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Latin american journal of aquatic research

versión On-line ISSN 0718-560X

Lat. Am. J. Aquat. Res. v.36 n.2 Valparaíso  2008

 

 

Lat. Am. J. Aquat. Res., 36(2): 159-181, 2008
DOI: 10.3856/vol36-issue 2-fulltext-3

RESEARCH ARTICLE

 

Coupling between the environment and the pelagic resources exploited off northern Chile: ecosystem indicators and a conceptual model

Acoplamiento entre el ambiente y los recursos pelágicos explotados en el norte de Chile: un modelo conceptual

 

Eleuterio Yáñez1, Samuel Hormazábal2, Claudio Silva1, Aldo Montecinos2, M. Angela Barbieri1, Alexander Valdenegro2, Alejandra Órdenes1 & Fabián Gómez2

1Pontificia Universidad Católica de Valparaíso, P.O. Box 1020, Valparaíso, Chile
2
Universidad de Concepción, P.O. Box 160-C, Concepción, Chile


ABSTRACT. The eastern boundary of the Chile-Peru Current System constitutes one of the most biologically productive ecosystems in the world, due largely to coastal upwelling and the horizontal advection of nutrients. In this ecosystem, El Niño events are of great importance in the interannual variability of the environment. A change was observed in the environmental regime at the beginning of the 1970s with the onset of the 1972-1973 El Niño, marking an important decrease in the anchovy fishery (Engraulis ringens). After the mid-1970s, sardine (Sardinops sagax) landings increased noticeably. A second regime shift at the end of the 1980s was seen mostly in the noticeable recovery of anchovy and the decline of sardine. Herein, we present an integrated conceptual model of the different local and large-scale phenomena that affect the marine environment off northern Chile and the distribution and abundance of pelagic resources. The model considers an analysis of environmental and bio-fishery data on different scales and describes how the interdecadal (associated with re-gime shifts) and interannual (associated with El Niño events) fluctuations in the Equatorial Pacific are mani-fested in the eastern South Pacific and, therefore, in the northern zone off Chile, affecting the annual eyele, the dynamic of the coastal trapped waves, and coastal upwelling. In this framework, interdecadal fluctuations play an important role in the anchovy-sardine-anchovy replacement sequence.

Keywords: pelagic fishery, environmental variability, conceptual model, northern Chile.


RESUMEN. El borde oriental del Sistema de Corrientes de Chile-Perú constituye uno de los ecosistemas de mayor productividad biológica del mundo, debido principalmente a la surgencia costera y advección horizontal de nutrientes. En este ecosistema, los eventos El Niño son de mayor importancia en la variabilidad interanual del ambiente. No obstante, un cambio de régimen ambiental es observado a inicio de los 70's el que hubiera comenzado con El Niño 1972-73 y que marca la gran disminución de la pesquería de anchoveta (Engraulis ringens); en tanto, desde mediado de los 70's los desembarques de sardina (Sardinops sagax) aumentan notablemente. Un segundo cambio de régimen se observa a finales de los 80's, representado principalmente por la notable recuperación de anchovy y disminución de sardina. Se presenta un modelo conceptual integrador de los diferentes fenómenos locales y de gran escala que afectan el ambiente marino del norte de Chile, y la distribución y abundancia de recursos pelágicos. El modelo considera análisis de datos ambientales y bio-pesqueros a distintas escalas, y describe como las fluctuaciones interdecadales (asociadas a cambios de régimen) e interanuales (asociadas a los eventos El Niño) del Pacífico Ecuatorial se manifiestan en el Pacífico Sur Oriental y por tanto en la zona norte de Chile, afectando el ciclo anual, la dinámica de las ondas atrapadas a la costa y la surgencia costera. En este marco, las fluctuaciones interdecadales estarían jugando un rol importante en la secuencia del reemplazo anchoveta-sardina-anchoveta.

Palabras clave: pesquerías pelágicas, variabilidad ambiental, modelo conceptual, norte de Chile.


INTRODUCTION

The eastern boundary of the Chile-Peru Current Sys-tem is a highly productive marine ecosystem due to the transport of nutrients by large-scale horizontal advection and persistent coastal upwelling (Bernal et al, 1983). Henee, Perú and Chile have become primary fishery countries world-wide. In Chile, annual landings fluctuate around 5 million tons; 90% of these are made up by the combined pelagic resources, with the landings from northern Chile constituting 42% (SERNAPESCA, 1957-2004). In this área, the fishery is based successively on anchovy (Engraulis ringens) and sardine (Sardinops sagax), with noticeable changes related to fishing effort and environmental fluctuations (Yáñez et al., 2001).

Ocean-climate changes have been related to alterations in the marine ecosystems on several spatial-temporal scales (Alheit & Bernal, 1993; Haré et al., 2000; Chavez et al, 2003; Yáñez et al, 2003; Alheit & Niquen, 2004). Physical forcing is obvious on the seasonal scale, with variations in temperature and coastal upwelling, for example, controlling productivity, growth, and migration. On an interannual scale, this forcing is associated with El Niño-La Niña events; that is, when the tropical ecosystems are directly affected, the eastern boundary of the Pacific is remotely forced by the concomitant oceanic-atmospheric teleconnections (Shaffer et al, 1999; Pizarra et al, 2002). A high degree of uncertainty exists regarding the mechanisms involved in the interdecadal changes (Bakun&Broad,2001).

The oceanographic variables that most influence the fishery resources seem to be temperature, mixing layer depth (MLD), thermocline depth, upwelling intensity, and the surface current fields (Bakun & Broad, 2001). Since many physical processes cause changes in the sea surface temperature (SST), such anomalies may be symptomatic rather than causal. The MLD, the nutricline, and the depth of the photic zone can influence primary production, affecting the availability of nutrients and the intensity of the light to which the autotrophic species are exposed. Long-term changes in the boundaries of the thermocline depth can directly influence habitats, change the characteristics of mesoscale eddies and filament formation, and affect upwelling.

Observations of currents over the continental slope off Chile have revealed a complex structure of variability in the intraseasonal, seasonal, and interannual bands (Shaffer et al, 1999). This variability is remotely forced in the equatorial Pacific by Madden-Julian oscillations in the intraseasonal band (Shaffer et al, 1997; Hormazábal et al, 2002), equatorial winds in the seasonal band (Pizarra et al, 2002), and the El Niño-Southern Oscillation (ENSO) eyele in the interannual band (Shaffer et al, 1999; Pizarra et al, 2002). This variability, which is connected to the equatorial dynamic, makes it possible to predict the area's oceanographic conditions months in advance.

In the Peru-Chile Current System, coastal trapped waves, Rossby waves, and mesoscale eddies and meanders have been identified as the dominant physical processes of variability (Hormazábal et al, 2002, 2004; Pizarra et al, 2002). These processes have diverse spatial and temporal scales. In other regions, they have been observed to be factors that contribute to the balance between the source of nutrients towards the photic layer and primary production (Chávez et al, 1998; McGillicuddy et al, 1998; Cipollini et al, 2001; Uz et al, 2001). Some theories attribute great importance to the variability of currents and mesoscale structures in the distribution and abundance of fishery resources in their different developmental stages (Sakagawa, 1989).

The oceanographic processes that affect the life-style of the species act on different spatial-temporal scales, on a continuum from hours to centuries and from centimeters to thousands of kilometers (Haury et al, 1978; Levin, 1992). Perry et al. (2000) proposed that fish catchability can fluctuate on diverse spatial-temporal scales. These variations can reflect changes in the physical and biological environment that oceur on similar scales. These authors then proposed a conceptual diagram on spatial (1 to 10,000 km) and temporal (day to century) scales, with characteristics of processes in the physical and biological marine environment and in the fishery activities.

Chávez et al. (2002) studied the impact of the 1997-1998 El Niño events on central California and defined a conceptual model of the associated ecosystem changes. The model describes, on an interannual scale, the relationships between resource abundance and distribution in different developmental stages and the levels of temperature, salinity, chlorophyll, sea level, and zooplankton, amongst others. Later, Chavez et al. (2003) studied the long-term changes in anchovy and sardine associated with important changes in the condition of the Pacific Ocean. The resulting conceptual model refered to the variability of the ecosystem proxies and to changes in the regime on a multidecadal scale similar to that covered in the present work (1950-2002). Finally, Bertrand et al. (2004) proposed an integrated context of factors that, oceurring on different spatial-temporal scales, could be used to interpret the effect of an El Niño event on the pelagic fish populations. Those temporal scales ranged from days to decades and the spatial scales from local to oceanic.

The ecosystem in question is affected by ENSO events (Philander, 1990) and El Niño long-term environmental changes (Cañón, 1986; Haré & Mantua, 2000; Chávez et al, 2003). El Niño events cause horizontal and vertical migration in the fishery resources and the more intense events also affect reproduction and survival (Sharp & McLain, 1993; Tarazona & Castillo, 1999; Bakun & Broad, 2001), generating strong socio-economic impacts (Glantz, 1998). However, this phenomenon is connected to large-scale environmental changes, making it difficult to isolate its effect on the resources and to evalúate how the related biological processes interact with exploitation, causing short-term disturbances and fishery collapses (Myers et al, 1995; Hutching, 2000; Yáñez et al, 2003). Under these circumstances, the fishery regulations face serious difficulties (Bernal, 1990; Bakun & Broad, 2001; Sepúlveda et al, 2003).

Thus, the objective of the present study is to ana-lyze the fluctuations of the environment and the pelagic resources off northern Chile, and to propose an integrated conceptual model of these.

MATERIALS AND METHODS

Analyzed information

Data on the landings (1950-2004) come from Chilean (SERNAPESCA, 1950-2004) and Peruvian (www.imarpe.gob.pe) fishery logbooks. Both data bases report on seiners and refer to fishing days and days with catch off northern Chile (1963-2002). They also consider biomass and recruitment of anchovy (1984— 2002) and sardine (1974-2002) and estimates of anchovy eggs and larvae (1993-2002) off northern Chile. These fisheries have been subjected to bans on extraction during reproductive and recruitment periods since 1983 (Table 1).


Monthly averages (1950-2002) of the following variables, recorded at coastal stations in the study área, were also analyzed: SST, average sea level (ASL), air temperature (AT), and wind direction and magnitude for estimating Ekman transport (ET; Bakun et al, 1974) and the turbulence index (TI; Elsberry & Garwood, 1978). The monthly averages of the South-ern Oscillation índex (SOI; Alian et al, 1991) and SST in El Niño regions 1+2 and 3.4 (web NOAA-NCEP) were also considered. The depth of the base of the thermocline (Zb) was also used; this was located on a 5o x 2o (longitude x latitude) grid and centered on 24°S and 72.5°W; the Zb was calculated with expandable and mechanical bathythermographic data (Pizarro & Monteemos, 2004).

Data collected during research cruises (financed by the Fondo de Investigación Pesquera) carried out between 1993 and 2002 were analyzed for the vertical distribution (0-200 m) of temperature (T), salinity (S%), oxygen (O), and chlorophyll-a (CLOA), available from the Servicio Hidrográfico y Ocanográfico (SHOA) of the Chilean Navy, along with SST data from NOAA satélites (1987-2002) and CLOA data from SeaStar and Térra & Aqua satellites (1997-2002).

Data treatment

The anchovy and sardine resources were analyzed through landings, fishing effort, and the catch per unit effort (CPUE) abundance index (Espíndola et al, 2005). Moreover, we considered estimates of abundance and recruitment for virtual population analyses (VPA), hydro acoustic analyses (sA), and the egg production method (EPM). The relationship between CPUE and biomass was analyzed according to Harley et al (2001), such that the coefficient of catchability (q) can vary with abundance, resource behavior, fishing strategy, and the environment.

Fluctuations in the catch and CPUE were analyzed with the CLIMPROD system, which consider standard fishing effort and environmental variables to be explanatory (Freon et al, 1993). In order to isolate the most important modes in the variability of the oceanographic and fishery series, we used both standard empirical orthogonal functions (EOF) (Tanco & Berri, 2000) and complex EOF analyses (Horel, 1984). With the latter, it is possible to determine the relationships of the phases between the variables and, as with the standard EOF, reconstruct the series based on the modes.

We used the environment-resource interactions and bibliographic information to develop an integrated conceptual model that sustains the working hypothesis on different spatial-temporal scales. For example, on the interannual scale, it considers the relationships between biological-fishery changes and El Niño events whereas, on the interdecadal scale, it establishes relationships between the anchovy and sardine fluctuations and large-scale environmental changes.

RESULTS

Landings and CPUE

Anchovy landings in Chile and Perú follow practically the same tendeney, including the variations associated with El Niño events (Fig. 1a). This fishery collapsed at the beginning of the 1970s, and was replaced by the sardine fishery off northern Chile and Perú (Fig. 1b). After 1985, however, the sardine fishery showed an important decline, first in Chile and later in Perú (Fig. 1b), at the same that the anchovy fishery recovered notably (Fig. 1a).


By examining the relationship CPUE = q B , in which the anchovy biomass is estimated with the EPM, the parameter (3 = 0.55 indicates a bias when considering the CPUE proportional to the abundance (Fig. 2a). The estimates of the sardine CPUE seem to be more reasonable, with growth until the mid-1980s and then a clear decrease (Fig. 2b).


In order to evalúate the seasonal anchovy and sardine pattern, we excluded the period following 1984 due to the effects of the application of extraction bans (Table 1). Moreover, for anchovy, we did not consider the 1976-1984 period since the fleet was mainly targeting sardine at this time (Fig. 1). Thus, the seasonality of the CPUE and the anchovy landings increased, particularly in summer although also in winter (Fig. 3a). For sardine, the seasonal component was greater during the second semester and minimal in spring-summer, with a secondary increase in March (Fig. 3b).


Environment defined by local winds

A study of the seasonality of the meteorological variables showed that the TI was at a minimum in June and peaked from October to January (Fig. 4a). The ET cycle, however, was inverse; offshore (westward) transport was greater in spring-summer and lesser in June (Fig. 4b). The depth of the mixing layer in the coastal and oceanic sectors exhibited máximum values in winter, related with the disappearance of the seasonal spring-summer thermocline (Fig. 5).



CLIMPROD models were previously adjusted for the anchovy (1957-1977) and sardine (1973-1995) fisheries off northern Chile (Yáñez et al., 2001). For anchovy, we considered a lineal-lineal model, taking into account the fishing effort and TI as variables that explained the drop in the CPUE (r = 0.91). For sardine, we used a linear-quadratic model in which fishing effort and SST explained the increase and later decrease in the CPUE (r2 = 0.93).

In the present work, we again analyzed the sardine (1973-2002) and anchovy (1984-2002) fisheries for the study area by adjusting non-conventional CLIMPROD models. The relationship between sardine landings and fishing effort were lineal-positive, increasing until peaking between 1983 and 1987 and then returning to the original values (Fig. 6a). The exponential-positive relationship between landings and the ET was greater in 1982-1987 (Fig. 6b). The sardine CPUE also showed positive, although not lineal, relationships with fishing effort and ET; the values increased from 1973 to 1985 and later de-creased again (Figs. 7a and 7b). The important relationship detected between these two explanatory variables should be pointed out (Fig. 8a), as should the strong relationship between ET and turbulence (Fig. 8b).




For anchovy, the direct relationship between re-cruitment and biomass was noteworthy (Fig. 9a). The relationship between recruitment and ET was inverse (Fig. 9b), as was the relationship between biomass and ET (Fig. 10a) and the more disperse relationship between standard fishing effort and ET (Fig. 10b). Although not as clear as that of sardine, the relationship between anchovy landings and fishing effort was also positive (Fig. 11).




For the anchovy fishery (1957-2002), the EOF analysis of combined variables allowed us to extract a first principie component (PC1) that explained 58% of the variability (Fig. 12a). The variables individually explained: catch (54%), fishing effort (59%), ET (65%>), and TI (55%). The analysis of the sardine fishery (1974-1995) considered a PC1 that explained 62% of the variability (Fig. 12b). In this case, the variables individually explained: recruitment (77%), biomass (83%), SST (48%), and ET (52%).


Environment defined by remote equatorial forcing

An index of the environmental fluctuations was defined using the PC 1 of the smoothed and normalized series (by the standard deviation) of the SST, ASL, Zb, and AT (Fig. 13). This mode explained 91% of the variance and was related to the normalized anchovy and sardine landings. According to the analysis, the CP1 explained 68% of the variance (Fig. 14a) and was similar to the environmental index (Fig. 13a), with a correlation of 0.92.


The PC2 explained 21% of the total variance, showing a positive tendency between the 1960s and the 1990s (Fig. 14b). As can be seen in Fig. 14c, on the main axis, the anchovy and sardine fisheries presented opposing variations, favoring a "positive" environment for sardine landings and a "negative" environment for anchovy landings. On the other hand, these two fisheries presented similar fluctuations on the second axis, increasing their landings when the values on the axis were positive.


Conceptual model

Considering the different analyses carried out and the bibliographic information mentioned, we adapted an integrated conceptual model of the changes that were produced on different spatial-temporal scales (Fig. 15).


Interdecadal scale

Evidence of regime shifts was clearly observed when studying the decadal variability of the anchovy and sardine abundance proxies off northern Chile. Consequently, in this ecosystem, we observed alternating periods dominated by anchovy and sardine (Fig. 1). We defined regime shifts to be the critical transition periods between an ecosystem dominated by anchovy and another dominated by sardines (Fig. 12).

The first period (1950-1971) was favorable to the anchovy fishery, given the high fishery efforts (FE AN) and landings (LAN AN) recorded in the 1960s and 1970s. This period was characterized by positive SOI values and a comparatively cold environment (reflected in the AT and SST), with an ASL that tended to decrease, modérate winds (reflected in the ET and T1), and a shallow Zb (Fig. 12, Fig. 13, and Table 2a). A shift in the ecosystem regime was observed in the beginning of the 1970s with the onset of the 1972-1973 El Niño events and associated with heavy fishing pressure. As a result, the anchovy abundance decreased due to low recruitment caused by the diminished parental biomass.

The second period (1973-1987) favored the sardine fishery, as seen in the high fishery efforts (FE SAR) and landings (LAN SAR); anchovy landings were also important as of 1986 (Figs. 1a and 12). In this period, the SOI in general decreased, the environment was comparatively warmer (reflected in the SST and AT), the ASL was greater, and the Zb deepened; local winds increased and, therefore, so did the ET and the TI (Fig. 13, Table 2b). A second regime shift was observed in the ecosystem at the end of the 1980s, with a shift from a sardine-dominated system to an ancho vy-dominated system; the latter prevailed through the writing of this study (July 2007).


The third period (1989-2002) was again favorable for anchovy, with high biomass (B-AN), recruitment (R-AN), fishery efforts (FE AN), and landings (LAN AN), unlike those observed for sardine (Table 2c). Although the AT did not tend to recover its previous cold levels and the SOI tended to recover positive values only at the end of the period (except for 1988-1989), the characteristics corresponded more to those of a cold environment, with local variables (SST, NM, ET, TI) that clearly tended to decrease.

Interannual scale

During El Niño events, the SOI decreased, tempera-tures were high at the Equator regions El Niño 1-2 and El Niño 3-4, and in the study area (SST, AT), the ASL increased, and the Zb deepened (Table 3a). Under these conditions, the anchovy abundance (as measured by acoustics (sA)), the quantity of eggs (EGGS), decreased (at least in 1997) along with the and the dis-tance from the coast (DC); henee, the species dispersal and vertical distribution increased. At the same time, the anchovy fishery efforts (FE AN) and landings (LAN AN) diminished noticeably.

During normal-cold years, these conditions were completely inverted (Table 3b). The SOI strengthened, temperatures at the Equator (El Niño 1-2, El Niño 3-4) and in the study area (AT, SST) dropped, the ASL declined, and the Zb became shallower. Under these conditions, particularly during long-term cold periods (prior to 1976, after 1990), the anchovy biomass, egg quantity, DC, FE-AN, and LAN-AN increased; anchovy vulnerability also increased due to the effect of the shallower Zb.


Seasonal scale

In the study área, the average monthly SST was higher during El Niño events (Figs. 16a and 16b). In summer, the SST was high, tended to decrease in autumn, reached its lowest levels in winter, and then recovered in spring. During normal-cold years, chlorophyll was higher in summer and mínimum in winter; during El Niño events, the summer máxima decreased noticeably.

The thermal horizontal gradients (GRT) in the anchovy distribution were greater in spring-summer and lesser in winter, particularly during normal-cold years (Fig. 16a). In the sardine distribution, the gradients were steeper in summer-autumn, particularly in normal-cold years; after this they decreased and remained low until the end of the year (Fig. 16b). With the exception of summer, the gradients during El Niño events were lesser, particularly in winter-spring.


Anchovy were mainly distributed within 25 nauti-cal miles (nm) of the coast in summer-autumn; in winter-spring, the distance from the coast (DC) increased noticeably, with specimens reaching 35 nm (Fig. 16a). During El Niño events, the distribution was rather coastal in summer and later surpassed 25 nm from the coast on occasions. In normal-cold years, the average DC for sardine was between 25 and 35 nm from the coast in summer-autumn, and from 45 to 55 nm in winter-spring (Fig. 16b). During El Niño events, sardine were also found from 25 to 35 nm in summer-autumn, but in winter-spring their distribution was more coastal (30-45 nm).

The anchovy CPUE did not show a clear seasonal-ity in the study period, although it was slightly greater in summer (Fig. 16a). During El Niño, the CPUE was slightly greater in summer-autumn-winter than during normal-cold years. The sardine CPUE in normal-cold

periods showed its highest values in spring (Fig. 16b). During the El Niño events, the CPUE was noticeably elevated in summer-autumn-winter, indicating a greater availability due to the approaching warmer water s.

Interannual and seasonal scale

During El Niño events, the isotherms and the distribution of anchovy deepened. The greater anchovy densities (aggregation fish core) were typically found at an average depth of 15 m, with a vertical excursión that did not exceed 60 m during El Niño periods (Fig. 17). The anchovy tend to expand their vertical distribution during these periods, reaching an average máximum depth near 140 m (Fig. 17).


During the study period the fluctuations of ASL and the subsurface Peru-Chile current were strongly modulated by the dynamic of the equatorial waves (Pizarro et al., 2002; Hormazabal et al., 2002). At intraseasonal frequencies, the coastal-trapped waves are dominating (Shaffer et al., 1997; Hormazabal et al., 2002); on lower frequencies, during warm periods equatorially forced Rossby waves are dominating and during cold periods these waves appear to be forced by a combination of coastal wind and equatorial forcing (Hormazabal et al., 2002; Hormazabal, 2003).

Daily-weekly scale

Off northern Chile, coastal upwelling prevailed throughout the year due to the predominance of south and south-west winds that had maximum values in summer and minimum values in winter (Pizarro et al., 1994). Upwelling events lasted 4 to 15 days and their extension reached 40 nm; the longer events were produced at the end of summer and beginning of autumn, and the shorter events occurred in winter-spring (Ba-bieri et al, 1995).

In spring-summer, upwelling was stronger due to the greater intensity of the favorable winds, the shallower thermocline (Zb), the more marked thermal gradients, and the productivity that was more concentrated at the coast (Fig. 18). During the long-term cold periods, these spring-summer environmental conditions had a positive effect on the anchovy concentration and a negative effect on sardine. During long-term warm periods, the spring-summer environmental conditions negatively affected the anchovy concentration and positively affected sardine. On the other hand, in winter, the winds were weaker, the Zb deepened, the thermal gradients decreased in intensity, and productivity was more disperse. These conditions were accentuated during the cold years and were no ticeably modified during El Niño and long-term warm periods.

DISCUSSION

Landings and CPUE

The anchovy yields showed negative anomalies associated with El Niño events (Yáñez & Barbieri, 1988), as reflected in their lower landings (Fig. 1a). More-over, the drop in this fishery at the beginning of the 1970s coincided with the collapse of the recruitment observed before the strong 1972-1973 El Niño event

(Mendelsohn, 1989; Mendelsohn & Mendo, 1987). Anchovy biomass and catches off Perú also decreased prior to the 1972-1973 El Niño event (Alheit & Ñiquen, 2004). However, the sardine fishery began to grow in the mid-1970s associated with the decrease in the long-term SOI (Cañón, 1986; Fig Ib).

Off northern Chile, the succession of pelagic species, including ichthyoplankton (Loeb & Rojas, 1988), began in the early 1970s in association with a change in the local and global environmental indexes. After 1985, however, changes were observed in some local indexes such as SST and ASL and, although not clearly, mthe SOI (Yáñez, 1998). In fact, after 1975, a warm period persisted in the area that was related to the increased abundance of sardine, heavy predators of anchovy eggs (Santander et al., 1983). Towards the end of the 1980s, the colder conditions were reestablished that favored the recovery of anchovy and the decline of sardine in spite of El Niño events in 1987, 1991-1992, and 1997-1998 (Ymez et al., 2001).

We recommend prudence with the CPUE esti-mates, due mainly to the difficulty in separating the variations in abundance from availability. Further-more, the operational information from the vessels considered days with fishing but did not identify the sites of activity of the vessels that returned without catch. Nonetheless, the annual anchovy CPUE for northern Chile (1986-2003), calculated considering trips with fishing and total trips, presented a similar decreasing tendency (Anónimo, 2004).

In spite of the El Niño events of 1987, 1991-1992, and 1997-1998, the environmental conditions in the north of Chile presented a cooling tendency towards the mid-1980s that favored anchovy (Yáñez et al, 2003). The relationship between the CPUE and anchovy biomass may have been influenced by, among others things, increments in vulnerability when faced with adverse environmental scenarios (Alheit & Niquen, 2004) and technological changes in the fleet that were not entirely included in the standardization models (Punt, 2000).

It should be pointed out that the anchovy CPUE off northern Chile (Anónimo, 2004) presented a different tendency than the VPA-estimated biomass (Serra et al., 2004). In fact, in an examination of the CPUE = q Bp relationship, using anchovy biomass estimated with EPM, the parameter (3 = 0.55 indicated a bias when considering the CPUE proportional to the abundance.

On the other hand, the sardine CPUE estimates seemed to be more reasonable, with growth to the mid-1980s followed by a clear decrease (Anónimo, 2004). However, the VPA-estimated biomass indi-cated that the decrease began earlier (Serra & Canales, 2003), which also implied a certain degree of hyper stabilityoftheCPUE.

Anchovy are largely distributed in the first 30 nm from the coast, are small (18 cm), short-lived (4 years), and have high fecundity. On the other hand, sardine are larger (42 cm), live longer (11 years), have lower fecundity, and are more oceanic in distribution (Serra, 1983; Serra & Tsukayama, 1988; Yáñez et al, 2001).

The summer increments in anchovy CPUE and landings are associated with the secondary spawning and the important process of recruitment (Braun et al., 1995; Castillo et al, 2002), protected since 1986 by bans on extraction (Table 1). The increase in winter is associated with the secondary recruitment period; the main spawning period occurs after a break in sexual activity from March to May (Braun et al, 1995). It should be noted that the bans applied since the mid-1980s are effective from July to September (Table 1).

Sardine normally present prolonged spawning from July to March, with an intense period from August to September and another less intense period from February to March (Serra & Tsukayama, 1988; Martínez et al, 1992). Since 1983, these periods have been protected by extraction bans (Table 1).

Under extreme environmental conditions, such as those associated with the El Niño phenomenon, drastic alterations are produced in the magnitude and season of the spawning peak of these resources off northern Chile (Martínez et al, 1986). When subtropical waters do not invade the región, waters with subantarctic origins domínate and favor upwelling, creating a suitable situation for anchovy. Under normal conditions, anchovy do not generally surpass 50 m depth and, horizontally, they are found within 50 nm of the coast given the warm conditions and less food available in the exterior waters (Yáñez, 1989). The intrusion of subtropical waters modifies the environmental conditions, affecting the anchovy behavior. Thus, El Niño events diminish anchovy catches, affecting resource availability and vulnerability. These events affect fecundity and egg viability and the survival of larvae, post larvae, and juveniles; they also decrease recruitment and stock biomass (Chavez et al, 2003).

The seasonality of the TI and ET are associated with coastal winds that are less intense in winter and more intense in spring-summer. Nonetheless, the annual cycle of oceanic wind obtained from satellite data differs from that of the coastal wind, being more intense in winter and less intense in summer, producing negative Ekman pumping in summer (Hormazábal et al, 2001). This limits the extensión of the upwelling events to a narrow coastal band, concentrating the anchovy near the coast and making them more vulnerable.

This is related to the annual cycle of satellite chlo-rophyll-a, which, along the coast, is at its highest in the summer and at its lowest in the winter (Yuras et al, 2005). In the oceanic sector, the cycle is the oppo-site, allowing a greater horizontal extensión of the área of anchovy distribution in winter due to higher food availability. The anchovy landings and CPUE drop from July to October, possibly associated in part with the greater depth of the mixing layer. An increase in the MLD implies a greater vertical extensión of the resource, which makes it less accessible to the fishing fleet.

Environment defined by the local winds

Pizarra & Monteemos (2004), Monteemos & Pizarra (2005), and Monteemos et al (2005) established that the fluctuations in the environment off northern Chile, on an interdecadal scale, are mainly forced by changes in the Trade Winds in the equatorial Pacific. Thus, oceanic changes off northern Chile are produced by variations in the Zb that are related to equatorial fluctuations. The deepening (rising) of the Zb implies an increase (decrease) of the ASL and an increase (de-crease) of the SST due to a decrease (increase) in the vertical advection of the relatively cold waters.

After certain levels, increased turbulence and Ekman transport, which are closely correlated, do not favor anchovy production (Yáñez et al, 2001). Egg and larvae mortality are greater and, therefore, recruitment is lower due to the dispersal and modification of the food quality (Bernal et al, 1983; Santander & Flores, 1983), and an increase in the seaward transport of the reproduction produets (Parrish et al, 1983). Nevertheless, an effect on the catchability cannot be excluded, particularly on older individuáis able to modify their distribution.

The augmented sardine abundance off northern Chile was associated with a regime shift that began in the early 1970s and was reflected in increased local variables and a decreased SOI (Yáñez, 1998). This warm period likely favored the increased intensity of sardine spawning and recruitment until wind intensities surpassed those that maximize anchovy recruitment (Yáñez et al, 2001). These healthy recruitments favored the increase of 5 to 8-year-old sardine individuáis, the most represented in the catches. Food availability for these specimens is good due to the greater Ekman transport and the absence of a considerable anchovy biomass.

Nonetheless, since 1985, large anchovy egg and larval abundances have been seen again (Rojas, 1986; Loeb & Rojas, 1988), along with an extraordinary growth in their catches, which were affected by the El Niño events of 1987, 1991-1992, and 1997-1998 (Fig. 1a). This recovery was related to the tendency towards environmental cooling after 1987, the decrease in ET and TI to average values, and the decreased sardine abundance (Yáñez et al., 2003). In fact, after the peak observed in 1984, the sardine recruitment decreased linearly with the parental biomass and with the average SST of year i to i-2 (Yáñez et al., 1998), which was then reflected in the noteworthy decrease in the catches (Fig. 1b). On the other hand, zooplankton volumes recorded in 1973 and 1985 were low but recovered notably towards the end of the 1980s (Ayon & Guevara, 2004).

The two periods favorable to the anchovy fishery were separated by a negative period that coincided with an environment that favored sardine abundance and the development of its fishery (Fig. 12). The same regime shifts identified herein were proposed for explaining the succession of pelagic species off northern Chile, as well as the changes in common sardine, anchovy, hake, jack mackerel, and swordfish off central Chile (Yáñez et al., 2003). Chavez et al. (2003) indi-cated that the fluctuations of anchovy and sardine in the Pacific were associated with large-scale changes in oceanic temperature, identifying two cycles with two cold phases (1900-1925, 1950-1975) and two warm phases (1925-1950, 1975-1995).

Environment defined by remote Equatorial forcing

Eighty-nine percent of the interdecadal variability of the landings, representing an abundance index on this temporal scale, could be explained by the variability of an environment defined basically by fluctuations in the Zb (68%) and fishing effort (21%). Nonetheless, although it was possible to find reasonable explanations for the relationship between the fisheries and the fluctuations of the environmental variables, we must consider that the magnitudes of the changes in these variables were small.

In fact, these amplitudes were on the order of 0.8°C in the case of SST, 10 cm for ASL, and 10 m for Zb. The question then arises as to whether or not it is possible to produce a change in the pelagic fisheries with these environmental variations. This requires a further the analysis of the life cycle of these pelagic species and determining which phase of this cycle is more susceptible to being modified by the environmental variations on the interdecadal scale.

Integrating conceptual model

A conceptual model was adapted to show how the interdecadal (associated with regime shifts) and inter-annual (associated with El Niño events) fluctuations in the equatorial Pacific are manifested in the southeast-ern Pacific and, therefore, off northern Chile affecting, amongst others, the dynamic of the equatorial waves, the annual cycle, and coastal upwelling.

In the interdecadal period analyzed, three periods were observed that are associated with two regime shifts. The first, early in the 1970s, began along with the 1972-1973 El Niño event; the second, produced around the 1990s, began in the mid-1980s. The fluctuations in the bio-fishery and environmental variables available for this interdecadal period characterize the three periods in question (Table 2). The most notable biological fluctuation is the substitution sequence of the species under study (Fig. 1).

On the interannual scale, significant changes were produced by El Niño events, which "heated" an environment with rather cold characteristics (Table 3). Long-term warm and cold periods were also observed on the interdecadal scale. The El Niño phenomenon was amplified when developed during long-term warm periods (1973-1987), which can be seen in the SST, ASL, and AT of the 1982-1983 event, without detracting firom the high values that these variables presented during the strong 1997-1998 El Niño (Yáñez etal, 2003).

The high sardine landings were more related to the establishment of a long-term warm period (1973-1987) than with El Niño events that occurred within this period (Fig. 1b). In fact, after the 1972-1973 El Niño event, sardine increased noticeably in the spawning and larval distribution áreas, even establishing a spawning center off Talcahuano (Bernal et al., 1983).

Bernal et al. (1983) indicated that eastern boundary systems, aside firom the low-frequency fluctuations, have a clear and definable seasonality. In the present work, this variability is conceptualized by analyzing the fishery (CPUE, distance from the coast) and satellite data, duly geo-referenced for the 1987-2002 period (Fig. 16). As effects, we considered the resources in normal-cold years and during El Niño events of the study period. The most outstanding fact is the spatial segregation of the species and their concentration at the coast during El Niño events, particularly in winter-spring, when the thermal gradients are lower.

The deepening of the thermocline during warm pe-riods produced a change in the vertical distribution of the anchovy, significantly decreasing their availability to the fishing gear, as reflected in the decreased catches normally observed during these warm periods (Fig. 1a). This modification of the vertical excursión of the anchovy is accompanied by a significant deepening of the isotherms and the upper limit of the oxygen minimum zone, as shown off Iquique in works by Morales et al. (1999) and Thomas et al. (2001), amongst others. This is related to the decreases associated with the low recruitment produced by the negative effect these warm periods have on reproduction and eggs and larval survival.

The normal spring-summer conditions favored anchovy reproduction, the recruitment that is produced the year before, and vulnerability to the fishing gear (Tabla 4). Given normal winter conditions, anchovy are distributed in a wider and more disperse zone, being less vulnerable.


El Niño events increase anchovy mortality during the first stages due to changes in the quality of food caused by less effective upwelling; their vulnerability also decreases due to changes in their horizontal and vertical distribution. These effects are reflected in the noticeably lower catches during such events (Fig. 1a). The recovery of the catches the following year is due to the great reproductive capacity of the anchovy under favorable conditions and greater stock vulnerability.

Long-term warm periods accentuate these conditions, which are unfavorable for anchovy, by increasing turbulence (that disperses the food) and Ekman transport (that advects reproductive elements west-ward). The nearness and permanence of warm waters in the zone favor the approach of sardine, their increased abundance under favorable conditions and, therefore, the development of the fishery.

ACKNOWLEDGEMENTS

This work was developd within the framework of Fondo de Investigación Pesquera project 2003-33: Análisis integrado histórico ambiente - Recursos I - II regiones.

 

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Received: 14 November 2006; Accepted: 29 April 2008