Fish Biomass
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Increase fish biomass
No change observed
Gökova Bay to Cape Gelidonya
Türkiye
Problem: Unregulated and unsustainable fishing in the Mediterranean Basin has led to a sharp decrease in the last 20 years in the catch size and number of target species in local fisheries, and a loss of native fish abundance and trophic complexity.
Action: No-Fishing Zones (NFZs) have been established by the government in collaboration with AKD and local fishers to promote sustainable fishing practices and allow refuges for fish to spawn and grow. AKD are increasing areas within NFZs that are patrolled and engaging with fishers to deter illegal fishing activities within NFZs.
Indicator: An increase in total fish biomass indicates that fish are benefiting from the NFZs, allowing them to grow larger (increasing biomass) before they are caught by fishers. This also indicates a benefit to fishers, who can get a higher price for the larger fish they are able to catch and sell.
Methods
Biomass across trophic levels was monitored within three project areas, both within and outside No-Fishing Zones (NFZs): Gökova (eight inside NFZ sites, three sites outside NFZs), Kaş (six inside NFZ sites, seven outside NFZs), Göcek (four inside NFZ sites, six sites outside NFZs), and Bozburun (three inside 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). 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.
Fish biomass for the transect was then divided by the total survey area of the transect to produce values in g/m2.
Diver completes in-water fish census transect to monitor fish biomass. Photo: AKD.
Results
In both Gökova and Kaş, total fish biomass was significantly higher inside than outside NFZs, although the difference was much larger for Gökova than for Kaş (Fig. 1). In Göcek, there was no statistically significant difference in total fish biomass inside and outside NFZs.
In Kaş the total fish biomass varied significantly among years, but there was no overall directional trend. No statistically significant variation over time was found for Gökova or 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: Fish biomass (g/m2) inside and outside No-Fishing Zones across four project areas. Note that axis scales are different between graphs.
Interpretation
The highest fish biomass from 2019 – 2024 was recorded in Gökova, where NFZs have been established and enforced for longest. Biomass did not change significantly over the period. The reason it did not increase may be the opening of some NFZs to amateur fishing in 2019, which may also have led to illegal fishing activities under the guise of amateur fishing.
New protected areas in Göcek were designated in August 2020 (although they are open to amateur fishing), and enforcement began in 2021, coinciding with an increase in total fish biomass within NFZs, contrasting with trends outside NFZs. AKD are still negotiating with the government to close the protected areas in Göcek to amateur fishing, and this is an important step to ensure professional fishers support or respect the restrictions too. Future data collection will show whether fish biomass recovers in the NFZs over the longer term.
Dusky Grouper within Turkish no-fishing zone, eastern Mediterranean. Photo: AKD.
Enforcement activities in Kaş began in late 2019, and following this total fish biomass increased within the NFZ. 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 within NFZs. Despite the lack of enforcement in recent years, looking across all years fish biomass was higher inside than outside the NFZ, which may be driven by the years when the NFZ was enforced and biomass increased.
In Bozburun, active enforcement within the NFZ only began in the summer of 2024 and fish biomass has not yet had time to recover. Future data collection will help to study the effects of the NFZ on fish biomass.
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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