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Common carp (Cyprinus carpio L.) are a freshwater pest fish in Australia. To efficiently plan and implement an integrated carp management program, we need to understand when, where and why carp move. This allows control efforts to be targeted and more efficient.
Direct observation of fish by divers or video provides information on habitat use, movements and biotic interactions, but it is not possible to record physiological data. Diving and video observations are restricted to shallow waters with adequate light and/or small areas.
Capturing, radio-tagging and releasing carp to act as ‘tracker’ or ‘Judas’ fish reveals the location of carp aggregations
Mark-recapture experiments involve capturing live fish and physically marking or tagging them with a personal identification number before they are released. This generally involves the use of internally implanted magnetic or biological tags, or external tags (eg T-bar tags). Mark-recapture experiments can provide information on population dynamics and large-scale movement and migration patterns, but tagging can present problems related to handling the animal, deaths related to tagging, or tag-shedding. Their use is also limited because data on the location and biological state of the tagged animals are recorded irregularly, limiting the range of behaviours we can study.
The tagged animal must also be recaptured, meaning that these methods can only be used where the fishing effort allows an acceptable recapture rate.
Telemetry is a step forward in the study of animal behaviour in the wild. Telemetry involves the wireless transfer of information by radio, digital, ultrasonic/acoustic or infrared signals from transmitters attached or implanted into animals to a remote receiver system. Telemetry improves our understanding of carp distribution, habitat preferences, home ranges and migration requirements by allowing us to monitor their location, behaviour and physiology continuously and individually in uncontrolled environments. Tracking systems can either be:
Environmental or habitat variables (eg depth, temperature, salinity and vegetation) and movement parameters (eg speed, distance, orientation and timing) can be measured using telemetry.
In telemetry, information is stored either with archival tags, or tags that can send their information to a satellite or hand-held receiving device. Fixed-location techniques use stationary transducers to monitor passing fish. The recent introduction of electronic data storage tags or archival tags allows for continuous monitoring of the animal’s immediate environment, or the microenvironment, and has increased reliability and decreased the cost.
Systems have been developed based on passive integrated transponder (PIT) and code-activated transponder (CAT) tags. A PIT tag is an electronic microchip encased in biocompatible glass, which protects the electronic components and prevents tissue irritation. The tag is usually between 10 and 14 mm long and 2 mm in diameter. PIT tags are injected with a needle or inserted by surgical incision under the skin of the fish. Studies of juvenile individuals are now possible because of the small size of PIT tags. PIT tags are ‘passive’ because the tag is inactive until activated by a close-range, electromagnetic field of a hand-held receiver. In contrast, CAT tags require two incoming pulses spaced at an appropriate interval to be activated (ie pulse-position coding).
Both acoustic and radio telemetry are suitable for use in aquatic environments, but there are costs and benefits associated with each. For the majority of freshwater studies, radio telemetry is the method of choice. The wide range of sophisticated transmitting, receiving and data-logging systems available to researchers, coupled with the relative ease with which radio telemetry equipment can be deployed and the studies conducted, has resulted in its wide-spread use.
Despite the wealth of potential applications of telemetry, sensory information tags are expensive compared to acoustic tags. Consequently, most studies have used simple ‘pingers’, or acoustic transmitters, only to obtain information on patterns of movement, dispersal/migrations and habitat use by fish. In most cases, the characteristics of the environment occupied by the tagged individuals have been recorded by samplings coinciding with the movement data. The problem of recapture still persists with telemetry tags. Compared with many other fish species, carp are difficult to tag. This is because carp often expel radio transmitters in water temperatures above 20˚C as a result of bacterial infection at the wound site. The use of slow-release antibiotics in carp will reduce the influence of bacterial infection on transmitter expulsion1. The size of the study area depends on the range of transmitters and receivers.
A radio telemetry study in River Murray, Victoria, has confirmed that carp make annual, predictable migrations between main-river channel for overwintering and shallow wetlands for summer-spawning. Significantly, over 90% of carp moved from the Murray River into adjacent floodplain habitats during flooding2. This suggests that carp management efforts in regulated river-floodplain environments should target the key floodplain access points and overwintering habitats. Aggregation of carp at migration barriers (eg weirs and carp screens) may persist for several months, which can allow for targeted removal. The ‘Judas carp’ technique, where males are radio-tagged and act as ‘tracker’ fish, can be used to target such aggregations as part of carp harvesting operations.
Nonetheless, carp movement patterns can be complex and may vary with individuals. For instance, the Victorian study found that the majority of carp (65%) stayed within 100 m of their home sites, but the remainder moved further, with two fish moving approximately 650 km downstream2. Carp movement patterns can also be different in other regions, and alternative management strategies may be more appropriate for other habitats. For example, a New Zealand study found that adult koi carp were active all year round and moved a total of 164 km from the tagging locations, with individuals travelling as far as 232 km in a year at rates of up to 13 km per day3.
The Invasive Animals Cooperative Research Centre (IA CRC) funded a long-term project to investigate carp movement around the Murray-Darling Basin (MDB). The innovative project used combined telemetry technologies, including passive integrated transponder (PIT) tags, code-activated transponder (CAT) tags and embedded remote sensors. This allowed the researchers to better understand carp stock-structure within the MDB, quantify movement among stocks and explore ‘triggers’ for movement. This ground-breaking project has also helped identify how carp movement behaviour at specific times throughout their life story may be exploited in carp control (eg aggregation, passage through fishways etc).
Radio telemetry has been used with success for carp in Tasmania4. Since 1997, the Inland Fisheries Service has been capturing, radio-tagging and releasing carp to act as tracker fish, allowing them to identify the location of carp aggregations. These aggregations can then be targeted using traditional harvesting methods, such as netting or electrofishing. The resulting carp database has been a useful management tool in the eventual eradication of carp from Lake Crescent and continues to assist in the eradication of carp from Lake Sorell.
Radio telemetry is also being tested in Lake Cargelligo in the lower Lachlan catchment of New South Wales as part of the IA CRC’s River Revival Lachlan River Carp Cleanup project. The aim is to inform commercial carp fishers of where carp aggregate or move depending on various environmental conditions, such as water temperature, wind and air pressure. This knowledge will hopefully maximise the efficiency of their catch in terms of time and the amount of carp captured. Radio telemetry in itself is not a ‘silver bullet’ for carp control, but can assist in determining the best targeted management strategies for specific environments and increase the efficacy of carp control over a sustained period of time.
The authors of these documents have taken care to validate the accuracy of the information at the time of writing. This information has been prepared with care but it is provided “as is”, without warranty of any kind, to the extent permitted by law. The views and opinions expressed in this publication are those of the authors and do not necessarily reflect those of the institutions the authors work for or those who funded the creation of this document.
Centre for Invasive Species Solutions, 2014. Radio tracking as a support tool of carp control methods. Factsheet. PestSmart website. https://pestsmart.org.au/toolkit-resource/radio-tracking-as-a-support-tool-of-carp-control-methods accessed 25-09-2021