Đinh Văn Khương
Senior Member
Impact of El Niño events on pelagic fisheries in Peruvian w
Impact of El Niño events on pelagic fisheries in Peruvian waters
Miguel Ñiquen
Department of Pelagic Research, Instituto del Mar del Perú, P.O. Box 22, Callao, Peru
Received 20 February 2003; accepted 3 May 2004. Available online 28 August 2004
Abstract
Using data from stock assesment surveys on pelagic resources during El Niño events of 1972/73, 1982/83, 1997/98, we analyze biological changes on pelagic ecosystems and pelagic fisheries during different stages of development of El Niño phenomenon: emergence, full, final and post-Niño. Results indicate changes in spatial distribution of resources, their concentration and size structure. In anchovy (Engraulis ringens) a decrease in biomass was observed, which was estimated at 1.2 million tons in September 1998, the lowest throughout the 1990s. This resource showed an asymmetric distribution towards the south of Peru. Other pelagic resources increased their biomass during or after Niño events, primarily sardine (Sardinops sagax), jack mackerel (Trachurus murphyi), pacific mackerel (Scomber japonicus), and longnose anchovy (Anchoa nasus). At the end of the El Niño phenomenon we found less productivity but more diversity in the pelagic ecosystem. During the 1997/98 El Niño, the diversity index (Manual de Ecologia, 1a Edition, Editorial Trillas, Mexico, 267pp) increased from 0.87 to 1.23–1.70.
In both the emergence stage and fully developed stages of El Niño we found large numbers of sardine and longnose anchovy present simultaneously. Size structure of sardine, jack mackerel, and pacific mackerel showed an increase in juveniles. Anchovy during El Niño showed a single modal group composed of adults, but the post-Niño phase indicated an increase in juveniles with an average length of 6–7 cm. In El Niño conditions spawning among anchovy was low, but among sardine and pacific mackerel it was high. We observed, for the first time during full spawning, juvenile sardines with a total length of 18–20 cm. The anchovy spawning season during the post-Niño phase was considerably lengthened, from April to December 1998. Drastic change occurred in fisheries when monospecific fisheries, based on anchovy before El Niño, became multispecific fisheries based on sardine, jack mackerel, pacific mackerel, longnose anchovy and other species typical of tropical and warm oceans. Landings of anchovy decreased substantially, but those of longnose anchovy, jack mackerel, pacific mackerel, and sardine increased.
1. Introduction
The El Niño phenomenon generates substantial changes in oceanographic and meteorological conditions in the Pacific Ocean, with manifestations impacting the Peruvian coast (Zuta et al., 1976); this has mainly affected pelagic resources, producing alterations in their biological processes, behaviour, and gradual decrease in their population levels (Valdivia, 1976).
One of most sensitive species to these changes is anchovy, which in the 1960s reached biomass levels of 15–20 million tons, providing annual landings that exceeded 10 million tons in 1970. From 1972, abundances diminished drastically due to a combination of high exploitation and anomalies caused by powerful 1972–73 El Niño (Tsukayama, 1983). This situation worsened in early 1980s, with the occurrence of another exceptionally strong El Niño phenomenon in 1982–1983 (Csirke et al., 1994).
This paper describes the effects on pelagic resources of strong or exceptionally strong El Niño events occurring during the periods 1972–73, 1982–83 and 1997–98, predominantly translated into biological alterations and vertical and latitudinal changes in distribution and concentration. These effects are of particular interest because they are related to biological processes such as their reproductive cycle and recruitment, as well as the ecosystem approach to fishery management.
2. Methods
The study area is the Peruvian coast between 03° 30′–18° 20′S and 72°–84°W, which is dominated by the Humboldt–Peru eastern boundary current system, characterized by a complex system of surface and sub- surface currents associated with variations of coastal upwelling, and increases in southern trade winds (Zuta and Guillen, 1970). The main pelagic species are anchovy (Engraulis ringens), sardine (Sardinops sagax), jack mackerel (Trachurus murphyi), and pacific mackerel (Scomber japonicus) (Tsukayama, 1983). For species identification Chirichigno and Velez (1998) was used.
Stock assessments of the main pelagic resources were made by hydroacoustic surveys of pelagic stocks, using a SIMRAD EK 500 and EK 400 echosounder operating at 120 and 38 Khz to a depth of 250 m.
Sea-surface temperature and salinity were taken from land oceanographic stations every 10 min during the operations at sea and from coastal stations at Chicama and Chimbote, provided by the Oceanographic Forecasts and Marine Hydrophysical Department of IMARPE.
Biometrics and biological samplings of pelagic species were made, and species composition of catch, biological aspects, size structure, individual weight, sex and sexual maturity were recorded. The diversity was estimated using the Shannon–Wiener Index (Franco, 1985) on the commercial catch data.
3. Results and discussion
The El Niño events reveal the following changes, summarized in Fig. 1 and described below, in the environmental conditions and stocks of the main pelagic resources between the pre-El Niño period and the end of the event.
fig1:Effects of El Niño events on pelagic resources
3.1. Environmental conditions
One characteristic of the El Niño events was a very marked temperature change between Paita and Chimbote (5°–10°S) caused by the intrusion of warm equatorial surface waters, producing extreme thermal anomalies of up to 8 °C above the average in the phase of maximum development. However, between Callao and Ilo (12°–17°S) variations were smaller, and anomalies reached+6 to+2 °C (Zuta et al., 1984).
Each El Niño had different features (Bjernes, 1966). The data, however, shows that in 1971, 1982 and 1996, there was a period of cold weather prior to the phenomenon, which lasted until the early months of the following year (Fig. 2).
fig 2: Sea-surface temperature anomalies observed at Chicama (07°42′S–79°26′W) before, during and after El Niño events.
The period of higher temperature observed in 1972 was quite protracted (Zuta et al., 1976), while in 1982–83 it was short and much more intense. By contrast, the temperature rise of 1997–98 was both long in duration and strong in its intensity. In all three cases, two development peaks were observed, with the second having a higher intensity (Fig. 2).
3.2. Changes in Latitudinal and Vertical Distribution
Historical data analysis indicates displacement, from north to south and west to east, of several marine species (Chirinos de vildoso, 1976; Ñiquen et al., 1999; Kameya, et al., 2001). Some of them, originating from Panamanian waters, were observed in the north-central region of the Peruvian coast during the 1997/98 El Niño event (Ñiquen et al., 1999; Bouchon et al., 2001). This displacement can be viewed as the shift of a system with its integral species, as the thermal anomaly intensified (Fig. 3). This is how fish, such as tunas, billfishes, pacific mackerel, and jack mackerel, appeared in Peruvian waters, followed by longnose anchovy (Anchoa nasus), strand machete (Opisthonema libertate), skipjack (Katsuwonus pelamis), and finally mictophidae (Diogenichthys laternatus). The fish beard of corn (Bregmaceros bathymaster) has been detected (a species of restricted distribution) usually found off Panama Gulf (Chirichigno and Velez, 1998), together with ayamarca (Cetengraulis mysticetus), mackerel scad (Decapterus afuerae), pacific cutlassfish (Trichiurus nitens), and crabs (Euphilax sp). Meanwhile, resident Peruvian species like anchovy became patchier and asymmetrically distributed towards the southern Peruvian coast, instead of being widely distributed over the whole Peruvian coast as was usually the case. Sardine shoals also were displaced from the north-central to the south-central area.
Fig. 3. Species migration by the of El Niño 1997–98 event along the Peruvian coast
Incidence of warm conditions in the north-central region in the primary stages of El Nino events determines anchovy movement, first towards the coast, and mostly within 30 Km of inshore waters. These movements result in high concentrations, making this species very vulnerable to fishing. Almost immediately, the highest concentrations begin moving south of Chimbote (09°S) and deeper schools of anchovy also have been detected in the north-central region, located between depths of 20 and 80 m, when the El Niño was fully developed. This behaviour resulted in a change in fishing strategy, with the largest catches made at night when fish moved closer to the surface.
With regard to sardine, the largest concentrations began to move, from a normal distribution off Paita (05°S), towards Pimentel and Chicama (07°–08°S), simultaneously approaching the coast. In the most notable case they reached Ilo (17°S) in the south, with higher concentrations off Chimbote (09°S), close to the coast, as was the case during 1997–98 El Niño event (Fig. 4). A similar pattern was found during the El Niño event of 1972–73 (Zuzunaga, 1985)
Fig. 4. Changes in distribution and size structure of anchovy and sardine during April 1997–June 1998.
These interannual migrations depend on the magnitude of environmental changes. For example, in summer 1998 (the highest peak of 1997–1998 El Niño), most of the anchovy biomass was detected south of 13°S (Fig. 4) and the bulk of the catches occurred between 16° and 18°S. while in summer 2000 (cold conditions), the catches were spread out over Peruvian coast, with the highest value off northern Peru (Fig. 5).
Fig. 5. Latitudinal changes of anchovy catch relative to sea-surface temperature (SST).
A complementary aspect of distribution and availability of anchovy is shown by the observation of sea birds, which left islands in the north of Peru in April 1997, migrating mainly to the central and southern areas (IMARPE, 1997).
3.3. Changes in Size Structure
During the 1972–73, 1982–83 and 1997–98 El Niño events, adult anchovy prevailed and young fish were generally few or absent (Fig. 4 and Fig. 6), which impact recovery and population growth. However, at the beginning of the post-Niño phase, this behaviour varied notably with a strong increase in young and juvenile fish (IMARPE, 1998), coming from spawnings starting at the end of the El Niño event and disappearance of adults, which were probably concentrated near the coast and therefore undetectable by echosounders and not catchable by purse seiners
Fig. 6. Anchovy Size structure during El Niño events.
Sardine changed from predominantly adults to mostly young fish during the 1997–98 El Niño event (Fig. 4). In general, an increase in the abundance of juvenile sardine has been observed during El Niño events (Fig. 7), with mainly individuals between 1 and 2 y of age, which were detected throughout the whole El Niño period. The presence of warm conditions has favoured this stock increase.
Fig. 7. Size structure Sardine during “El Niño” events.
It is important to highlight the fact that large numbers of young fish were observed among sardine, longnose anchovy, pacific mackerel, and also hake (Merluccius gayi), lumptail searobin (Prionotus stephanophrys) and Vinciguerria spp, all of which were favoured by the strength of the 1997/98 El Niño. This has been detected in different phases of El Niño, especially at the middle of the event in September 1997 (Fig. 8). This situation indicates a recovery in the stocks of the main pelagic species, because at the end of El Niño event, a new species composition is observed, as a result of the entrance of other pelagic species, different to anchovy, which mainly have a juvenile size structure, and whose development depends on the environmental conditions. If the conditions are warm, the change will be long lasting, and if the environment is cold, other pelagics will tend to disappear, originating a rapid return of anchovy.
Fig. 8. Proportion of young marine species before, during and after the El Niño 1997–98 event.
In case of longnose anchovy, fish beard of corn (Bregmaceros bathymaster) and sea-catfish (Galeichthys peruvianus), where adults prevail the stocks grew during the whole 1997–98 El Niño event, in which these adults had almost completed a full life cycle during 18 months of oceanographic anomalies.
3.4. Changes in the Reproductive Process
During the 1997–98 El Niño, the anchovy reproductive cycle was disrupted and spawning diminished in intensity (Perea et al., 1998), while the opposite happened to the pacific mackerel and sardine: spawning increased in intensity at the beginning of El Niño, and during El Niño they approximated the historical average (Fig. 9). Furthermore, spawning juvenile sardine and pacific mackerel were detected during the final period of El Niño. Sexually mature sardines are predominantly 26–27 cm in length, but at this time significant numbers of sexually mature sardines with lengths of 18–20 cm were observed, which suggests that El Niño conditions favoured the reproduction of this species.
Fig. 9. Gonadosomatic index (IG) of pelagic resources during El Niño 1997–98.
This is probably related to better environmental conditions for sardine, allowing these juveniles to develop their full reproductive potential. It is worth mentioning that the current range of sardine sizes corresponds to that found in sardine off California, where it is known that 50% is sexually mature at a length of 19 cm. Macewicz (1996) and Parrish et al. (1989) mention that California sardine and Peru–Chile sardine probably have a common origin and constitute only one species. This evidence of early spawning of sardine would indicate that sardines on the Peruvian coast during El Niño years behave in a similar way to the Californian sardines, during normal years. It is unknown if this evidence has temporary or permanent character in other years. It should be kept in mind that this was the first opportunity to sample juvenile sizes in situ in the appropriate pre-spawing or spawning season, which allowed for the correction of a lack of samples in these sizes at spawning time. Another interpretation of the observed size distribution is a reduction in sardine growth rate, influenced by poor feeding conditions, which would be reflected by the presence of small, but older than expected sardines.
The intensity and reproductive activity of other pelagic resources such as pacific mackerel and longnose anchovy were increased (Fig. 9).
3.5. Changes in Biomass of Pelagic Resources
During an El Niño event, a remarkable decrease in anchovy biomass and increase in the biomass of other pelagic species has been observed (Fig. 10 and Fig. 11). However, the magnitude of impact depends on the intensity of El Niño events and the species composition during the period before the development of the phenomenon. In 1971–73, at the beginning of the El Niño, anchovy dominance was almost total, decreasing drastically throughout the course of the event. At the end of the event increases in stock as of sardine, jack mackerel and mackerel were noticeable. With regard to the 1982–83 El Niño, other pelagic resources dominated the period before the event, and represented 95% of total biomass by the end of it. The 1997–98 El Niño was an intermediate case between the two previous ones, where anchovy biomass was relatively important before the event and decreased dramatically during its development (Ñiquen and Gutierrez, 1998), while the biomass of other pelagic species, which was substantial before the event, increased or tended to remain stable throughtout.
Fig. 10. Pelagic biomass before, during and after El Niño events.
Fig. 11. Anchovy biomass and other pelagic during El Niño 1997–98.
In three analyzed cases, a decrease in fish productivity is general, mainly due to a decrease in the biomass of anchovy. The 1982–83 El Niño event could be seen as an exception in that the total biomass increased during the last year, but this increase, in fact, was due to the increase in jack mackerel (as a migratory resource) abundance (53% of the total).
During the last El Niño event (1997–98) a decrease in anchovy biomass was observed during the mature phase of the phenomenon, which only started to show weak signs of recovery at the end of 1998, while the biomass of other pelagic species increased during the first half of the El Niño event and decreased during the second half of it and during the post-Niño phase (Fig. 11).
3.6. Changes in Pelagic Fisheries
As a consequence of stock variations in the main pelagic resources, of three El Niño events analysed, the Peruvian anchovy fishery was seriously affected and was forced to diversify its catches, towards other pelagic resources (Fig. 12) such as sardine, jack mackerel, pacific mackerel and longnose anchovy. It also took advantage of the availability of unusual species such as round sardine, ayamarca, strand machete, golden fish and others.
Fig. 12. Anchov and other pelagic landings during El Niño 1997–98.
From 1990 until the beginning of the 1997–98 El Niño events, we observed an increase in anchovy catches due to good spawning and a trend towards cold conditions. This was observed in purse-seine landings, where anchovy was the principal component during the summer of 1994 and the fall of 1997, making up 90% of total catch on the north and central Peruvian coast (03°30′–16°S). In the winter of 1997, a decrease in anchovy catch from purse-seine fleets was observed, while the catches in other pelagic species remained relatively stable before large fluctuations increasing during the post-Niño period (Fig. 11). As a result, the proportion of the different species in the landing varied considerably over this period, with an abrupt change in the first quarter of 1998, when the anchovy fishery was closed (Fig. 13).
Fig. 13. Changes in the seasonal species composition in purse-seiner landings off north-central Peru, 1994–1998.
The relationship between SST of Chimbote (09°S) and Shannon–Wiener Index during January 1997 and September 1998 (Fig. 14), showed a direct relationship until April 1998. Then, while SST decreased, Shanon–Wiener index remained high. In this case other factors have a bearing on the diversity index, elevated salinity being a likely principal factor (Moron et al., 1998). Nonetheless, one must keep in mind that this diversity index was computed from commercial fishery data and therefore is likely to be underestimated and distorted by fishery strategies and fishery regulations such as the closure of the fishery for anchovy.
Fig. 14. Relationship between Shannon–Wiener index (H′) and sea-surface temperature (SST) off northern-central Peru, during the 1997–98 El Niño.
4. Conclusions
Observations carried out during El Nino events reveal the following changes in the stocks of the main pelagic resources (Fig. 1):
1. Environmental changes: During El Niño events, very marked increases in sea-surface temperature take place, especially between Paita and Chimbote (5°–10° S), with thermal anomalies of up to 8 °C above average during stages of maximum development.
2. Changes in distribution: anchovies, instead of being widely distributed over the whole Peruvian continental shelf, became more coastal, within 30 Km, more patchy, and asymmetrically distributed towards the southern Peruvian coast, instead of being widely distributed over the whole Peruvian coast, while sardine shoals were displaced from north-central to south-central area. Both species displayed a trend to deepen to below 20 m.
3. Changes in size structure: Anchovy size structure showed a predominance of adults, with a low incidence of juveniles. Only in the post-Niño phase, the number of young and juvenile anchovy increases notably. Sardine changed from mostly adult to mostly young fish.
4. Changes in the reproductive process: Reproductive process of anchovy are affected in its intensity and disrupted during maximum El Nino intensity, awaiting better environmental conditions to recover. Meanwhile, reproductive activity of other pelagic resources such as sardine, pacific mackerel and longnose anchovy increased, and this became evident during the beginning of El Niño period, indicating that El Niño events had caused quick renovation of these stocks.
5. Changes in biomass structure of pelagic resources: From anchovy dominance, to a prevalence of other pelagic types such as sardine (except during the 1997–98 event), jack mackerel, pacific mackerel, longnose anchovy, vinciguerria, lumptail searobin, pacific cutlassfishes and mictophydae.
6. Changes in landings: From a monospecific fishery to a multispecífic fishery, with an increased percentage of other pelagic resources and a reduced percentage of anchovy.
Finally, impacts of El Niño events on pelagic resources reflect a clear spatial tendency toward a more southerly displacement and a temporal sequence in the changes of the biological processes (reproduction, recruitment, abundance). Moreover, we can identify some response patterns of such changes, whose knowledge is very important for the improvement of pelagic fisheries management.
Acknowledgements
We appreciate the kindness and collaboration of Dr. Pierre Freon and we are grateful with people of Pelagic Resource Department of IMARPE. Special thanks to the anonymous reviewers of this paper because of their suggestions.
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Impact of El Niño events on pelagic fisheries in Peruvian waters
Miguel Ñiquen
Department of Pelagic Research, Instituto del Mar del Perú, P.O. Box 22, Callao, Peru
Received 20 February 2003; accepted 3 May 2004. Available online 28 August 2004
Abstract
Using data from stock assesment surveys on pelagic resources during El Niño events of 1972/73, 1982/83, 1997/98, we analyze biological changes on pelagic ecosystems and pelagic fisheries during different stages of development of El Niño phenomenon: emergence, full, final and post-Niño. Results indicate changes in spatial distribution of resources, their concentration and size structure. In anchovy (Engraulis ringens) a decrease in biomass was observed, which was estimated at 1.2 million tons in September 1998, the lowest throughout the 1990s. This resource showed an asymmetric distribution towards the south of Peru. Other pelagic resources increased their biomass during or after Niño events, primarily sardine (Sardinops sagax), jack mackerel (Trachurus murphyi), pacific mackerel (Scomber japonicus), and longnose anchovy (Anchoa nasus). At the end of the El Niño phenomenon we found less productivity but more diversity in the pelagic ecosystem. During the 1997/98 El Niño, the diversity index (Manual de Ecologia, 1a Edition, Editorial Trillas, Mexico, 267pp) increased from 0.87 to 1.23–1.70.
In both the emergence stage and fully developed stages of El Niño we found large numbers of sardine and longnose anchovy present simultaneously. Size structure of sardine, jack mackerel, and pacific mackerel showed an increase in juveniles. Anchovy during El Niño showed a single modal group composed of adults, but the post-Niño phase indicated an increase in juveniles with an average length of 6–7 cm. In El Niño conditions spawning among anchovy was low, but among sardine and pacific mackerel it was high. We observed, for the first time during full spawning, juvenile sardines with a total length of 18–20 cm. The anchovy spawning season during the post-Niño phase was considerably lengthened, from April to December 1998. Drastic change occurred in fisheries when monospecific fisheries, based on anchovy before El Niño, became multispecific fisheries based on sardine, jack mackerel, pacific mackerel, longnose anchovy and other species typical of tropical and warm oceans. Landings of anchovy decreased substantially, but those of longnose anchovy, jack mackerel, pacific mackerel, and sardine increased.
1. Introduction
The El Niño phenomenon generates substantial changes in oceanographic and meteorological conditions in the Pacific Ocean, with manifestations impacting the Peruvian coast (Zuta et al., 1976); this has mainly affected pelagic resources, producing alterations in their biological processes, behaviour, and gradual decrease in their population levels (Valdivia, 1976).
One of most sensitive species to these changes is anchovy, which in the 1960s reached biomass levels of 15–20 million tons, providing annual landings that exceeded 10 million tons in 1970. From 1972, abundances diminished drastically due to a combination of high exploitation and anomalies caused by powerful 1972–73 El Niño (Tsukayama, 1983). This situation worsened in early 1980s, with the occurrence of another exceptionally strong El Niño phenomenon in 1982–1983 (Csirke et al., 1994).
This paper describes the effects on pelagic resources of strong or exceptionally strong El Niño events occurring during the periods 1972–73, 1982–83 and 1997–98, predominantly translated into biological alterations and vertical and latitudinal changes in distribution and concentration. These effects are of particular interest because they are related to biological processes such as their reproductive cycle and recruitment, as well as the ecosystem approach to fishery management.
2. Methods
The study area is the Peruvian coast between 03° 30′–18° 20′S and 72°–84°W, which is dominated by the Humboldt–Peru eastern boundary current system, characterized by a complex system of surface and sub- surface currents associated with variations of coastal upwelling, and increases in southern trade winds (Zuta and Guillen, 1970). The main pelagic species are anchovy (Engraulis ringens), sardine (Sardinops sagax), jack mackerel (Trachurus murphyi), and pacific mackerel (Scomber japonicus) (Tsukayama, 1983). For species identification Chirichigno and Velez (1998) was used.
Stock assessments of the main pelagic resources were made by hydroacoustic surveys of pelagic stocks, using a SIMRAD EK 500 and EK 400 echosounder operating at 120 and 38 Khz to a depth of 250 m.
Sea-surface temperature and salinity were taken from land oceanographic stations every 10 min during the operations at sea and from coastal stations at Chicama and Chimbote, provided by the Oceanographic Forecasts and Marine Hydrophysical Department of IMARPE.
Biometrics and biological samplings of pelagic species were made, and species composition of catch, biological aspects, size structure, individual weight, sex and sexual maturity were recorded. The diversity was estimated using the Shannon–Wiener Index (Franco, 1985) on the commercial catch data.
3. Results and discussion
The El Niño events reveal the following changes, summarized in Fig. 1 and described below, in the environmental conditions and stocks of the main pelagic resources between the pre-El Niño period and the end of the event.
fig1:Effects of El Niño events on pelagic resources
3.1. Environmental conditions
One characteristic of the El Niño events was a very marked temperature change between Paita and Chimbote (5°–10°S) caused by the intrusion of warm equatorial surface waters, producing extreme thermal anomalies of up to 8 °C above the average in the phase of maximum development. However, between Callao and Ilo (12°–17°S) variations were smaller, and anomalies reached+6 to+2 °C (Zuta et al., 1984).
Each El Niño had different features (Bjernes, 1966). The data, however, shows that in 1971, 1982 and 1996, there was a period of cold weather prior to the phenomenon, which lasted until the early months of the following year (Fig. 2).
fig 2: Sea-surface temperature anomalies observed at Chicama (07°42′S–79°26′W) before, during and after El Niño events.
The period of higher temperature observed in 1972 was quite protracted (Zuta et al., 1976), while in 1982–83 it was short and much more intense. By contrast, the temperature rise of 1997–98 was both long in duration and strong in its intensity. In all three cases, two development peaks were observed, with the second having a higher intensity (Fig. 2).
3.2. Changes in Latitudinal and Vertical Distribution
Historical data analysis indicates displacement, from north to south and west to east, of several marine species (Chirinos de vildoso, 1976; Ñiquen et al., 1999; Kameya, et al., 2001). Some of them, originating from Panamanian waters, were observed in the north-central region of the Peruvian coast during the 1997/98 El Niño event (Ñiquen et al., 1999; Bouchon et al., 2001). This displacement can be viewed as the shift of a system with its integral species, as the thermal anomaly intensified (Fig. 3). This is how fish, such as tunas, billfishes, pacific mackerel, and jack mackerel, appeared in Peruvian waters, followed by longnose anchovy (Anchoa nasus), strand machete (Opisthonema libertate), skipjack (Katsuwonus pelamis), and finally mictophidae (Diogenichthys laternatus). The fish beard of corn (Bregmaceros bathymaster) has been detected (a species of restricted distribution) usually found off Panama Gulf (Chirichigno and Velez, 1998), together with ayamarca (Cetengraulis mysticetus), mackerel scad (Decapterus afuerae), pacific cutlassfish (Trichiurus nitens), and crabs (Euphilax sp). Meanwhile, resident Peruvian species like anchovy became patchier and asymmetrically distributed towards the southern Peruvian coast, instead of being widely distributed over the whole Peruvian coast as was usually the case. Sardine shoals also were displaced from the north-central to the south-central area.
Fig. 3. Species migration by the of El Niño 1997–98 event along the Peruvian coast
Incidence of warm conditions in the north-central region in the primary stages of El Nino events determines anchovy movement, first towards the coast, and mostly within 30 Km of inshore waters. These movements result in high concentrations, making this species very vulnerable to fishing. Almost immediately, the highest concentrations begin moving south of Chimbote (09°S) and deeper schools of anchovy also have been detected in the north-central region, located between depths of 20 and 80 m, when the El Niño was fully developed. This behaviour resulted in a change in fishing strategy, with the largest catches made at night when fish moved closer to the surface.
With regard to sardine, the largest concentrations began to move, from a normal distribution off Paita (05°S), towards Pimentel and Chicama (07°–08°S), simultaneously approaching the coast. In the most notable case they reached Ilo (17°S) in the south, with higher concentrations off Chimbote (09°S), close to the coast, as was the case during 1997–98 El Niño event (Fig. 4). A similar pattern was found during the El Niño event of 1972–73 (Zuzunaga, 1985)
Fig. 4. Changes in distribution and size structure of anchovy and sardine during April 1997–June 1998.
These interannual migrations depend on the magnitude of environmental changes. For example, in summer 1998 (the highest peak of 1997–1998 El Niño), most of the anchovy biomass was detected south of 13°S (Fig. 4) and the bulk of the catches occurred between 16° and 18°S. while in summer 2000 (cold conditions), the catches were spread out over Peruvian coast, with the highest value off northern Peru (Fig. 5).
Fig. 5. Latitudinal changes of anchovy catch relative to sea-surface temperature (SST).
A complementary aspect of distribution and availability of anchovy is shown by the observation of sea birds, which left islands in the north of Peru in April 1997, migrating mainly to the central and southern areas (IMARPE, 1997).
3.3. Changes in Size Structure
During the 1972–73, 1982–83 and 1997–98 El Niño events, adult anchovy prevailed and young fish were generally few or absent (Fig. 4 and Fig. 6), which impact recovery and population growth. However, at the beginning of the post-Niño phase, this behaviour varied notably with a strong increase in young and juvenile fish (IMARPE, 1998), coming from spawnings starting at the end of the El Niño event and disappearance of adults, which were probably concentrated near the coast and therefore undetectable by echosounders and not catchable by purse seiners
Fig. 6. Anchovy Size structure during El Niño events.
Sardine changed from predominantly adults to mostly young fish during the 1997–98 El Niño event (Fig. 4). In general, an increase in the abundance of juvenile sardine has been observed during El Niño events (Fig. 7), with mainly individuals between 1 and 2 y of age, which were detected throughout the whole El Niño period. The presence of warm conditions has favoured this stock increase.
Fig. 7. Size structure Sardine during “El Niño” events.
It is important to highlight the fact that large numbers of young fish were observed among sardine, longnose anchovy, pacific mackerel, and also hake (Merluccius gayi), lumptail searobin (Prionotus stephanophrys) and Vinciguerria spp, all of which were favoured by the strength of the 1997/98 El Niño. This has been detected in different phases of El Niño, especially at the middle of the event in September 1997 (Fig. 8). This situation indicates a recovery in the stocks of the main pelagic species, because at the end of El Niño event, a new species composition is observed, as a result of the entrance of other pelagic species, different to anchovy, which mainly have a juvenile size structure, and whose development depends on the environmental conditions. If the conditions are warm, the change will be long lasting, and if the environment is cold, other pelagics will tend to disappear, originating a rapid return of anchovy.
Fig. 8. Proportion of young marine species before, during and after the El Niño 1997–98 event.
In case of longnose anchovy, fish beard of corn (Bregmaceros bathymaster) and sea-catfish (Galeichthys peruvianus), where adults prevail the stocks grew during the whole 1997–98 El Niño event, in which these adults had almost completed a full life cycle during 18 months of oceanographic anomalies.
3.4. Changes in the Reproductive Process
During the 1997–98 El Niño, the anchovy reproductive cycle was disrupted and spawning diminished in intensity (Perea et al., 1998), while the opposite happened to the pacific mackerel and sardine: spawning increased in intensity at the beginning of El Niño, and during El Niño they approximated the historical average (Fig. 9). Furthermore, spawning juvenile sardine and pacific mackerel were detected during the final period of El Niño. Sexually mature sardines are predominantly 26–27 cm in length, but at this time significant numbers of sexually mature sardines with lengths of 18–20 cm were observed, which suggests that El Niño conditions favoured the reproduction of this species.
Fig. 9. Gonadosomatic index (IG) of pelagic resources during El Niño 1997–98.
This is probably related to better environmental conditions for sardine, allowing these juveniles to develop their full reproductive potential. It is worth mentioning that the current range of sardine sizes corresponds to that found in sardine off California, where it is known that 50% is sexually mature at a length of 19 cm. Macewicz (1996) and Parrish et al. (1989) mention that California sardine and Peru–Chile sardine probably have a common origin and constitute only one species. This evidence of early spawning of sardine would indicate that sardines on the Peruvian coast during El Niño years behave in a similar way to the Californian sardines, during normal years. It is unknown if this evidence has temporary or permanent character in other years. It should be kept in mind that this was the first opportunity to sample juvenile sizes in situ in the appropriate pre-spawing or spawning season, which allowed for the correction of a lack of samples in these sizes at spawning time. Another interpretation of the observed size distribution is a reduction in sardine growth rate, influenced by poor feeding conditions, which would be reflected by the presence of small, but older than expected sardines.
The intensity and reproductive activity of other pelagic resources such as pacific mackerel and longnose anchovy were increased (Fig. 9).
3.5. Changes in Biomass of Pelagic Resources
During an El Niño event, a remarkable decrease in anchovy biomass and increase in the biomass of other pelagic species has been observed (Fig. 10 and Fig. 11). However, the magnitude of impact depends on the intensity of El Niño events and the species composition during the period before the development of the phenomenon. In 1971–73, at the beginning of the El Niño, anchovy dominance was almost total, decreasing drastically throughout the course of the event. At the end of the event increases in stock as of sardine, jack mackerel and mackerel were noticeable. With regard to the 1982–83 El Niño, other pelagic resources dominated the period before the event, and represented 95% of total biomass by the end of it. The 1997–98 El Niño was an intermediate case between the two previous ones, where anchovy biomass was relatively important before the event and decreased dramatically during its development (Ñiquen and Gutierrez, 1998), while the biomass of other pelagic species, which was substantial before the event, increased or tended to remain stable throughtout.
Fig. 10. Pelagic biomass before, during and after El Niño events.
Fig. 11. Anchovy biomass and other pelagic during El Niño 1997–98.
In three analyzed cases, a decrease in fish productivity is general, mainly due to a decrease in the biomass of anchovy. The 1982–83 El Niño event could be seen as an exception in that the total biomass increased during the last year, but this increase, in fact, was due to the increase in jack mackerel (as a migratory resource) abundance (53% of the total).
During the last El Niño event (1997–98) a decrease in anchovy biomass was observed during the mature phase of the phenomenon, which only started to show weak signs of recovery at the end of 1998, while the biomass of other pelagic species increased during the first half of the El Niño event and decreased during the second half of it and during the post-Niño phase (Fig. 11).
3.6. Changes in Pelagic Fisheries
As a consequence of stock variations in the main pelagic resources, of three El Niño events analysed, the Peruvian anchovy fishery was seriously affected and was forced to diversify its catches, towards other pelagic resources (Fig. 12) such as sardine, jack mackerel, pacific mackerel and longnose anchovy. It also took advantage of the availability of unusual species such as round sardine, ayamarca, strand machete, golden fish and others.
Fig. 12. Anchov and other pelagic landings during El Niño 1997–98.
From 1990 until the beginning of the 1997–98 El Niño events, we observed an increase in anchovy catches due to good spawning and a trend towards cold conditions. This was observed in purse-seine landings, where anchovy was the principal component during the summer of 1994 and the fall of 1997, making up 90% of total catch on the north and central Peruvian coast (03°30′–16°S). In the winter of 1997, a decrease in anchovy catch from purse-seine fleets was observed, while the catches in other pelagic species remained relatively stable before large fluctuations increasing during the post-Niño period (Fig. 11). As a result, the proportion of the different species in the landing varied considerably over this period, with an abrupt change in the first quarter of 1998, when the anchovy fishery was closed (Fig. 13).
Fig. 13. Changes in the seasonal species composition in purse-seiner landings off north-central Peru, 1994–1998.
The relationship between SST of Chimbote (09°S) and Shannon–Wiener Index during January 1997 and September 1998 (Fig. 14), showed a direct relationship until April 1998. Then, while SST decreased, Shanon–Wiener index remained high. In this case other factors have a bearing on the diversity index, elevated salinity being a likely principal factor (Moron et al., 1998). Nonetheless, one must keep in mind that this diversity index was computed from commercial fishery data and therefore is likely to be underestimated and distorted by fishery strategies and fishery regulations such as the closure of the fishery for anchovy.
Fig. 14. Relationship between Shannon–Wiener index (H′) and sea-surface temperature (SST) off northern-central Peru, during the 1997–98 El Niño.
4. Conclusions
Observations carried out during El Nino events reveal the following changes in the stocks of the main pelagic resources (Fig. 1):
1. Environmental changes: During El Niño events, very marked increases in sea-surface temperature take place, especially between Paita and Chimbote (5°–10° S), with thermal anomalies of up to 8 °C above average during stages of maximum development.
2. Changes in distribution: anchovies, instead of being widely distributed over the whole Peruvian continental shelf, became more coastal, within 30 Km, more patchy, and asymmetrically distributed towards the southern Peruvian coast, instead of being widely distributed over the whole Peruvian coast, while sardine shoals were displaced from north-central to south-central area. Both species displayed a trend to deepen to below 20 m.
3. Changes in size structure: Anchovy size structure showed a predominance of adults, with a low incidence of juveniles. Only in the post-Niño phase, the number of young and juvenile anchovy increases notably. Sardine changed from mostly adult to mostly young fish.
4. Changes in the reproductive process: Reproductive process of anchovy are affected in its intensity and disrupted during maximum El Nino intensity, awaiting better environmental conditions to recover. Meanwhile, reproductive activity of other pelagic resources such as sardine, pacific mackerel and longnose anchovy increased, and this became evident during the beginning of El Niño period, indicating that El Niño events had caused quick renovation of these stocks.
5. Changes in biomass structure of pelagic resources: From anchovy dominance, to a prevalence of other pelagic types such as sardine (except during the 1997–98 event), jack mackerel, pacific mackerel, longnose anchovy, vinciguerria, lumptail searobin, pacific cutlassfishes and mictophydae.
6. Changes in landings: From a monospecific fishery to a multispecífic fishery, with an increased percentage of other pelagic resources and a reduced percentage of anchovy.
Finally, impacts of El Niño events on pelagic resources reflect a clear spatial tendency toward a more southerly displacement and a temporal sequence in the changes of the biological processes (reproduction, recruitment, abundance). Moreover, we can identify some response patterns of such changes, whose knowledge is very important for the improvement of pelagic fisheries management.
Acknowledgements
We appreciate the kindness and collaboration of Dr. Pierre Freon and we are grateful with people of Pelagic Resource Department of IMARPE. Special thanks to the anonymous reviewers of this paper because of their suggestions.
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