Rabbitfish
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Decrease the proportion of invasive rabbitfish, relative to overall fish biomass
Improving
Gökova Bay to Cape Gelidonya
Türkiye
Problem: Invasive herbivorous rabbitfish are abundant in the project area and have detrimental effects through overgrazing. They outcompete native fish species and impact important vegetated habitats such as seagrass and macroalgae.
Action: No-Fishing Zones have been established, which should help reduce the dominance of invasive fish by building ecological resilience, including reducing fishing pressure on native carnivorous fish, which predate invasive rabbitfish.
Indicator: A reduction in the proportion of invasive rabbitfish to overall fish biomass recorded within No Fishing Zones indicates a better functioning marine ecosystem.
Invasive species of rabbitfish, Siganus rivulatus, grazing algae. Photo: AKD.
Diver completes in-water fish census transect to monitor fish biomass. Photo: AKD.
Methods
Rabbitfish proportion of total fish biomass was monitored within three project areas, both within and outside No-Fishing Zones (NFZs): Gökova (eight within NFZ sites, three sites outside NFZs), Kaş (six within NFZ sites, seven outside NFZs), Göcek (four within NFZ sites, six sites outside NFZs), and Bozburun (three within NFZ sites, three sites outside NFZs). Sites were spaced at least 500m apart. Surveys were conducted annually from 2019 – 2024, although not all sites were surveyed in all years.
SCUBA surveys were used to conduct visual fish censuses along transects at 15-20m depth, following standardised MedPAN methodology for rocky reef monitoring in the Mediterranean (MedPAN 2013). The proportion of rabbitfish was measured as the ratio of rabbitfish biomass to overall fish biomass. One 50 x 10m transect was completed at each site. The diver swam one way along the transect at a constant speed, recording the species and size of each fish encountered. Total fish length was estimated visually in 5-10 cm increments. Fish biomass (wet mass) was then estimated from size data using existing length-weight relationships from Morey et al (2003) and FishBase.
Results
In both Gökova and Göcek, the proportion of rabbitfish biomass compared to total biomass monitored between 2019 and 2024 was significantly lower within NFZs compared to outside NFZs (Fig. 1). However, for both areas zoning explained a relatively small proportion of the variation in proportion of rabbitfish biomass (14% for Gökova, 23% for Göcek), suggesting that other factors are also important. In Kaş, there was no statistically significant difference in proportion of rabbitfish biomass inside and outside NFZs.
In Gökova, there was a significant trend in proportion of rabbitfish biomass over time, with an increase in rabbitfish particularly outside NFZs. Proportion of rabbitfish biomass in Kaş also showed a significant, but non-linear, trend over time. There was no trend in proportion of rabbitfish biomass over time in Göcek.
Analysis was not yet possible for Bozburun, since data collection there only started in 2022 and the NFZ has only been enforced since late 2024.
Figure 1: Rabbitfish biomass as a percentage of overall fish biomass, inside and outside No-Fishing Zones, across four project areas.
Interpretation
The finding that proportion of rabbitfish biomass was significantly lower inside than outside NFZs in the two most well-enforced areas, Gökova and Göcek, is an encouraging sign that NFZs may increase seascape resilience to the effects of invasive alien species, due to their more complete trophic relationships and the expected higher proportion of predatory fish which can help to suppress rabbitfish numbers.
In Bozburun, active enforcement of NFZs only began in the summer of 2024, meaning that current patterns in rabbitfish abundance do not reflect NFZ protection effects. The full impact of protection will take time and is expected to become evident over the next 3 to 5 years.
In Kaş, NFZ enforcement led by AKD began in late 2019. By 2020 and continuing into 2021, total fish biomass within NFZs increased sharply while rabbitfish biomass proportions in NFZs were lower than outside NFZs. However, at the end of 2022, AKD ceased enforcement activities in Kaş due to logistical and political constraints. Since then, total fish biomass has declined, and the biomass proportions of rabbitfish within and outside NFZs have converged.
Publications
Kızılkaya, Z., Saydam, E., Walker, K. et al. Progress and challenges of multi-habitat marine restoration in the eastern Aegean Sea, Türkiye. npj Ocean Sustain 4, 12 (2025). https://doi.org/10.1038/s44183-025-00114-9
Citations
MedPAN, 2013. Reef Life Survey – Standardised Survey Procedures for Monitoring Rocky & Coral Reef Ecological Communities.
Morey et al., 2003. Weight-length relationships of littoral to lower slope fishes from the western Mediterranean. Fisheries Research, 61(1), 89-96. https://doi.org/10.1016/S0165-7836(02)00250-3
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