Bottom trawling for crustaceans in Portuguese coastal waters is an important fishery in terms of revenue, despite its negative impacts on deep-sea ecosystems. This fishery catches large amounts of unwanted species that were discarded for various reasons before the introduction of the Landing Obligation, which banned the discarding of regulated species. However, where it can be demonstrated that a species has an acceptably high likelihood of survival, exemptions to this ban may be granted. In this study, time-to-mortality was used to estimate immediate mortality rates and identify important biological characteristics determining the susceptibility of 14 by-catch species, most with commercial interest (Conger conger, Galeus melastomus, Helicolenus dactylopterus, Lepidorhombus boscii, Lophius budegassa, Lophius piscatorius, Merluccius merluccius, Micromesistius poutassou, Mullus surmuletus, Phycis blennoides, Scyliorhinus canicula, Trigla lyra, Trachurus trachurus and Trachurus picturatus). Species with potential to survive after the discarding process were identified and a short-term survival assessment of conger eel (Conger conger) was performed. The results suggest that species with scales, gas bladder and high metabolic rates have higher post-discard mortality. Size was a critical factor determining survival in some species, with smaller individuals dying faster. The short-term survival rate of conger eel was determined to be 84% (95% CI: 75.5 to 93.3%). The methodology and results from this study can help identify species that may survive the discarding process and factors influencing their survival.
Actual conditions of discards of by-catch from small-scale trawl fishery in Ariake Sea