools across multiple configurations.

Findings

Fish swim more efficiently in schools than alone.

Efficiency gains occur in various configurations, except in dense phalanx.

Hydrodynamic interactions alone can explain efficiency without wake exploitation.

Abstract

There is increasing evidence that fish gain energetic benefits when they swim in a school. The most recent indications of such benefits are a lower tail (or fin) beat at the back of a school and reduced oxygen consumption in schooling fish versus solitary ones. How such advantages may arise is poorly understood. Current hydrodynamic theories concern either fish swimming side by side or in a diamond configuration and they largely ignore effects of viscosity and interactions among wakes and individuals. In reality, however, hydrodynamic effects are complex and fish swim in many configurations. Since these hydrodynamic effects are difficult to study empirically, we investigate them in a computer model by incorporating viscosity and interactions among wakes and with individuals. We compare swimming efficiency of mullets of 12.6 cm travelling solitarily and in schools of four different configurations at several inter-individual distances. The resulting Reynolds number (based on fish length) is approximately 1150. We show that these fish always swim more efficiently in a school than alone (except in a dense phalanx). We indicate how this efficiency may emerge from several kinds of interactions among wakes and individuals. Since individuals in our simulations are not even intending to exploit the wake, gains in efficiency are obtained more easily than previously thought.