Documenting effects of coastal reef restoration (DK)

In this study, acoustic telemetry is applied to produce high accuracy 3D positioning of Atlantic cod (Gadus morhua) on and around a rocky reef in Denmark. The rocky reef was established in Tybrind Vig in 2023 and is protected from fisheries. The presence and behaviour of Atlantic cod will be examined on and around the rocky reef and compared to the presence of cod in a nearby control area. The rocky reef is expected to benefit Atlantic cod and increase the presence of cod locally.

Background

The Western Baltic Sea used to sustain high commercial catch rates of the Atlantic cod. The fisheries peaked in the 1990s but catches have since deteriorated. Today, the population of Atlantic cod in the Western Baltic Sea is severely degraded and target fisheries have been banned. The decline of Atlantic cod is caused by anthropogenic pressures, including habitat degradation and overfishing (read more here).

Stone fishing or marine rock removal has been normal practice in Danish coastal waters for centuries. The rocks were used for construction on and near harbor areas, e.g. for pier construction. Stone fishing ended in 1999 in Denmark and was officially banned in 2009. It is estimated that 55 square kilometers of rocky reef habitat was removed from the Danish coastlines between 1900 and 2000. Marine rock removal also happened before 1900, but the activities remain poorly documented. Only few rocky reefs have been reestablished.

Rocky reefs create structures on the seabed where organisms such as mussels and macro algae can settle and grow. This can provide shelter and, over time, a beneficial food source for fish and other organisms. Atlantic cod often occur near structures on the seabed, and rocky reefs constitute a suitable and natural habitat. Thus, marine rock removal has resulted in significant habitat degradation for cod along the Danish coastlines.

In this study, a cavernous rocky reef has been established inside the bay Tybrind Vig in the Little Belt, in central Denmark. The rocky reef was deployed in 2023 and consists of approximately 3500m3 of rocks. The rocky reef is located on a depth of 6-8 meters and measures 60 x 160 meters (see Figure 1). More information is available here.

The rocky reef in Tybrind Vig is protected against fisheries during the entire study period. The protection provides optimal conditions to study the direct effects of the rocky reef on the presence of cod.

A previous study have demonstrated that rocky reefs can significantly increase the presence of Atlantic cod in the Baltic Sea. Therefore, the rocky reef in Tybrind Vig is expected to increase the presence of cod significantly. The goal of this study is to examine and document the positive effects of the rocky reef on the presence of Atlantic cod.

High resolution 3D tracking of Atlantic cod

We will apply acoustic telemetry to examine the presence and behaviour of Atlantic cod on and around the rocky reef. Hydrophones and transmitters are produced by Thelma Biotel in Norway. All applied hydrophones are the model TBR800 release. The applied transmitters can measure acceleration and depth and emit acoustic signals at medium power.

A network of 12 hydrophones has been deployed on and around the rocky reef. Another network of 12 hydrophones has likewise been deployed on a control site inside Tybrind Vig. The depth at the control site corresponds to the depth where the rocky reef is located. The seabed in the control area has no structure and is completely flat.

Up to 100 Atlantic cod are captured inside Tybrind Vig and tagged with acoustic transmitters in the spring 2024. Half of the tagged cods will be released on the rocky reef and the other half will be released inside the control area. The cod will be tracked for six months in 2024.

All hydrophones will be retrieved after six months of data collection. We will use YAPS to estimate tracks of the tagged cod and quantify their movement behaviour at and around both the rocky reef and the control site.

The results from this study can be used to assess the effects of protected, coastal rocky reefs as a management tool for Atlantic cod in the future.

Project partners

We thank several project partners involved in the study. The project partners have provided essential assistance to the project in Tybrind Vig. Project partners include:

• Ib Jensen - Føns Vig Fritidsfiskerforening
• Allan Buch - Bælternes Fiskeriforening og Danmarks Fiskeriforening
• Commercial fisher Jens Henriksen
• Niels Ole Præstbro - Naturpark Lillebælt

Funding

Funding is provided by

• the Velux Foundation
• the Nordea Foundation
• the national Rod and Net Fish License Funds of Denmark