Habitat niche differentiation of Ballan wrasse (Labrus bergylta) and Corkwing wrasse (Symphodus melops) revealed by acoustic telemetry: Implication for an urban restoration project

Abstract

Anthropogenic changes to natural habitats impact species behavior and habitat use, changing ecological interactions and ecosystem dynamics. Human influenced ecosystems can be found all along the Norwegian coast, where the Bergen city centre fjord is a good case. This urban fjord has been repurposed as a harbour for hundreds of years, with high and increasing boat and human activities, resulting in a highly polluted seabed of Store Lungegårdsvannet of heavy metals (PCB7 and PAH16) and organic pollutants. This is considered threatening with negative influences on both the marine ecosystem and human health. Therefore, Store Lungegårdsvannet is in an ongoing state of garbage removal and following a restoration planned for 2023, including that a proportion of the seafloor is going to be covered by sand sediments to reduce the effect of polluted marine substrate. Wrasses are an abundant fish family inhabiting the Norwegian coast and they are often used as bioindicator species for evaluating anthropogenic impacts. In order to use wrasse as a bioindicator for the effects of seafloor restoration, however, we need to understand their spatiotemporal habitat use. At a local scale, the different wrasse species are expected to use the habitat in slightly different ways, also known as habitat niche partitioning. A consequence of fine scale niche partitioning is that different species of wrasse may respond differently to anthropogenic changes and habitat restoration. To investigate possible differences in behaviour of two wrasse species, 34 individuals of ballan wrasse (Labrus bergylta) and corkwing wrasse (Symphodus melops) in Store Lungegårdsvannet (Bergen, Norway) were implanted with acoustic tags in June 2021 and July 2022. These tags were instrumented with sensors for measuring temperature and depth, providing information on wrasse behaviour for over a year. Results from this study revealed that (1) ballan wrasse inhabited deeper depths than corkwing wrasse, that (2) the experienced temperatures of the two species were similar despite differences in depth use, that (3) changes in behaviour when temperature decreased during winter was clearly present for both species, by moving to deeper waters and that (4) ballan wrasse started activity earlier than corkwing wrasse during summer and later in autumn. The results support existence of habitat niche differentiation for ballan wrasse and corkwing wrasse, making them differently impacted by habitat disturbance from a restoration process.