A novel method for investigating fish social behaviour in the field

Field studies of social behaviour are challenging due to having to observe multiple individuals simultaneously. In aquatic environments, these challenges are often amplified by environmental conditions such as habitat complexity, turbidity or darkness, which are often the conditions of interest in the context of studying anthropogenic impacts. Many fish species rely on social interactions for at least part of their life cycle, and while these interactions are known from laboratory studies to be impacted by environmental factors, approaches to quantify social behaviour under natural conditions are limited. Here, we present a novel method whereby multiple funnel traps are deployed simultaneously within a pond to quantify the aggregation and assortment of wild fish populations, using three-spine sticklebacks (Gasterosteus aculeatus) as a model species. The number of sticklebacks caught decreased in lower temperatures and through the seasons, from May to November. There was some evidence for decreases in fish numbers with reduced light intensity and higher dissolved oxygen levels. No effect of turbidity was found. The only evidence for changes in aggregation was related to the proportion of breeding males caught, where sticklebacks were less aggregated during the breeding season. These trends are expected based on previous work and knowledge about stickleback biology, validating the method. However, in contrast to previous studies, we found little evidence for phenotypic assortment. This study establishes a new cost-effective technique for investigating the social behaviour of wild fish, with some important benefits over existing techniques.