Trophic Web
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Increase proportion of fish in higher trophic levels
Improving
Gökova Bay to Cape Gelidonya
Türkiye
Problem: Decreasing abundance of higher trophic level fish indicates a collapsing trophic web. Larger, carnivorous fish require sufficient prey (smaller, herbivorous or planktivorous fish) to thrive. As seagrass meadows decline so does abundance of these lower trophic level fish species, resulting in fewer carnivorous fish. The higher trophic level fish are also the only natural check on invasive fish species, so their ecological function is critical.
Action: No-Fishing Zones have been established and are being patrolled to provide areas where the abundance of species from all trophic levels can recover.
Indicator: Increase in the proportion of species from higher trophic levels (carnivores and apex predators) will indicate a reversal of recent trends of decline and demonstrate a recovering trophic structure.
Methods
Biomass across trophic levels 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 herbivorous and carnivorous fish was measured as the ratio of biomass of each group 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.
Fish biomass for the transect was then divided by the total survey area of the transect to produce values in g/m2. Species were classified into the following four trophic groups, using FishBase classifications: herbivorous, zooplantivorous, carnivorous and apex predators.
Diver completes in-water fish census transect to monitor fish biomass. Photo: AKD.
Dusky Grouper within Turkish no-fishing zone, eastern Mediterranean. Photo: AKD.
Results
In Gökova, the proportion of fish from higher trophic levels (carnivores and apex predators) was significantly higher inside than outside NFZs. Zoning explained 20% of the overall variation in high-trophic-level biomass, suggesting other factors were also important. There was no statistically significant variation over time.
The proportion of high-trophic-level biomass in both Göcek and Kaş did not show a statistically significant difference inside versus outside NFZs, although this may have been influenced by the fact that inside-NFZ data only existed for 2021 onwards. In Göcek there was significant variation between years, but no overall directional trend, while in Kaş there was no statistically significant variation over time.
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: Proportion of total fish biomass made up by fish from higher trophic levels (apex predators and carnivores) inside and outside No-Fishing Zones, across four project areas. Note that x-axis scales are different between plots.
Interpretation
In Gökova, the significantly higher proportion of carnivores and apex predators inside compared to outside NFZs aligned with expectations, as the MPA here is well-established and well-enforced, supporting a more mature ecological balance.
In Göcek, enforcement began in 2021, but zones remain open to amateur fishing. In Bozburun, the NFZ was only enforced in 2024. In both areas, more time may be needed to detect significant recovery of fish biomass across trophic levels, to mirror the effect found in Gökova.
In Kaş, high-trophic-level biomass began to rise steadily following enforcement implemented in 2019. However, enforcement ceased in 2022 due to logistical and political constraints, and this was followed by a decline in fish biomass. This shift may highlight importance of enforcement on maintaining higher trophic levels and ecological intactness.
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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