Characterisation of discards generated by Maltese bottom otter trawl fishing vessels

Abstract

Discarding is regarded as an economically wasteful and ecologically harmful practice and has been recognised as a major issue in trawling ever since its conception. This dissertation aimed to advance the knowledge of the discards generated by Maltese bottom otter trawlers, which are unmonitored under the EU Data Collection Framework (DCF) or the General Fisheries Commission for the Mediterranean (GFCM) Data Collection Reference Framework (DCFR). The unmonitored discards of a total of 17 hauls were analysed, originating from five fishing trips undergone between March and September of 2018, covering areas towards the Northwest, North and Northeast of the Maltese islands. A total of 101 species were identified, of which 38 were Osteichthyes, 4 Chondrichthyes, 24 molluscs, 9 crustaceans and 26 other invertebrates. The five defining species for shallow-water hauls (120-140m), by number of individuals and in descending order were: Argentina sphyraena, Arnoglossus laterna, Lepidotrigla cavillone, Serranus hepatus and Lepidotrigla dieuzeidei. Similarly, the defining species for deep-water hauls (330-590m): Munida rutllanti, Pasiphaea sivado, Coelorinchus caelorhincus, Chlorophthalmus agassizi and Nezumia sclerorhynchus. 18% of the C. agassizi analysed were suffering from the generalist cymothoid ectoparasite Ceratothoa steindachneri, which could potentially spread to farmed species. Multiple fragments of the critically endangered bamboo deep-water coral Isidella elongata, and one count of the giant tun Tonna galea, both protected under the Barcelona Convention, were amongst the unmonitored discards, suggesting that some vulnerable species are going unnoticed by DFA observers. This potentially unrecorded aggregation of I. elongata highlights the necessity for enhancing local data collection measures. There was no statistically significant difference between shallow and deep-water discard rates, both in terms of weight and individuals (P > 0.05) but there was a significant difference in the species richness between the two depth categories (P < 0.05).