Networking under pressure: Environment and population density shape fish grouping structures

Grouping behaviour balances benefits such as predator avoidance against costs from intraspecific competition and is shaped dynamically by both biotic and abiotic factors. Yet the specific drivers of grouping in fish remain poorly understood. A better understanding of grouping behaviour could inform our knowledge of predator–prey dynamics and competition and aid the management and conservation of certain species. In this study conducted in Lake Ormstrup, Denmark, we investigated how abiotic and biotic factors influence grouping behaviour in fish, using roach, Rutilus rutilus , an important zooplanktivorous fish in lake environments, as a study organism. Individuals were tagged with acoustic transmitters and monitored using a fine-scale telemetry system. Groupings were quantified via proximity-based network analysis using relative density, the proportion of actual versus potential connections, as a response variable. We applied extreme gradient boosting regression trees to identify the key environmental drivers of grouping. We found that grouping behaviour increased in the morning or at midday during high temperatures and was highest in winter at low temperatures. Water clarity was a strong positive driver of grouping, potentially reflecting predator avoidance behaviour. Clear water often coincided with high zooplankton availability in summer. This abundance of food may mitigate intraspecific competition in roach, thus minimizing the cost of grouping. Interestingly, lower roach densities were associated with tighter groupings, likely due to increased predation risk and reduced competition. Our study illustrates the complex dynamic trade-offs associated with grouping behaviour and sheds light on how food web structure and behaviour interact to shape ecosystems. • Environmental factors shape grouping trade-offs, but long-term studies are rare. • Telemetry, network analysis and boosted models revealed drivers of fish grouping. • Grouping rose by day, at extreme temperatures, in clear water and at low densities. • Abiotic and biotic factors alter grouping trade-offs and predator–prey dynamics.