Tracking Common Bream and Northern Pike in the Norfolk Broads
By Emily Winter, PhD Student, Department of Life and Environmental Sciences, Bournemouth University, 4th January 2019
Bream and pike are two ecologically very different, but important fish species of the Norfolk Broads. They are abundant throughout the rivers, dykes and broads and provide high quality catch-and-release angling, but something could be about to stir in a hidden corner of the ecosystem.
Hoveton Great Broad is undergoing huge restoration, involving sediment removal and biomanipulation, which will include fish removal and the disconnection of the broad from the wider river system. To find out how these engineering works could be affecting the behaviour and ecology of the resident fish populations, we are tracking their movements using two types of telemetry.
Acoustic telemetry is a tracking technique that uses two principal pieces of equipment: acoustic transmitters and receivers. The small, electronic transmitters (or ‘tags’) produce uniquely coded signals in the form of ultrasonic ‘pings’ that can travel over 500 m underwater. The tags have a battery life of 2.5 to 3 years. They are surgically implanted into the body cavity of the live fish and allow us to monitor fish movement patterns. This is achieved using receivers, which are our underwater ‘listening’ stations. They listen continuously and record the precise date and time of each detection, along with the tag’s unique code. With many receivers positioned all over the Norfolk Broads system, we can examine bream and pike behaviour across the catchment, both before and after the restoration project, and determine any impact on their ecology.
Our second tracking technique is passive integrated transponder (PIT) telemetry. Essentially, PIT tags are larger versions of the everyday microchips that are used to identify pet cats and dogs. They are surgically implanted into the fish and are detected at ‘listening’ stations, which are comprised of electrical antennae that encircle the river channel, creating an electromagnetic field. Upon entering the electromagnetic field, i.e. when a fish swims through the antennae, a PIT tag is energised and transmits its unique identity code. So, PIT telemetry uses different technology to acoustic telemetry, however the outcome is very similar. The main difference is the spatial scale over which the telemetry can be performed. Acoustic tags can be detected over 100s of metres and are suited for use in large rivers and lakes, while PIT tags are detected over 0.5 – 1.0 m, meaning they are more suited to smaller dykes and drainage channels. In addition, since PIT tags are not battery powered, they theoretically have an infinite lifespan. By using a combination of acoustic and PIT telemetry we can exploit the advantages of each technique, meaning bream and pike movements can be monitored over a variety of habitats within the Broads.
So, with the technology explained, the question ‘Why choose bream and pike?’ still burns. These two species are not only economically important in terms of angling revenue (estimated to be worth over £100M to local economy each year!), but are ecologically important too. Bream primarily feed on invertebrates, plankton and plant matter in the sediment and water column. They are a social shoaling species and are highly mobile. They are considered to be ‘ecosystem engineers’, because their feeding activity stirs up the sediment and reduces water clarity, which can contribute to a reduction in aquatic plants. For this reason, they are often seen as the villains in the lake restoration story. Pike are markedly different in their behaviour and ecology. They are ruthless sit-and-wait predators at the top of the fish food chain and are even known to be cannibalistic. Consequently, they are typically solitary, and have relatively low activity levels outside the spawning season.
All fish used in the study were caught by rod and line angling by generous volunteers. Fish were anaesthetised before surgery and were released at the site of capture after full recovery. Surgery was performed under UK Home Office Licence after approval by the Animal Welfare and Ethical Review Body of Bournemouth University. Neither bream or pike have been tracked or their movements studied in such detail before in the Norfolk Broads, so we are excited that this project will provide new and valuable information on how the fish community uses the diverse river ecosystem and responds to human-induced environmental change.