By-catch in the Patagonian Scallop fishery: composition and damage to main non-target species

ESCOLAR MARIANA; SCHWARTZ, MATÍAS; MARECOS A.; RONALDO DÍAZ; HERRERA SUSANA; LAURA SCHEJTER; CAMPODONICO SILVANA; BREMEC C.

Florianopolis

Workshop; 19th. International Pectinid Workshop; 2013

Introduction Trawling fisheries using mobile gears inevitably impact on non-target species, both by encounter with the gear on the seabed and by capture and discarding, which may result in injury or death (Hall 1996). These encounters would affect the habitat and benthic community in long-term (Bradshaw et al. 2002). The Patagonian scallop fishery is developed since 1996 in the Southwestern Atlantic (Lasta & Bremec 1998). In this fishery, the catch composed of scallops, other benthic invertebrates and shell hash (Bremec & Lasta 1997) are mechanically processed onboard; non-target species and non-commercial size scallops (< 55 mm) are separated by metal drums and discarded. Therefore, damage may be caused by the initial encounter with the gear, impaction or crushing by the mesh or by other organism of the catch (damage by trawling) or by sorting process on the fishing vessel (damage by process). Species loss was not recorded, but some species changed its abundance in fishing areas after almost twenty years of trawling (Bremec et al. 2010; Escolar et al. 2009; Schejter et al. 2008). However, specific analysis of the damage produced by the fishery on by-catch species lacks. The aim of the present study was to analyze the direct effects of the scallop fishery on non-target species, distinguishing between damage caused by trawling and by mechanical process on board. We test two predictions: 1-the damage caused by trawling is lower than the damage produced by the mechanical sorting and 2–fragile or large organisms suffer more damage than hard or small ones. Material and methods The fleet of Patagonian scallop fishery (4 vessels) operates non-selective bottom otter trawls with booms (head and foot rope, between 17 and 22 m long). Samples were collected from the catch during a stock assessment of Patagonian scallop on board of F/V “Atlantic Surf I” in 2012. A total of 10 hauls were examined. In order to discriminate between damage caused by gear and mechanical sorting, random samples (3 replicates, vol. 10 l basket) were collected: 1- immediately after the catch was dropped onto deck (damage by trawling) and 2- after selector drums and before it fell in the sea (damage by process). The samples (N = 60) were handled carefully to avoid additional damage and stored at -20°C; once in the lab, samples were kept for 12 h at room temperature then species were identified, counted and weighed (± 0.01 g wet wt). The damage sustained by the main species also was recorded according to four point scale using criteria appropriate to each taxonomic group (see Jenkins et al. 2001 and Veale et al. 2001). The species composition on deck and discards were analyzed using multivariate techniques (SIMPER, ANOSIM). A mean damage index (MDI) for main non-target species was calculated per haul according to Jenkins et al. (2001). T-test to dependent samples was used to compare mean damage index between deck and discard. To evaluate the relationship between the MDI and body-size a Spearman’s rank correlation test was applied. Results and discussion The specie composition was significantly different between deck and discard (R= 0.894, P< 0.01). Echinodermata, Porifera and Gastropoda dominated on deck, but an important decrease of Gastropoda were recorded in discard (Fig. 1). The species that most contributed to this difference were the target specie Zygochlamys patagonica, Adelomelon ancilla, Odontocymbiola magellanica and Fusitriton magellanicus (gastropods), Diplopteraster clarki, Diplasterias brandti, Gorgonocephalus chilensis and Austrocidaris canaliculata (echinoderms) and Ascidiacea, which presented less biomass in discard than in deck. From the nine species in which statistical analysis could be applied, two species of echinoids, two of ophiuroids and one gastropod showed significant increase in MDI after the mechanical sorting. Four species MDI did not change, among them Z. patagonica and the echinoderms Ctenodiscus australis, D. brandti and Ophiura lymani. The sea urchin Sterechinus agassizii was the specie more affected, its MDI increased from 1.77±0.47 to 3.91±0.21 after the process on board (t = -13.04, p < 0.01), as well as them species that for its size were retained (A. ancilla, O. magellanica, D. clarki and G. chilensis). A low or mid-low MDI (MDI < 2.5) was recorded to the rest of species. MDI and body-size were positive, negative or non-correlated depending on the species. Therefore, our predictions are not totally fulfilled because 1-in some species MDI did not change after the process on board and 2-body-size not always was related to MDI, even some species showed lower MDI to larger size. The damage sustained by non-target species was species-specific and related to its morphology. It is important point out that the damage depend on the benthic composition in the area hauled (Pranovi et al. 2001). In areas where the most important fishing activities are developed, the benthic community is dominated by Z. patagonica, with low biomass contribution of non-target species (authors’obs. pers.). This was not the case of our study area, which was selected by the high presence of non-target species. This study was framed in the action plan of present MSC certification obtained by this scallop fishery, and will allow us make comparisons among vessels with different processes on board (actually in progress) in order to reduce the damage and number of non-target species affected.