Fate of discards from deep water crustacean trawl fishery off the south coast of Portugal

Abstract

Trawling for crustaceans takes place on the south coast of Portugal at depths between 200 and 800 m. Large amounts of discards are released back to sea, reaching the bottom in the general area where trawling occurs. The objective of this work was to study the time taken for decomposition of the discards, to identify the most important scavenging species involved, and to evaluate the impact on the species targeted by the fishery. We conducted a series of 22 trials, using traps baited with samples of the discards. The time of immersion varied between 1 and 40 h. The amount of tissue removed from the discards was evaluated on a qualitative scale of 1-3, and quantitatively. The species caught in the traps were identified. Considerable consumption of the bait had occurred after 5 h of immersion. After 24 h of immersion only fish bones were left; past 40 h, consumption was complete. The conger eel (Conger conger) was the most common fish species present in the traps. Two species, the amphipod Scopelocheirus hopei and the isopod Natatolana (Cirolana) borealis, were identified as most important for the recycling of organic matter in the system. The stomach contents of a sample of species caught in trawls were analysed for the presence of small scavengers. Preliminary results show that S. hopei and N. (C.) borealis are part of the diet of some of the target species of this fishery, such as the Norway lobster, Nephrops norvegicus and the rose shrimp Parapenaeus longirostris.

Deep-Sea Mega-Epibenthic Assemblages from the SW Portuguese Margin (NE Atlantic) Subjected to Bottom-Trawling Fisheries

“…The presence of abundant motile predators or scavengers in HT segments is consistent with previous observations reporting a rapid response after disturbance of such species (e.g., Dannheim et al, 2014;Almeida et al, 2017) but also experimental works performed in the deep sea (Bluhm, 2001;Gerdes et al, 2008). In fact, there is often an increased food availability for these trophic groups in recurrently trawled areas, which results from both the on-site mortality or injured fauna, but also from discarding practices (Ramsay et al, 1996;NRC, 2002;Castro et al, 2005). The low commercial value of many by-catch species (e.g., Henslow's crab) at the WIM often leads to discarding of an average of 40-70% of the fished biomass by crustacean trawlers (Borges et al, 2001;Monteiro et al, 2001).…”

Contribution of bait to lobster production in an oligotrophic marine ecosystem as determined using a mass balance model

“…Addition of organic material to marine ecosystems has been shown to affect marine ecosystem function across a range of systems globally (Wassenberg and Hill, 1987;Ramsay et al, 1997;Groenewold and Fonds, 2000;Dempster et al, 2002;Castro et al, 2005;Catchpole et al, 2006;Tuya et al, 2006). For instance, addition of organic matter has been shown to enhance secondary production, particularly in areas where trawl fishing occurs such as the North Sea (e.g.…”

Crustacean fauna (Stomatopoda: Decapoda) associated with the deepwater fishery of Heterocarpus vicarius (Decapoda: Pandalidae) along the Pacific coast of Costa Rica

“…The ecology of deepwater shrimp species, their fishery and impact on the environment have been especially well studied in the Mediterranean (Marsan et al 2000, Belcari et al 2003, Sardà et al 2003a, b, 2004, Castro et al 2005, Chilari et al 2005. In contrast, our knowledge regarding these topics in Latin America is far from complete.…”