Incidence of fisheries impact and purification treatment in the survival rate of wedge clams caught in the northern Alboran Sea
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1
Oceanographic Center of Málaga, Spanish Institute of Oceanography (IEO), Spain
The wedge clam Donax trunculus Linnaeus, 1758 is one of the most demanded commercial bivalve molluscs targeted by the artisanal fleet using mechanized dredges in the western Mediterranean Sea. Recently, the damage caused by this type of fishing gear in the shell and foot of wedge clams, as well as the survival of the species after treatment in a purification centre for the detoxification of specimens (in accordance with Annex II of Regulation (EC) No 854/2004), was evaluated by the Spanish Institute of Oceanography.
Samples were collected on board commercial vessels using mechanized dredges in two sites (Fuengirola and Caleta de Vélez, southern Spain; Figure 1), and in the case of Fuengirola on two types of bottoms that differed in the hardness of the substrate (called as "hard bottoms" and "soft bottoms" by fishermen), and at three towing speeds: i) the usual as during normal commercial fleet operations (0.3 knots); ii) at a slower speed (0.1 knots); and iii) at a faster speed (0.5 knots). All hauls were replicated (n= 3) for a total of 36 hauls. Individuals displaying any external impact signal (e.g. dislocated or fragmented shells) and/or any damage in the foot were recorded and quantified. Additionally, a determination of the benthic fauna inhabiting fishing grounds was done from discard samples collected in the analyzed commercial hauls (n= 12). Finally, sediment samples (n= 3) were collected from the "soft" and "hard" bottoms for granulometric characterization. On the other hand, the survival of the commercial wedge clams was monitored upon arrival at port, where the number of live and dead (specimens with open, dislocated or fragmented shells, and/or that did not respond to mechanical stimulation) wedge clams was determined. Samples were then transferred to the purification centre, where they remained for 24 hours (as usual), after which they were checked once again, and kept refrigerated (4°C) for 96 hours, making survival checks every 24 hours. Kruskal-Wallis analyses were used to compare results between substrates, sites and towing speeds.
The wedge clams caught in Caleta de Vélez displayed a higher number of specimens with dislocated or fragmented shells (3.6%) than in Fuengirola (0.25%), with the highest values observed for normal and fast towing speeds (between 8% and 3%) in hauls carried out on substrates with a high pebbles content (ca. 80% of the inert material collected by the dredge).
Regarding the damage in the foot of wedge clams, a significantly higher proportion of damage was observed in individuals caught in Caleta de Vélez (n= 360; 22.03%) than in Fuengirola (n= 360; 9.93%), for hauls carried out at low and fast towing speeds (Kruskal-Wallis: p< 0,001). This damage ranged from small shallow cuts to total amputations of the foot (Figure 2). In both sites, the proportion of damaged individuals was lower in the case of slow towing speeds. In relation to the type of substrate ("hard" versus "soft"), results did not display significant differences (Kruskal-Wallis: in all cases p>0.05). The amputation of the foot can be mainly due to 1) predation by certain decapod crustaceans (crabs); and 2) self-mutilation induced by stress caused by capture. Our observations confirms the incidence of these two factors, since there are injuries compatible with an amputation caused by a fast and powerful closing of the valves (comarginal cuts at the edge of the shell), as well as deep perpendicular cuts at the margin of the foot that are characteristic of the mechanical action of a crab´s chelae. In this line, a greater abundance of predatory crustaceans was found in samples from Caleta de Vélez (ca. 19% of the total biomass corresponding to Liocarcinus vernalis and Portumnus latipes) in comparison to samples from Fuengirola (ca. 6%).
The proportion of living individuals after treatment in the purification centre ranged between 98.8% and 100% in all cases. Additionally, three subsamples of commercial wedge clams caught in this study were kept in purification for 72 hours, displaying in all cases a survival rate higher than 99%. Similarly, the survival rate ranged between 94% and 100% after three days in refrigeration (4°C). On the other hand, controls (subsamples of wedge clams with no purification treatment) that were kept in refrigeration during four days displayed survival rates between 92% and 100%.
The results of this study show that (1) a higher proportion of damaged shells could be related to a higher trawling speed and higher pebbles content on the seabed; (2) the damage presented in foot, whether caused by amputation by predation or by self-mutilation by stress, does not influence the survival of most specimens within 96 hours after fishing, which is derived from the high survival rate observed in this experience (>95%); (3) the purification treatment applied to the wedge clams does not seem to affect its survival rate, with very high values that did not decrease after refrigeration at the usual temperature of storage of shellfish resources. If the cold chain is kept intact and the shipping conditions are correct, wedge clams should remain in good condition for consumption until at least five days after capture.
Figure 1. Location map of the study area in the northern basin of the Alboran Sea (A). Location of the bottoms where the wedge clam D. trunculus was sampled in Fuengirola (B: "hard" bottoms are indicated in 1; "soft" bottoms are indicated in 2) and in Caleta de Vélez.
Figure 2. Different degrees of foot damage observed in the wedge clam D. trunculus caught in the northern Alboran Sea. Undamaged foot (A); different slight damages that include small cuts in the dorsal (B), ventral (C) and anterior (D) edges, as well as comarginal cuts at the edge of the shell, probably caused by the closing of the valves (E-F: same individual); and with severe damages corresponding to the partial (G) or complete (H) absence of the foot.
Acknowledgements
We would like to thank the artisanal fishing boats "Nuevo Hermanos Madueño" and "El Lele" for their collaboration in this study. To José Manuel Escobedo, owner of the Escobedo-Living Seafood purification and dispatch centre, for giving up his facilities for the purification process. The FEMP-AND-01 project is funded by the European Maritime and Fisheries Fund.
Keywords:
Donax trunculus,
artesanal fishery,
Bivalve,
mechanized dredges,
Foot damage
Conference:
XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019.
Presentation Type:
Poster Presentation
Topic:
Fisheries, Aquaculture and Biotechnology
Citation:
Marina
P,
Urra
J,
Garcia
T and
Baro
J
(2019). Incidence of fisheries impact and purification treatment in the survival rate of wedge clams caught in the northern Alboran Sea.
Front. Mar. Sci.
Conference Abstract:
XX Iberian Symposium on Marine Biology Studies (SIEBM XX) .
doi: 10.3389/conf.fmars.2019.08.00153
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Received:
24 May 2019;
Published Online:
27 Sep 2019.
*
Correspondence:
Dr. Pablo Marina, Oceanographic Center of Málaga, Spanish Institute of Oceanography (IEO), Málaga, Andalusia, 29016, Spain, pablo.marina@ieo.es
Mx. Teresa Garcia, Oceanographic Center of Málaga, Spanish Institute of Oceanography (IEO), Málaga, Andalusia, 29016, Spain, teresa.garcia@ieo.es