4. Fin-Tastic New Homes: Artificial Reefs as Biodiversity Boosters

Habitat Creation at Varbjerg
The Storå river is home to various fish species including the brown trout (Salmo trutta) and European eel (Anguilla anguilla). This river's mouth at Varbjerg Strand is the site of a unique conservation project. It was here that the world's first smolt reef was implemented, which was specifically designed to increase the survivability of brown trout smolt.

Smolt refers to juvenile trout and salmon undergoing smoltification, a crucial process that allows these fish to transition from freshwater rivers to the saline environment of the sea. During smoltification, their bodies become more streamlined and silver, losing the camouflage parr marks beneficial only in rivers. Internally, their osmoregulatory system (an internal system that regulates the balance of water and solutes in an organism's body fluids) transforms to cope with the sea's salinity. Additionally, they begin to display more schooling behaviour rather than territorial, and exhibit an increased downstream migratory drive.


The image above shows three different stages of brown trout:
A) A juvenile trout.
B) A juvenile trout during the smoltification process.
C) A smolt.
When a brown trout undergoes the smoltification process and migrates to the sea, it becomes known as a sea trout (Salmo trutta).

The artificial reef likely provides a vital refuge for vulnerable trout smolt, protecting them from predators such as cormorants (Phalacrocoracidae) and grey herons (Ardea cinerea). Its strategic placement at the river mouth is crucial because this area's brackish water – less saline than the open sea but not true freshwater – is ideal for the smolt's transition. The reef also mitigates wave movement and protects the coastline from further erosion, thus protecting homes and businesses. 


Trout Smolt Surveys
In May 2025, I took part in night-time brown trout smolt surveys at the smolt reef at Varbjerg Strand. The aim of this survey was to determine the success of the artificial reef by comparing the number of smolt counted to a previous survey conducted in 2022, at which time no reef was present. 

The surveys were conducted at the darkest hours (between 11pm and 1am) when the smolt are most active. They tend to be most active at night to minimise predation from diurnal species (those that are active during daytime). The aerial image shows the trapezium/horseshoe-shaped artificial reef - which is visible as the dark shape against the lighter seabed. A total of 60 25-metre line transects were conducted, with 20 on each reef "arm" and 20 in the middle area between the reef arms. Each transect was separated by 7 metres.

The results show that there was an increase in smolt density in 2025, compared to 2022. This indicates that the artificial reef is a contributing factor to the protection of vulnerable smolt. These findings provide a positive case study for the potential use of similar reefs at other coastlines to help in smolt survival, and other small, vulnerable fish species.

During spring 2025, hydrophones were also placed at and around the reef. The hydrophone data can be used to show the movement of acoustically tagged fish, showing their individual movements. If the tagged smolt "ping" off the hydrophones multiple times over several days or weeks, it can show that they are staying within the reefs influence, implying the reef is successful. At the time of writing, the 2025 data is currently being compared to the 2022 data (when there was no reef present).

Location: Varbjerg Strand, Denmark

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