Chapter 24. THE BAY OF BISCAY: THE ENCOUNTERING OF THE OCEAN AND THE SHELF (18b,E) ALICIA LAVIN, LUIS VALDES, FRANCISCO SANCHEZ, PABLO ABAUNZA Instituto Español de Oceanografía (IEO) ANDRE FOREST, JEAN BOUCHER, PASCAL LAZURE, ANNE-MARIE JEGOU Institut Français de Recherche pour l’Exploitation de la MER (IFREMER) Contents 1. Introduction 2. Geography of the Bay of Biscay 3. Hydrography 4. Biology of the Pelagic Ecosystem 5. Biology of Fishes and Main Fisheries 6. Changes and risks to the Bay of Biscay Marine Ecosystem 7. Concluding remarks Bibliography 1. Introduction The Bay of Biscay is an arm of the Atlantic Ocean, indenting the coast of W Europe from NW France (Offshore of Brittany) to NW Spain (Galicia). Tradition- ally the southern limit is considered to be Cape Ortegal in NW Spain, but in this contribution we follow the criterion of other authors (i.e. Sánchez and Olaso, 2004) that extends the southern limit up to Cape Finisterre, at 43∞ N latitude, in order to get a more consistent analysis of oceanographic, geomorphological and biological characteristics observed in the bay. The Bay of Biscay forms a fairly regular curve, broken on the French coast by the estuaries of the rivers (i.e. Loire and Gironde). The southeastern shore is straight and sandy whereas the Spanish coast is rugged and its northwest part is characterized by many large V-shaped coastal inlets (rias) (Evans and Prego, 2003). The area has been identified as a unit since Roman times, when it was called Sinus Aquitanicus, Sinus Cantabricus or Cantaber Oceanus. The coast has been inhabited since prehistoric times and nowadays the region supports an important population (Valdés and Lavín, 2002) with various noteworthy commercial and fishing ports (i.e. Brest, Nantes, Lorient, Saint- ________________ The Sea, Volume 14, edited by Allan R. Robinson and Kenneth H. Brink ISBN 0–674– ©2004 by the President and Fellows of Harvard College 933 934 THE BAY OF BISCAY: THE ENCOUNTERING OF THE OCEAN AND THE SHELF Nazaire, La Rochelle and Bordeaux in France; Ondarroa, Bilbao, Santander, Gi- jón, Avilés and A Coruña in Spain). Marine studies have been performed extensively in the Bay of Biscay, where some of the oldest marine laboratories in the world are located (e.g. the French Laboratoire de Zoologie et de Physiologie Maritime of Concarneau and Station Marine de la Société Scientifique d’ Arcachon funded in 1857 and 1867 respec- tively, and the Spanish Estación de Biología Marina de Santander funded in 1886 by Dr. A. G. de Linares, and the Sociedad Oceeanográfica de Guipúzcoa funded in 1908 in San Sebastian). Despite having these laboratories on the fringes, oceanog- raphy has developed slowly and in the older maps of the North Atlantic circulation a gap always appears in the Bay of Biscay showing the lack of knowledge of the area. Activities were always restricted to some PhD studies inside ‘rades’ or bays. In the 1970s activity began to develop all over the bay, not only in the shelf (such as the POLYGAS and PHYGAS cruises) and a large increase occurred in the 1980s. In the physical oceanographic studies on the Bay of Biscay there is a re- markable figure from Britain - Dr. Robin Pingree. Dr. Pingree developed a great deal of work either alone or together with some colleagues and has a large number of publications on the area, 16 of which are cited in references in the present chap- ter. He has strongly improved the knowledge of the oceanography of the Bay of Biscay and we would like to honor him in this chapter. The Bay of Biscay is a well-differentiated geomorphological unit in the north- east Atlantic. The abyssal basin has a mean depth of 4,800 m. The shelf in the south of the bay (Spanish coast) is quite narrow whereas on the French coast it is much wider, increasing with latitude. In the Bay of Biscay there are various deep- sea canyons that have generally narrow, steep-sided, linear and sinuous channels (http://www.univ-brest.fr). The deep-sea valleys allow continental sediments to be transported to oceanic basins. Most of the water masses occupying the bay have a North Atlantic origin or are the result of interaction between waters formed in the Atlantic with water of Mediterranean origin. The hydrodynamics of the bay are dominated by: a) a weak anticyclonic circulation in the oceanic part, b) a pole- ward-flowing slope current, c) coastal upwelling, d) the northward flow of Medi- terranean water, e) the shelf circulation and f) the cross-shelf transport along the axes of submarine canyons (OSPAR, 2000). Most of these features show a marked seasonality (Koutsikopoulos and Le Cann, 1996). The Bay of Biscay is a region of large tidal amplitudes and strong thermohaline forcing (Piraud et al., 2003). It is well known for its energetic internal tides, caused by the combination of summer stratification, steep shelf-edge topography, and strong (cross-slope) tidal currents, especially at spring tides (Lam et al., 2003). In a temperate sea like the Bay of Biscay the annual phytoplankton and zoo- plankton seasonal cycle is governed by the alternation between mixing and stratifi- cation of the water column (Fernández and Bode, 1991; Valdés et al., 1991). The highest planktonic biomass appears in spring followed by a secondary peak in autumn. This cycle can be modified by the development of shelf-edge fronts and coastal upwelling (García-Soto and Pingree, 1998; Díez et al., 2000). Mesozoo- plankton biomass is highly variable, following the production of phytoplankton, which is mainly determined by the seasonally changing environmental conditions. The primary production levels and the topographic characteristics of the shelf basins have allowed the development of important fisheries and mariculture areas LAVIN, VALDES, SANCHEZ, ABAUNZA, FOREST, BOUCHER, LAZURE AND JEGOU 935 of oysters and mussels. Living marine resources exploited in the Bay of Biscay include a wide range of organisms, from seaweeds to shellfishes (i.e. Norway lob- ster, cephalopods), fishes (i.e. mackerel, hake, anchovy, sole) and whales. Indeed, it has been reported that molluscs were exploited in the bay as early as the Palaeo- lithic period and whales in the Middle Ages (Valdés and Lavín, 2002). The study of the exploitation of some pelagic fishes in the Bay of Biscay started in the middle of the last century under the auspices of ICES (International Council for the Explo- ration of the Sea) (Cendrero, 2002). The economic and social importance of these and other fisheries and the objective of obtaining appropriate yields led to the development of studies to explain the variability of fish abundance and to predict the strength of incoming recruitment. In this way, important developments have been made in research fields related to marine primary production, ichthyoplank- ton, zooplankton and applied oceanography. Many of the exploited resources in the Bay of Biscay are managed as a particular unit or stock. In spite of the amount of information about the biology of these species, there is much to work on species interactions, including man, and in their relationships with the variable environ- ment. Thus, the study of ecosystems should be one of the milestones for future research in the Bay of Biscay. There are many descriptive studies on different aspects of the Bay of Biscay as we will show throughout this contribution and recently a few reviews were pub- lished integrating all the information available. The main contributions are Quality Status Report from OSPAR (2000) and the work of Valdés and Lavín (2002), which considers the Bay of Biscay as a large marine ecosystem. Díez et al. (2000) reviewed the information on the southern part of the Bay of Biscay (the Can- tabrian Sea). The ocean is an inhomogeneous space. Therefore, the regionalization of the oceans arises to explain the discontinuities observed in the distribution of oceanic properties, substances and movements (Vanney, 2002). The aim of this chapter is to bring out the common characteristics from geography, geology, oceanography and ecology that are met with in the Bay of Biscay and that allow it to be defined as a particular marine region. 2. Geography of the Bay of Biscay The North Atlantic Ocean has a general circulation made up of two enormous gyres: the anticyclonic subtropical gyre, and the cyclonic subpolar gyre. The Bay of Biscay is found at mid-latitudes between the limits of both gyres - the Azores cur- rent from the subtropical gyre and North Atlantic current from the subpolar gyre. 2.1 Topography and geology The Bay of Biscay is bounded zonally by the parallels 48∞N and 43∞N. The north- ern boundary roughly corresponds to the separation between the Armorican shelf and the Celtic Sea. The slope of the bay is formed by three main areas with differ- ent orientation, the Armorican slope NW-SE, the Aquitaine slope N-S and the Cantabrian slope with an E-W orientation. The southern boundary is composed of Cape Finisterre and the Galician Bank (Fig. 24.1). 936 THE BAY OF BISCAY: THE ENCOUNTERING OF THE OCEAN AND THE SHELF Figure 24.1 Bay of Biscay topography map (depth in m) based on the GEBCO97 dataset. The Biscay abyssal plain is adjacent to the Porcupine plain in the northern part but separated from the Iberian Abyssal Basin and the West Iberian Margin by the Charcot Seamounts and the Galician Bank (Fig. 24.1). The French Central Massif, the northern and western Pyrenees and the Iberian mountain systems correspond to the most prominent continental limits of a river catchment area, which is limited to the north by the hills of Normandy and Brit- tany.