Sterile 'Judas' carp - Surgical sterilisation does not impair growth, endocrine and behavioural responses of male carp

Use of ‘Judas’ fish to betray the locations of conspecifics is a powerful tool in management of invasive pest fish but poses a risk of contributing to recruitment. Our aim therefore was to generate surgically sterilised male common carp (Cyprinus carpio) and test whether they readily assimilate into wild populations, retain sexual behaviour and successfully betray the locations of feral carp. Male common carp were surgically sterilised (n = 44) adopting a two-point nip technique, using either a haemoclip, suture or electro cautery to tie each of the testicular ducts about 2.5 cm cranial to urogenital sinus—retaining all of the glandular testis tissue. Observed survival (95%) and success (>70%) rates were relatively high. Plasma steroids (11-keto testosterone and 17β-estradiol) were quantified by immunoassay. A subset of sterile and control male fish (n = 7 each) were implanted with radio-transmitters and released into Lake Sorell (50 km²) and their ability to betray the location of feral carp was assessed by radio tracking and targeted fishing. There was a statistically significant difference in 11-keto testosterone and 17β-estradiol levels over time (P < 0.05), but not between the sterile and control groups within each sampling time (P > 0.05), implying that surgery did not compromise the animals physiologically. The sterile Judas fish integrated well into the population—behaving similarly to control Judas males and assisted in the capture of feral carp. The study marks a significant breakthrough in the management of this pest fish with potential adoption to the management of other pest fish globally.