Trophic Levels


An increase in fish from higher trophic levels will indicate 1) reduced pressure on these populations, through the no-fishing zones; and 2) an increase in biomass of fish species from lower trophic levels, which provide prey for carnivorous fish. Further, an increase in fish biomass from the herbivorous classification indicates that the population can be supported by a sufficient amount of macroalgae (seagrass).


In the last three years, AKD has observed a 37% increase in carnvivorous fish biomass, however also a 23% decrease in apex predator biomass.


The proportion of herbivorous and carnivorous fish is measured as the ratio of biomass of each group to overall fish biomass. Data is collected through standard visual fish censuses along transects on rocky bottoms. Surveys occur at depths of between 12-18m, following the MedPAN standardised methodology for rocky reef monitoring in the Mediterranean. Divers conduct three 25 m-long and 5 m-wide transects at each four stations per site. The diver swims at a constant speed, identifying and recording the number and size of each fish species encountered. Fish sizes are estimated visually in 5 cm increments of total length (TL). Fish biomass (wet mass) is then estimated from size data by means of length-weight relationships from the available literature and existing database. Fish biomass for the transect is then divided by the total survey area of the transect to produce values in gr/m2. Species are classifed into the following four trophic groups (based on FishBase classifications): herbivorous, zooplantivorous, carnivorous and apex predators.

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