Factors affecting the variability of discards in Mediterranean bottom trawl fisheries

The current study examined the effect of environmental, operational, temporal and catch-related variables on the discard quantities of four species subject to the EU landing obligation (Merluccius merluccius, European hake, HKE; Parapenaeus longirostris, deep-water rose shrimp, DPS; Mullus barbatus, red mullet, MUT; Trachurus trachurus, horse mackerel, HOM) caught in Italian and Greek bottom trawl fisheries. Generalized Additive Models were applied to discards per unit of effort of each species in seven study areas distributed over different subdivisions of the Mediterranean Sea (Western Mediterranean: (i) Ligurian Sea and Northern Tyrrhenian Sea, (ii) Southern and Central Tyrrhenian Sea; Central Mediterranean: (iii) Southern Adriatic Sea, (iv) Western Ionian Sea; Eastern Mediterranean: (v) Eastern Ionian Sea, (vi) Central and South Aegean Sea, (vii) Crete) and were able to explain 21-96% of the deviance. Geographic coordinates and depth were included as significant covariates in the majority of the final models, showing that discards are affected by bathymetric preferences of the species and sizes, local patterns in productivity and/or possible existence of nursery grounds. Discard rates were higher at the shelf break for HKE and DPS, slightly shallower for HOM, while MUT discards occurred mainly in waters <100 m depth. Positive relationships between discards and volume of catches and negative ones between discards and the mean length (especially for HKE) highlighted the effect of the catch characteristics as well as fishers' discarding behaviour. Interannual effects seemed to be related to trends in species abundance and recruitment success, while seasonal patterns were usually linked to the recruitment period and/or seasonal fishing intensity. The length of the vessel, vessel age and haul duration were significant in few cases, but there were no common patterns across all regions/species. Overall, the results showed that discarding in Mediterranean bottom trawl fisheries is affected by a large number of variables that should be taken into account for a more effective mitigation of unwanted catches.