Tracking repeat spawning anadromous fish migrations over multiple years in a fragmented river suggests philopatry and sex-linked variation in space use

There is limited knowledge of how philopatry influences the spatial ecology of iteroparous anadromous species during their spawning migrations, but this knowledge is important in understanding population responses to interventions such as river reconnection. Here, acoustic telemetry was applied to twaite shad Alosa fallax and hybrids (n = 184) during their freshwater spawning migration, enabling quantification of philopatry across spawning migrations and assessment of the factors affecting space use. Tagged fish moved a median of 7 km day⁻¹. Their migration routes were tortuous (median ratio of total distance moved/upstream extent = 2.8), and included multiple upstream/downstream direction changes (median = 27) over a median freshwater movement distance of 247 km. Females occupied larger core areas than males, but previous spawning experience, body length, tagging status, and introgression with A. alosa did not predict core area size. Seventy-one fish returned a year after tagging, with a median freshwater residency of 33 days. Between years, intra-individual similarity in space use was significantly greater than inter-individual similarity, providing strong evidence of philopatry. These results provide insights into how spawning philopatry and phenotype influence riverine space use in a threatened anadromous species, and have implications for river reconnection efforts.