versión On-line ISSN 0717-6538
Gayana (Concepc.) v.68 n.2 supl.TIProc Concepción 2004
Gayana 68(2) supl. t.I. Proc. : 291-296, 2004 ISSN 0717-652X
MESOSCALE EDDIES AND PELAGIC FISHERY OFF CENTRAL CHILE (33-40°S)
S. Hormazabal1, S. Núñez2, D. Arcos2, F. Espindola1 & G. Yuras1
1. University of Concepción, Casilla 160-C, Concepción, Chile
The rich biological productivity within the Peru-Chile current system depends mainly on wind-driven coastal upwelling, which brings colder, nutrient-rich, subsurface waters into the illuminated upper layer, promoting high phytoplankton productivity whish is available for zooplankton and ultimately for fish. Off central Chile (29-39°S), where strong mesoscale eddies and meanders characterize the Coastal Transition Zone (CTZ), we are using hydrographic data, hydroacustic fish-biomass data and satellite information of wind stress, sea level anomalies and chlorophyll, to investigate linkages between mesoscale eddies and meanders and pelagic fish spatial variability. Satellite and in situ data showed that mesoscale structures have a strong association with primary production enhancement in the CTZ. In this region, mesoscale eddies have a coherent vertical structure above 600 m depth, and generate an offshore transport of 2x106 m3 s-1 which would be exerting an important influence on upper trophic levels in oceanic waters. A good relationship between mesoscale eddies and temporal-spatial fish distribution has been observed. The different responses of biological-physical interactions associated with mesoscale structure are also discussed. Grant from FIP, FONDECYT N°1040618 and Fisheries Research Institute.
In the eastern boundaries of the oceans, cold water filaments, mesoscale eddies and meanders are recurrent structures (Cáceres, 1992; Lutjeharms et al., 1991; Thomas et al., 2001). Intense eddies and meander currents generate a high variability region, between the coast and the deep ocean, known as Coastal Transition Zone (CTZ). Off the central zone of Chile (29-39°S), the CTZ is characterized by a wide band of high kinetic energy, which extends from the coast to 600-800 km (Hormazabal et al., 2004). In this zone the mesoscale eddies have typical spatial scales of about 200 km, remaining with a coherent spatial structure for several months, propagating predominantly offshore, and producing a transport of about ~2 Sv s-1 (Hormazabal et al., 2004). In this region, the high resolution numerical models indicate that barolinic instabilities of the coastal currents constitute the main source of eddies and meanders (Leth and Shaffer, 2001).
Studies done in the 1990s have suggested that the rise of nutrients to the illuminated layers, due to cyclonic mesoscale eddies (eddy pumping), may contribute the necessary nutrients for primary production (Falkowski et al., 1991; McGillicudy et al., 1998). On the other hand, mesoscale eddies with an anticyclonic gyre may produce an increase in the concentration of chlorophyll-a and in general in the concentration of planktonic organisms, through the convergent transport of particles toward the center of the gyre. It has also been observed that in some of the boundaries of the anticyclonic gyres a rise of the isopycns and a transport of waters rich in nutrients to the photic zone are produced, which causes an increase in the chlorophyll concentration (Mizobata et al., 2002). These physical mechanisms related to mesoscale structures promote the increase in the concentrations of available phytoplankton which support zooplankton and fish.
In different regions of the world a good association between pelagic species and diverse mesoscale structures has been observed, such as upwelling fronts, plumes, filaments, meander currents and eddies. For example, in the Kuroshio Current system, mesoscale eddies have been linked to favorable conditions for feeding and growth of anchovy eggs and larvae (Nakata et al., 2000). In the California current system a high survival rate and a significant growth of sardines have been observed, associated with mesoscale eddies (Logerwell and Smith, 2001; Logerwell et al., 2001). In the equatorial central pacific, off the coasts of Hawaii, high capture rates of sailfish and swordfish have been associated with regions that present strong fronts and thermal gradients, together with eddies and meander currents (Mitchum and Polovina, 2001; Seki et al., 2002). In the Chile-Peru current system, jack mackerel has been associated with regions of high chlorophyll concentrations and strong thermal gradients and inversions (Serra et al., 1994; Quiñones et al., 1994; Yañez et al., 1996). In the north zone off Chile, the highest number of captures of Spanish sardine and anchovy has been linked to regions of significant surface thermal gradients (Yañez et al., 1996). Recently, Hormazabal et al., (2004) have shown that mesoscale eddies and meanders constitute a distinctive element of great relevance in the dynamics of the surface currents off central Chile. This, along with studies done in other regions of the world, suggests that these mesoscale structures may have an important effect on the distribution of pelagic resources, such as jack mackerel. In this study some evidence indicating that in the central zone of Chile the spatial distribution of jack mackerel (Trachurus symetricus murphyi) is affected by mesoscale eddies and meanders is presented.
DATA AND METHODS
Individual captures of georeferenced mackerel records are used (tons, latitude, longitude), obtained from the purseine industrial fleet that operates in the south-central region off Chile (30-40°S). Information from acoustic cruises is also used, obtained through a systematic sampling of a variable size conglomerate, with sections extending from 5 to 200 mn off the coast, during June 1997. Acoustic information integrated between the surface and 500 m of depth is obtained with an ecointegrater SIMRAD EK-500 of 38 kHz. In the transects executed during the acoustic cruise, vertical profiles of temperature (°C), salinity (psu), dissolved oxygen concentration (ml/l) and chlorophyll-a (mg/m3) were carried out between the surface and 600 m of depth. In addition, for the area of study, satellite data of chlorophyll-a, estimated from SeaWIFS sensor, and surface current derived from merged TOPEX/Poseidon and ERS-1/2 satellite altimetry were used. The surface field of geostrophic currents and kinetic energy were estimated by following the methodology described by Hormazabal et al., 2004.
RESULTS AND DISCUSSION
1. CTZ and Mesoscale eddy variability
In the Humbolt current system the spatial distribution of kinetic energy, corresponding to a 7.5 year mean of surface currents, obtained through altimetry, shows a strong separation between the coastal transition zone located off the north of Chile (20-29°S) and the south-central zone (29-39°S). In this latter region mesoscale eddies and meanders are more intense than in the north zone, constituting dominant structures in the local variability (Figure 1). Eddies have a spatial scale of the order of 200 km; they extend vertically until the depth of 600 m (Figure 2), last for several months, and present typical westward movement. According to the spatial and temporal scales of these structures, it can be affirmed that they involve a westward transport of the order of 2106 m3/s; however, many of these eddies show rather stationary characteristics, where their associated transport is relatively weak.
Figure 1: Time mean, eddy kinetic energy calculated from 7.5 years of geostrophic velocity estimated from combined TOPEX/Poseidon and ERS-1 and ERS-2 altimeter measurements.
Figure 2. Surface current derived from merged TOPEX/Poseidon and ERS- satellite altimetry for periods a) May 19 and b) May 29, 1997. Left panel shows offshore sections of c) Temperature, d) Salinity, e) Density, f) Dissolved oxygen and g) Chlorophyll-a, at 36°30'S for period May 20-23, 1997. Positions of hydrographic stations are indicated by open circles in the surface current maps.
Various works have showed that the equatorial subsurface waters (ESSW), which are characterized by high salinity and nutrient concentrations, and low dissolved oxygen content, play an important role in the surface waters enrichment process by effect of coastal upwelling. These waters are transported toward the pole, over the continental slope/shelf margin, through the Peru-Chile undercurrent, which has a net flow toward the pole of 1106 m3/s (Shaffer et al., 1999). Mesoscale eddies, originated in the coastal zone, can transport a significant volume of subsurface equatorial waters toward the west, in this way extending the region that is directly affected by coastal upwelling. A situation of this kind is presented in Figure 2, where a cyclonic eddy is cut by an oceanographic section, leaving ESSW in its inside, separated from the poleward coastal flow. In this gyre, comparative colder and denser upwelling waters are evidenced, and so is a significant increment in the phytoplankton biomass in its inside, which achieves values higher than the ones observed at the coastal zone (Figure 2). These increases in the chlorophyll, associated with cyclonic eddies, can also be observed through satellite chlorophyll information, such as it is presented in Figure 3.
Figure 3: Surface chlorophyll density estimated from SeaWIFS ocean color for March 13, 2001. Black arrows show surface current derived from merged TOPEX/Poseidon and ERS- satellite altimetry.
2. Mesoscale eddies and jack mackerel distribution
Pelagic Jack mackerel biomass estimation (arbitrary unit) from hydrographic surveys and Jack mackerel fishing sets (positive captures) suggest a relationship between Jack mackerel distribution and mesoscale eddies (Figures 4 and 5). There, acoustic biomass estimation increases inside cyclonic eddies and strong coastal meandering currents (e.g. Figure 4, upper panel). This kind of distribution is also evidenced in fishing labor distribution with Jack mackerel captures, which predominantly appear around mesoscale eddies and meanders. A typical example of this kind of association with fishing labor is presented in Figure 5.
In the south-central zone off Chile currents associated with mesoscale eddies and meanders are of the order of 30 cm/s (Figures 2 and 5). These currents may well have a significant effect on the pelagic resource distribution, as it is suggested by Jack mackerel biomass estimation and capture distribution (Figures 4 and 5). However, Jack mackerel possesses the capability of developing a sustained swimming velocity of the order of 100 cm/s (Hunter, 1971; Wardle et al., 1996), whereby these structures could hardly draw up the boundaries of their motion. Then the association between mesoscale eddies and Jack mackerel must be defined by other factors, within which feeding behavior can be found. Mesoscale eddies, depending on their direction of rotation, intensity and temporal evolution, can produce a significant marine plankton concentration available for higher trophic levels.
Figure 4: Pelagic Jack mackerel biomass estimation (arbitrary unit) from hydrographic surveys between may 19 and june 08, 1997, off central Chile (34-40°S). Right panel shows surface current derived from merged TOPEX/Poseidon and ERS-1/2 satellite altimetry for biomass estimation period. Horizontal dotted line indicates cruise track.
Figure 5: Individual fishing sets (circles) and surface currents (arrows) derived from merged TOPEX/Poseidon and ERS- satellite altimetry off Central Chile (34-40°S), for period 04-june 2003.
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