Anthropogenic structures can form novel ecosystem niches. Invasive species are often particularly successful in occupying these habitats and utilize them as beachheads for further spread. The invasive round goby (Neogobius melanostomus, Pallas 1814), an inherently bottom-dwelling fish, uses vertical harbour walls as habitat, enabling them to reach boats (i.e. potential translocation vectors). To evaluate the relevance of vertical habitat use for population dynamics and translocation, we exemplary investigated a population of round gobies in a harbour ecosystem. Specifically, we investigated differences in trophic niche characteristics, individual trophic specialization, phenotypic traits, and breeding frequency in wall versus bottom dwelling round gobies. Habitat-characteristic dietary signatures indicated habitat partitioning during the breeding season. Trophic niches overlapped but were clearly distinguishable between the habitats: walls were inhabited by 1.4 times more trophic generalists than specialists, while the bottom was inhabited by 2.1 times more trophic specialists. Breeding frequency was 24 times higher on the walls than on the bottom. After the reproductive season, we found a higher similarity in trophic ecology of gobies inhabiting the two habitats, and differences in abundance, size, and condition. These results are in line with winter migrations to deeper habitats, which are common in round gobies in lentic and marine ecosystems. Our results suggest a high potential for microgeographic adaptation to either horizontal or vertical habitat use in invasive round gobies. We demonstrated that male gobies using the walls during the breeding season are larger and heavier, suggesting that wall-climbing may select for more competitive individuals. Additionally, the overall abundance of round gobies likely increases with the additional use of vertical habitat space, which may lead to higher propagule pressure. The ability to exploit anthropogenic habitats, and a higher translocation probability of competitive individuals, can contribute to the invasion success of round gobies in anthropogenically influenced aquatic systems.

Sample collection:

• Sampling site: commercial harbour Kleinhüningen in Basel, Switzerland (47°35'10"N 7°35'27"E)
• Two sampling periods:  19th August - 04th September 2020 (reproductive season); and 28th September - 23rd October 2020 (post-reproductive season)
• fishing gear round gobies: baited minnow traps
• baseline organisms: Dickerogammarus villosus and Dreissena polymorpha, kept in freshwater for > 24 h before processing

Sample processing:

• determination of sex, size (total and standard length), wet weight
• extraction of muscle filet (skin- and boneless) and liver
• tissues dried for > 48 h at 60°C and grinded to fine powder
• stable isotope analysis for δ¹³C, δ¹⁵N, C%, and N% using an isotope ratio mass spectrometer coupled to an elemental analyser (EA-IRMS)
• for lipid correction of livers: de-fatting of eight halves of livers based on a protocol of Bligh and Dyer (1959), modified by Turschak et al. (2014)
  • correction using mathematical formula based on Post, Layman et al. 2007

Isotope data is uploaded in several tables adjusted for the use in different parts of the analysis:

R script: Bussmann_etal_Vertical_habitat_use_R_analyses.R

• Defatting_Muscles-livers.csv is used in section "Conrol for fat content in livers" 
• Bussmann_etal_Vertical_habitat_use_data_NM.csv is used in section "Data wrangling", "Individual specialization", "paired t-tests for d13C & d15N (muscle/liver)", and "Population characteristics". 
• Iso-Data_Muscles_20210305.csv is used in section "Trophic niche: size, overlap and position"
• Iso-Data_Muscles_bootstrap_20210830.csv is used in section "Isotopic ranges (bootstrapped)"
• Iso_data_NM-and-Baseline_groupingVar2.csv is used in section "Niche position"

R script: Bussmann_etal_Vertical_habitat_use_Figures_for_publication.R

• Iso_data_NM-and-Baseline_groupingVar2.csv is used in section "Figure 2:  Bi-plot of δ13C and δ15N with standard ellipses"
• Iso-Data_Ind_20210818.csv is used in section "Figure 5: Individual trophic specialization"
• Bussmann_etal_Vertical_habitat_use_data_NM.csv is used in section "Figure 4: Trophic position", "Figure 7: Standard length", "Figure 7: weight", "Figure 7: condition", and "Figure 7: Sex"