Modelling dynamic effects of fishery on odontocetes through a food-web modelling approach in the Gulf of Taranto (Northern Ionian Sea)

This study is the first attempt at applying the Ecosim modelling approach to explore temporal effects of fishing effort (FE) changes on the odontocetes in the food web of the Gulf of Taranto. The implementation of the Ecosim approach was performed on the existent Ecopath food web model realized for an area covering 7745 km
²
in the Go
$T$
. The food web was described by 58 functional groups and 5 fishing gears, focusing the attention on four odontocetes: Stenella coeruleoalba, Tursiops truncatus, Grampus griseus, and Physeter macrocephalus. A total of 67 time series based on biomass (35) and fishing catches (32) were used to fit the model during the calibration step, adopting both the automated stepwise and manual fitting procedure. The best model selected was used to run the two decades forward projecting simulations (2018–2035) to predict the biomass of odontocetes, setting three different scenarios based on 1) constant FE as 2018 (baseline); 2) FE+25% between 2019 and 2023; 3) FE -25% between 2019 and 2023. T. truncatus showed an increase in biomass of 30.4% between 2018 and the endpoint in the baseline scenario, and it was the only species showing relevant differences in biomass between the three scenarios, with a decrease of 3.8% and an increase of 4.3% in the scenarios FE+25% and FE-25
^%
, respectively, compared to the baseline. S. coeruleoalba, G. griseus, and P. macrocephalus did not show any notable difference in biomass between the three scenarios at the endpoint of simulations. The scenario analysis underlines how changes in cetacean abundances could be influenced by fisheries through indirect prey controls determined by patterns of trophic interactions at different levels of intensity. Although an improvement in the fitting strategy and the representation of environmental data should be carried out, this Ecosim model can be considered a baseline to set future scenarios addressed to the conservation measures for cetaceans.