Detecting exploitable stages in the life history of koi carp (Cyprinus carpio) in New Zealand

Abstract

Koi carp are a highly coloured strain of common carp (Cyprinus carpio L.) and are an invasive alien species to New Zealand. The introduction of koi carp to the lower Waikato River during the 1980s has coincided with a dramatic decline in water quality within the regions riverine lakes. Removing koi carp from riverine lakes may improve water quality and preserve native biodiversity. This study used telemetry to examine the movements and seasonal habitat use of adult koi carp to find key locations to efficiently remove koi carp from the lower Waikato River basin. Common carp are notoriously difficult to monitor in water temperatures above 20˚C using telemetry due to transmitter expulsion. To prepare for telemetry based field experiments, three tank trials were conducted to determine the most appropriate surgical technique for the implantation and retention of telemetry transmitters in koi carp. To reduce transmitter loss, two methods were initially tested in tank trials including two treatment groups and two control groups of ten adult koi carp. In one treatment, radio transmitters were anchored to the pelvic girdle, and in a second treatment, a polymer coating was applied to acoustic transmitters to reduce tissue irritation. After 365 days, expulsion rates for uncoated acoustic transmitters were 60% (n = 6), coated acoustic transmitters 50% (n = 5), unanchored radio transmitters 60% (n = 6) and anchored radio transmitters 90% (n = 9). Bacterial infection of the wound appears to be the primary mechanism for transmitter expulsion. To reduce the influence of bacterial infection on transmitter expulsion, a slow-release antibiotic was tested to improve transmitter retention. Dummy acoustic transmitters were implanted in two groups of 20 adult koi carp, an untreated control group, and a group treated with a slow-release, single-dose antibiotic. After 176 days, the rejection rate of control fish (40%, n = 8) was twice that of fish treated with a slow-release antibiotic (20%, n = 4). Although we did not find a statistical difference between test groups, this was likely a function of sample size and we view the results as encouraging, justifying further investigation and use of slow-release antibiotics in koi carp. Koi carp were monitored on the lower Waikato River using both radio and acoustic telemetry for 18 months. Koi carp migrated throughout the basin and frequently used lateral habitat. At least 74% of fish that were monitored for at least 250 days migrated through the waterways between riverine lakes or wetlands and the Waikato River. Waterways connecting riverine lakes and wetlands to the lower Waikato River are ideal locations to intercept migrating koi carp. Fish released at Lake Whangape occupied a total linear range (TLR) of over 120 km in just 218 days, which represents 71% of the potential in river TLR within the lower Waikato River Basin. Adult koi carp were found to be active year-round, but peak movements occurred during spawning season (October) during both years of the study. With a long-term commitment and thoughtful planning, removing a biologically significant proportion of koi carp biomass from the riverine lakes and wetlands of the lower Waikato River is feasible.