Jordi Salat

Global warming-enhanced stratification and mass mortality events in the Mediterranean

Summer conditions in the Mediterranean Sea are characterized by high temperatures and low food availability. This leads to “summer dormancy” in many benthic suspension feeders due to energetic constraints. Analysis of the most recent 33-year temperature time series demonstrated enhanced stratification due to global warming, which produced a ≈40% lengthening of summer conditions. Many biological processes are expected to be affected by this trend, culminating in such events as mass mortality of invertebrates. Climatic anomalies concomitant with the occurrence of these events represent prolonged exposure to warmer summer conditions coupled with reduced food resources. Simulation of the effects of these conditions on a model organism demonstrated a biomass loss of >35%. Losses of this magnitude result in mortality similar to that noted in field observations during mass mortality events. These results indicate that temperature anomalies are the underlying cause of the events, with energetic constraints serving as the main triggering mechanism.

Seasonal changes of water mass structure and shelf slope exchanges at the Ebro Shelf (NW Mediterranean)

Among several field efforts undertaken in the framework of the EU MAST-III FANS project, three oceanographic cruises covering the Ebro shelf and slope regions (NW Mediterranean) were carried out on board R/V Garcia del Cid between November 1996 and July 1997. A major aim of these three cruises was to provide insight into the seasonal variability of the distribution of physico-chemical oceanographic parameters and the shelf/slope circulation. In this paper we discuss the observed changes in the hydrographic structure, the spatial distribution of nutrients, chlorophyll and suspended particulate matter and the local circulation in relationship to the seasonal variability of the Ebro river discharge rates and the water column stratification. The added effects of mesoscale circulation structures and of the exchanges across the shelf edge are also addressed.

Marine circulation along the Ebro continental margin

Abstract Current meter and hydrographic data have been used to derive the marine circulation pattern along the Ebro continental margin. The main flow pattern is found along the slope, linked to the water mass structure at basin scale. Circulation stability under all seasonal conditions has been proved by 2 years of current meter data recorded at 8, 50 and 100 m at a shelf break station. The data indicate a permanent barotopic flow to the southwest; this is only altered by inertial oscillations and some short periods of current reversals, probably related to instabilities in the shelf/slope front present in the area. The data collected during a hydrographic cruise (October 1984) confirm that the topography plays a major role in defining the circulation pattern. Because of a sharp change in the wide eastward projection of the northern Ebro continental margin, the slope waters penetrate onto the shelf through a concavity of the bottom topography and cause a permanent local upwelling northeast of the delta. Diluted waters from the Ebro discharge are used as indicators of the shelf circulation. Two different waters of continental origin have been identified on the shelf, a colder water found on the surface near the river mouth that spreads to the south, and a warmer water probably formed in the southern bay of the Ebro Delta. The buoyancy input introduced by these fresher waters, together with topographic effects, produces small current gyres on both sides of the delta and reinforces the shelf/slope front that drives the general circulation to the south.

Near‐inertial motion on the shelf‐slope front off northeast Spain

Near-inertial motion on the shelf-slope front off northeast Spain was monitored using surface drifters and moored current meters. On the shelf, strong inertial currents were generated by a wind burst. The inertial current amplitude was about 70 cm/s at the surface, 30 cm/s at the base of the mixed layer, and 10 cm/s in the interior. The observed near-inertial frequency on the shelf was about 10% lower than the local inertial frequency, suggesting that the near-inertial motion was embedded in region of strong anticyclonic shear. Also, the phase of near-inertial motion increased through the water column, indicating that the energy propagation was downward. By contrast, the surface inertial currents were only about 10 cm/s in the center of the shelf-slope front. Indirect evidence suggests that the observed small surface inertial currents were the result of rapid downward transfer of near-inertial energy in the front.

Early life history and spawning of Merluccius capensis castelnau in the northern Benguela Current

The eggs and the larval development of hake Merluccius capensis from the Benguela Current are described. The abundance and distribution of hake eggs and larvae found during several surveys made to northern Namibia between 1979 and 1981 at different times of the year are also given. Hydrographic relationships are discussed. Temperature and dissolved oxygen were found to be the parameters most significantly influencing the distribution of hake eggs and larvae. It is apparent from the literature that hake tend to have fixed spawning areas and seasons and that they have fairly well defined migratory habits at spawning time.

CVPA, AN EXPANDED VPA WITH CANNIBALISM. APPLICATION TO A HAKE POPULATION

Abstract A generalized method of VPA, called CVPA, including the effect of cannibalism is presented. In this method, natural mortality was split into two components, one depending on cannibalism and the other, which remains constant, due to all other natural causes. These are not evaluated, but the method enables us to include new mortality causes when they are quantified. This method was applied to Cape hake ( Merluccius capensis ) off the coast of Namibia, from latitudes 25 to 30°S. The results were compared to the values obtained using the standard VPA. Significant differences in number of fish at any given age and natural mortality were found between these two methods, showing that standard VPA underestimates the values for the youngest age groups. Biomass levels at a given age show the same trend somewhat attenuated.

Clues from the recent past to assess recruitment of Mediterranean small pelagic fishes under sea warming scenarios

Knowledge of the effect of environmental variables on the early life history of fishes is essential to assess the effect of future environmental changes on the recruitment of commercial species. We investigate the effect of sea warming on two small pelagic fishes (Engraulis encrasicolus and Sardinella aurita) in the NW Mediterranean Sea based on the analysis of ichthyoplankton data collected in two surveys of contrasting conditions: the exceptionally warm summer of 2003, which may be indicative of conditions under future climate change scenarios, and the summer of 2004, with temperatures within the climatic average for the period 2000-2012. We use fine-resolution environmental variables measured locally and Generalized Additive Models to assess the influence of environment on these two summer-spawning small pelagic fishes. We show that sea surface temperature is the main environmental factor explaining abundance, but other factors (food availability and water currents) have additional roles tuning the effect of temperature. In the hot summer of 2003 we observed a decline of local egg production of anchovy and an increase of larval advection from the colder Gulf of Lions compared to 2004. Round sardinella spawning was higher in 2003 than in 2004 and extended over a wider area, but larvae viability was compromised by the lower availability of trophic resources. We hypothesize that future changes in environmental forcing on these two co-occurring summer spawning species will determine differential larval survival, with cascading effects on the upper trophic levels which feed on these species, including negative impacts on their fisheries.

Water mass distribution and geostrophic circulation off Namibia during April 1986

Abstract Water mass distribution and geostrophic circulation off Namibia are presented for April 1986, a month of relatively strong warm water intrusion from the north and west. The main result is that mesoscale processes develop in the area during the season when the coastal upwelling is relaxed, and these processes may be an alternative contribution to the fertilization of the region. The main features observed are the following. Angolan waters entered the region through the surface layer, with a maximum flow at 50 m depth. A frontal zone, centered between 20°S and 22°S separated Angolan and Benguela waters, at the surface layer. Anticyclonic eddies with a radius of 15–30 miles occurred off central and southern Namibia. Water upwelling off Walvis Bay appeared to derive from the core of the Angolan water, which laid at a depth of 50–100 m. Anticyclonic gyres might contribute to the upwelling off Walvis Bay and to the occurrence of a local sinking in its centre, about 50 miles offshore, thus allowing long residence time and recirculation. The occurrence of eddies may have been related to the general relaxation of the Benguela Upwelling System in the preceding 3 months and the penetration of the Angolan waters, in the region north of Walvis Bay, while in the south, the meanders of the oceanic front may remain most important.

Horizontal and vertical distribution patterns of ichthyoplankton under intense upwelling regimes off Namibia

Horizontal and vertical distribution patterns of eggs and larvae of selected fish species in the waters off Namibia during intense upwelling seasons are discussed. The material was collected in the course of several cruises carried out off Namibia during winter and spring. The coastal area was under the influence of strong upwelling, characterized by surface isotherms running parallel to the coast and, except for some slope stations, the water column from the surface to 200 m deep showed almost no stratification. Analysis of vertical distributions showed that eggs and larvae of all species were widely distributed throughout the water column, with a maximum within the upper 100 m. Spatial patterns of ichthyoplankton distributions in the northern Benguela during intense upwelling seasons show clear segregation into coast and slope (oceanic) areas. Two species, lanternfish Lampanyctodes hectoris and lightfish Maurolicus muelleri, dominated the oceanic area. In the coastal area, the most abundant species was ...

The use of surface drifting floats in the monitoring of oil spills. The Prestige case

Comunicacion presentada en VERTIMAR-2005, Simposio sobre Vertidos Accidentales de Petroleo en el Medio Marino = Symposium on Marine Accidental Oil Spills, celebrada del 13 al 16 de julio de 2005 en Vigo (Espana)