Retention rate and biological effects of Floy and FRyID tags on juvenile longfin smelt (Spirinchus thaleichthys)

Tagging technologies are essential for fish population tracking, yet their physiological effects on small-bodied species remain underexplored. Juvenile longfin smelt ( Spirinchus thaleichthys ), a threatened estuarine species, provides a model for evaluating tagging-related stress in conservation aquaculture and release programs. This study assessed growth, condition, infection, behavior, and survival in juveniles subjected to three treatments: Handling Control (no tag), Floy tag (external, 26.6 mg), and FRyID (implantable radio-frequency identification, 1.7 mg) tag over 90 days. Fork length and body weight increased significantly over time (p < 0.001) across all treatments, with no significant effects of tag type or tag burden (p > 0.05). Growth scaling (log BW–log FL) showed a strong positive allometric pattern (p < 0.001) that remained consistent among treatments. Fulton’s condition factor (K) varied significantly across treatments (p < 0.01), with Floy-tagged fish exhibiting higher K than Control and FRyID groups, while tag burden negatively influenced K (p < 0.001). Tag retention was higher in Floy fish (90.0 %) than FRyID fish (63.3 %, p < 0.001) and was associated with infection and mortality (p < 0.05). Infection prevalence was highest in FRyID fish (31.7 %) and linked to emaciation, tag loss, and mortality (p < 0.01). Behavioral abnormalities were most frequent in Floy fish (∼1.45 events/day) and declined over time (p < 0.05). Survival decreased gradually but did not differ among treatments (p > 0.05). These findings highlight species- and size-specific trade-offs between tag retention, infection, and welfare, providing guidance for safer tagging practices in conservation aquaculture. • Growth increased over time, with no significant effect of tag type. • Tag burden negatively influenced Fulton’s condition factor. • FRyID tags increased infection risk, while Floy tags caused more emaciation. • Infection elevated mortality risk, but tag loss improved survival. • Results guide tag selection for small, delicate conservation species.