Sex-specific probability of PIT tag retention in a cyprinid fish

Abstract The estimation of age- and sex-specific mortalities is essential for constructing fish life tables and subsequent population modelling. However, the ecological data behind these parameter estimates acquired in nature may be systematically biased by the methodology of the study. To demonstrate mark retention bias by the widely used passive integrated transponder tags (PIT tags), we analysed long-term data showing the probability of tag retention within the body cavity in relation to fish sex. We used the cyprinid fish asp (Leuciscus aspius) as a model species with a cystovarian type of ovulation (eggs do not enter the body cavity). Altogether, 2312 fish were tagged with PIT tags in 2014–2018 and fin clipped and monitored for up to five years. In this period, 583 asp were recaptured at least one year after tagging, and fish that lost their tags were identified by regenerated fin. Based on recaptured fish, we show that the female tag retention rate is significantly lower than the male retention rate (85.3% in females vs. 97.8% in males). Furthermore, we used portable antenna to detect expelled PIT tags and localized a higher number of expelled female PIT tags on the spawning ground in comparison with those from males (123 female vs. 6 male PIT tags). This study demonstrates that systematic bias may potentially occur since PIT tags may penetrate gonads and leave the body during reproduction. We encourage designing PIT tag monitoring studies with respect to potential unequal sex-specific tag retention capabilities to avoid drawing inaccurate conclusions from biased data.