Fragmentation of river systems by dams and weirs is a major factor in the decline of riverine fishes, due to interruption of migratory pathways. The leading strategy for mitigating these impacts has been installation of fish passage structures. This strategy assumes that successful upstream migration is required to ensure reproduction and recruitment. However recent studies of Neotropical potamodromous fishes have shown that this strategy may be flawed, creating situations where fish may follow migration cues towards less favourable habitats for successful recruitment than those downstream, known as an ecological trap. Whilst the need for unrestricted movement in potamodromous fishes is recognized, the evaluation of fishways rarely considers how factors beyond upstream movement of fishes influence conservation success.
While data is lacking on factors such as migratory habits, habitat availability and migratory motivation, modelling hypothesised behaviours along migratory pathways in response to environmental cues, can assist with identifying potential recruitment scenarios. In this study, individual-based modelling (IBM) is used to explore how presence of fish passage infrastructure can act as an ecological trap depending on the availability of suitable spawning and nursery areas upstream and downstream of migration barriers. The model simulates hypothesised effects of facilitating fish passage on recruitment and abundance of Murray cod and Golden Perch in a regulated low-land river.
Behavioural rules and parameters are assigned to individuals, based on existing data and assumptions about migratory behaviour, habitat requirements and environmental parameters. Scenarios of recruitment are compared under two fish migration paradigms: 1) that successful recruitment and conservation is dependent on unrestricted upstream migration, or 2) that successful conservation is achieved through improved access to quality habitats. We anticipate that IBM will contribute to identifying knowledge gaps and testable hypotheses in the study of fish passage, migratory behaviour and recruitment of potamodromous fishes in regulated systems.