Batleb, J.M. (Joan Miquel)

Bueno-Pardo, J. (Juan)

Cabal, J. (Jesús)

González-Nuevo, G. (Gonzalo)

Nogueira, E. (Enrique)

Revilla-Álvarez, R. (Rafael)

Álvarez, E. (Eva)

English

Nogueira, Enrique

Batleb, Joan Miquel

Cabal, Jesús

González-Nuevo, Gonzalo

Revilla-Álvarez, Rafael

Álvarez, Eva

Bueno-Pardo, Juan

Digital.CSIC

Accumulation of northern krill (Meganyctiphanes norvegica) in a convergence zone at the Cap Breton Canyon (southern Bay of Biscay)

Repositorio Institucional Digital del IEO

Revista de Investigación Marina, 2008, 3  |  225Plankton & Nutrients poster session: R-113Accumulation of northern krill (Meganyctiphanes norvegica) in a convergence zone at the Cap Breton Canyon (southern Bay of Biscay).Enrique Nogueira*a, Joan Miquel Batleb, Jesús Cabal a, Gonzalo González-Nuevo a, Rafael Revilla a, Eva Álvarez a, Juan Bueno aa Avda Principe de Asturias 70 bis,Gijón, Spain. Fax: 34 98532 6277; Tel: 34 98532 8672; E-mail: enrique.nogueira@gi.ieo.esb Muelle de Poniente sn,Palma de Mallorca, Spain. Fax: 34 9740 4945; Tel: 34 97140 1561IntroductionPhysical-biological interactions generate spatial patterns in the distribution of plankton. In the last decade, it has being emphasized the importance of mesoscale physical structures in the distribution and production of plankton (Ressler and Jochens, 2003). Some authors have shown that organisms at the higher-trophic levels, including cephalopods, cetaceans and sea-birds also respond to these mesoscale features that control the availability of their prey (Davis et al. 2002).Northern krill (Meganyctiphanes norvegica) plays a pivotal role in the transfer of energy from the lower to the higher trophic levels of the food-web (Warren et al. 2004). During an oceanography-ﬁ sheries cruise conducted in the Bay of Biscay in autumn, a thin and large layer (ca. 15 m height on average, 1 nautical mile length) of northern krill was acoustically detected, and conﬁ rmed by a ﬁ shing haul, at the sub-surface layer (between 15-30 m depth) over the shelf-edge at the cap Breton Canyon. The position of this layer is concurrent with a converge zone associated to anticyclone eddy activity in this area. As far as we know, this is the ﬁ rst time that such a large layer of northern krill is reported in the Bay of Biscay.Figure 1. Oceanographic conditions in the Bay of Biscay between 19/09 and 04/10 of 2007 (a-b), and across the shelf at 2º 21’ longitude on day 21 (c-d). (a) Temperature at 5 dbar (ºC); (b) geopotential at 5 dbar (cm); (c) temperature and (d) ﬂ uorescence (arbitrary units) along the transect; (e) biovolume (µm3) of particles in the 0.5-1 mm equivalent spherical diameter (ESD) size-class at stations 1, 3 and 5 of the transect.MethodsThe PELACUS-1007 cruise was carried out on board RV Thalassa in the Bay of Biscay from September 19th to October 13th of 2007, to evaluate the spatial distribution and abundance of the pelagic community, from plankton to small pelagic fishes, and to study its relationship with the oceanographic conditions. Information on abundance and distribution of marine mammals and birds was also acquired. During day-time, under-way measurements of acoustic backscatter at 18, 38, 70, 120 and 200 kHz (SIMRAD ER60 scientific echo-sounder), opportunistic fishing hauls for identification of echo-traces, surface oceanographic conditions (temperature, salinity and fluorescence) and observations of top predators were carried out. At night, a profiler package was lowered at prescribed oceanographic stations from the surface down to a maximum 500 m depth. The profiler package consist of a rosette with twelve 5 l Niskin bottles for water sampling, a SBE 911 CTD (conductivity-temperature-depth), a Wet Lab Eco-AFL fluorometer, a SBE 43 oxygen probe, a Wet Lab transmisometer for measuring the amount of suspended particulate matter and a laser optical plankton counter (LOPC from BOT) for counting and sizing plankton between 90 and 20•103 µm size range. Meteorological information was acquired continuously during the cruise. Additional satellite-derived information (sea surface temperature, color and altimetry) was also gathered at different stages.Results and DiscussionThe general oceanographic conditions during the cruise revealed the occurrence of coastal upwelling in the Cantabrian Sea (Fig. 1a), anti-cyclonic eddy activity in the inner part of the Bay (Fig. 1b) and river plumes, and associated fronts and meanders, in the French shelf. In a coastal-offshore transect off 2º 21’ longitude, the tilt down of isotherms at the shelf edge were indicative of the presence of a converge area (Fig. 1c). Maximum fluorescence values were observed at the subsurface along the transect, but diminished notably at the outer stations (Fig. 1d). Contrastingly, zooplankton biovolumen in the 0.5-1 mm equivalent spherical diameter size-range was higher offshore than at the coast (Fig. 1e).Over the shelf edge of this transect, high acoustic backscattering was detected for the all the operation frequencies at a sub-surface layer located between 20-30 m depth, and with dimensions of 15 m height on average and 1 nautical mile length (Fig. 2a). A fishing haul with a pelagic net (2 cm mesh-size at the cod-end) confirmed that the layer corresponded to almost pure northern krill (Meganyctiphanes 226  |  Revista de Investigación Marina, 2008, 3XI International Symposium on Oceanography of the Bay of Biscay  (2-4 April, 2008. Donostia-San Sebastián)norvegica) of 21.4 mm average (16.7-26.2 mm range) standard length (from the posterior edge of the eye to the tip of the telson) (Fig. 2b).The aspect of the layer changed from the sunset to night (Fig. 3). It become more disperse and the center of gravity of the ascend up to 15 m depth. The occurrence of the layer was concurrent with a drop of sub-surface chlorophyll (fluorescence) and with the presence of marine mammals in the area (José Cedeira and Salvador García, pers. com.).AcknowledgementsThe authors whish to thank all the crew of the RV Thalassa for their continuous assistance during the cruise. EN thanks also all the participants in the cruise. The cruise was carried out within the frame of the IEO structural project ECOPEL  (Ecology of small pelagic fishes). G.G-N is a recipient of a predoctoral fellowship from the ‘Consejería de Educación y Cul-tu-radelPrincipadodeAsturias’.  ReferencesDavis, R.W., J.G. Ortega-Díaz, C.A. Ribic, W.E. Evans, D.C. Biggs, P.H. Ressler, R.B. Cady, E.J Harris, R.R. Leben, K.D. Mullin, B Würsig. 2002. Cetacean habitat in the northern Gulf of Mexico. Deep-Sea Research I, 49: 121-142.Ressler, P.H., A.E. Jochens. 2003. Hydrography and acoustic evidence for enhanced plankton stocks in a small cyclone in the northeastern Gulf of Mexico. Continental Shelf Research, 23: 41-61.Warren J.D., S.A. Demer, D.E. McGehee, R. Di Mento, J.F. Borsani. 2004. Zooplankton in the Ligurian Sea: Part II. Exploration of their physical and biological forcing functions during summer 2000. Journal of Plankton research, 26 (12): 1419-1427.Figure 2. (a) Echogram at 38 kHz corresponding to M. Norvegica; and (b) location of the identiﬁ cation ﬁ shing haul (number 7).Figure 3. Three different echograms acquired at 38 kHz for the same patch of M. norvegica, (a) at dusk (19 GMT), (b) at night (21 GMT) and (c) during the ﬁ shing haul (22 GMT). Vessel speed 10 knots for echograms (a) and (b) an 4 knots for (c).

Batle, J.M.

Cabal, J.

Santaella-Álvarez, E. (Eladio)

Accumulation of northern krill (Meganyctiphanes norvegica) in a convergence zone at the Cap Breton Canyon (Southern Bay of Biscay)

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Accumulation of northern krill (Meganyctiphanes norvegica) in a convergence zone at the Cap Breton Canyon (southern Bay of Biscay)

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