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Revista de Biología Tropical

versión On-line ISSN 0034-7744versión impresa ISSN 0034-7744

Rev. biol. trop vol.52  supl.1 San José sep. 2004


The red tide event in El Salvador, August 2001-January 2002


José Enrique Barraza1 , Julio Alberto Armero-Guardado2 & Zobeyda Marisol Valencia de Toledo3

1 Ministerio de Medio Ambiente y Recursos Naturales. Dirección de Patrimonio Natural, San Salvador, El Salvador. Fax: (503) 224 6926;,

2 Ministerio de Salud y Asistencia Social. Unidad de Epidemiología, San Salvador, El Salvador. Fax: (503) 222 5405;

3 Centro Nacional de Desarrollo Pesquero, Ministerio de Agricultura y Ganadería, Nueva San Salvador, El Salvador. Fax: (503) 228 0034;


Received 31-X-2002. Corrected 19-VI-2003. Accepted 11-XII-2003.



A red tide event occurred in El Salvador from August 2001 to January 2002. National health authorities usually measured toxin levels in Ostrea iridescens, however other species were analyzed during this microalgae bloom: Anadara similis, Anadara tuberculosa and Modiolus sp. El Salvador authorities consider 400 mouse units/100 g the highest value that is safe for human health. During this period toxin levels in O. iridescens and Modiolus sp. increased from values under 400 to 3 977 and 15 468 mouse units/100 g, respectively. Persistent and higher levels were recorded in oyster and mussel banks on the west part of the country. The Ministry of Health and Social Assistance treated 41 slight to moderate intoxications associated to bivalve mollusks consumption.

Key words: Anadara similis, Anadara tuberculosa, dinoflagellate, El Salvador, Modiolus sp., Ostrea iridescens, red tide, toxin.

Palabras clave: Anadara similis, Anadara tuberculosa, dinoflagelado, El Salvador, Modiolus sp., Ostrea iridescens, marea roja, toxina.


Red tides are caused by unicellular organisms (dinoflagellates) that can reach 50 000 individuals per milliliter of water during blooms. Recent episodes occurred in Central America during October-November 1989, March 1997, August 2001-January 2002. The most common of these organisms associated to Paralytic Shellfish Poisoning (PSP) include the genera: Pyrodinium, Alexandrium, Gymnodinium, and occur from tropical to temperate seas, such as Pyrodinium bahamense Plate 1906, bloom in the Pacific coast of Costa Rica in October 1989 (Mata et al. 1990). This dinoflagellate encompasses about 41% of all PSP events, whereas Gymnodinium species account 12% all over the world. (Azanza and Taylor 2001). Those organisms produce toxins that may cause severe or mortal neurological disturbances when humans eat seafood (usually bivalves), which has been exposed to these algal blooms, effects could appear 30 minutes after food intake (Armero-Guardado 2002) and also, creates ecological and economical impacts (Gocke et al. 1990). Azanza and Taylor (2001) mentioned that "harmful algal blooms during 1989 to 1999 have affected Asia (55%), Europe (16%), North America (12%), Australasia (6%), South America (8%), Central America (2%) and Africa (1%)".

This paper summarizes the main events registered and performed by national authorities (Health, Fisheries and Environment) during the red tide episode occurred in El Salvador coastal zone from August 2001 to January 2002.

From 1992 to 2000 the Bromatology laboratory of the Ministry of Health and Social Assistance (MSPAS, in Spanish) monitored dinoflagellate toxins from oysters (Ostrea iridescens) taken from three different sampling stations along the coastline. The median of analyses was 15 per year and the sample size varied from 7 to 50 specimens (Armero-Guardado 2002). Due to a significant toxin level increase in August 2001, Health Authorities decided to monitor red tide toxins in black clams (Anadara similis, Anadara tuberculosa), and rock mussels (Modiolus sp.). Samples were carried within ice boxes or bags, sometimes frozen, and 100 g of the bivalves soft tissue were required for toxin isolation after hot-acid digestion, filtration and dilutions, similar to the protocol described by Alonso-Picón (1989). Toxin dilutions were injected in white male mice (15-20 g), time and effects (including death) were observed.

Due to a red tide bloom at Las Lisas on the Pacific coast of Guatemala in August 2001, Salvadoran Health Authorities in coordination with the National Center for Promotion of Fisheries and Aquaculture, Ministry of the Environment and Natural Resources, National Police (CENDEPESCA, MARN, PNC, in Spanish, respectively) and Navy, focused on toxin levels in oyster samples from the west part of the country (Bocana El Zapote), close to the Guatemalan border (latter, the institutions mentioned before merged and created the Red Tide National Commission – CONAMAR - in December 2001). One aerial survey along the coastline (late August 2001) confirmed the elevated presence of sediments in water due to rainfall, and didn’t allow us to distinguish algal bloom patches on the ocean surface. Sediment runoff was very intense at the west part of La Libertad department (Mizata, Shutía, El Sunzal, El Tunco and La Libertad), where intertidal and subtidal rocky reefs are common. Previous red tide events have occurred in this area during the dry season (November 1989, March-April 1997), and the ocean turned maroon. During the 2001-2002 red tide event, results indicated levels of 509-612 mouse units/100 g, and represented a risk for the Salvadoran population, because they were above safe limits (400 mouse units/100g) according to MSPAS, national authorities banned the consumption and trade of marine mollusks.

Toxin levels detected in O. iridescens and Modiolus sp. are presented in Tables 1 and 2, respectively. Oysters from the western part of the country presented risk levels, however samples from the eastern part of the country didn’t reach such values, this reflects that the bloom affected the western coast only. Modiolus sp. presented the same pattern (Table 2), but toxin amounts were much higher, indicating the accumulation ability for this species, and also, a threat to public health during dinoflagellate blooms. The first species is subtidal and commercially exploited, and occurs in many reef banks along the coastline. The other species proliferates on the lower intertidal of vertical walls, and also, has been only observed on the western part of the country. Modiolus sp. is collected by locals as food, although biomass is really small, in comparison to oysters. We considered that the low amount of biomass inside mussels and also, their daily exposure during low tide, reduces the mussels ability to metabolize dinoflagellates quickly, therefore high toxin concentrations were detected.



The reason that samples from La Libertad reefs (Shutía, El Zonte, Mizata) remained high in toxin could be associated to the presence of many rivers coming from coffee crops in the mountains and this may contribute with a higher presence of chemicals and sediments than any other coastal area in El Salvador. The latter was confirmed during the aerial survey mentioned before, however further investigations are required. Also, the vertical disturbance caused by winds and currents proposed by Azanza and Taylor (2001) could be associated to this algal bloom.

Black clams (A. tuberculosa, A. similis) remained under safe levels (<400 mouse unit/100 g) during this red tide event, therefore the Red Tide National Commission focused the ban on oysters and mussels from El Zonte to Acajutla in December 2002. A. tuberculosa contained toxin levels during a similar event in Costa Rica (Mata et al. 1990). Toxin levels in oysters and mussels dropped and stabilized in January 2002, and CONAMAR lifted the ban completely in early February.

Armero-Guardado (2002) reported 41 intoxications associated to seafood poisoning, from August to November 2001 (Fig. 1). Most of them occurred in October (56%). Common symptoms appeared in a range from 1 to 24 hours, and they were: oral, arm and leg paralysis; also, intestinal disturbance. Human deaths did not occurred during this event. Most of the cases took place at La Libertad (88%), where people ingested mussels collected at Shutia, El Zonte and Mizata. In San Salvador, four persons (10%) got intoxicated when ingesting black clams at Estero de Jaltepeque, and one patient presenting similar symptoms was registered at the eastern part of the country (Jiquilisco Bay). These impacts on human health were slight when compared to former algal blooms in Costa Rica and Guatemala (Mata et al. 1990, Rodríguez et al. 1990, respectively). The two latter cases are contradictory to the results observed in this red tide episode, since A. similis and A. tuberculosa never reached risk values. Also, it is important to mention that some people along the country ingested black clams despite of the ban.

Investigation and standarization of monitoring of red tide would be required for the Central American area, so that appropriate regional decisions could be implemented to reduce health and economic impacts.



Manuel Mayén reviewed the English translation. Wilfredo Fuentes determined geographical positions.



Hubo una marea roja en El Salvador desde agosto de 2001 hasta enero de 2002. Las autoridades de salud usualmente medían niveles de toxinas en Ostrea iridescens, pero durante esta proliferación de microalgas se estudió también Anadara similis, Anadara tuberculosa y Modiolus sp. En El Salvador se consideran 400 unidades ratón/ 100 g como el límite sobre el cual el consumo de moluscos representa riesgo para la salud humana. Durante este período se detectó que los niveles de toxinas en O. iridescens y Modiolus sp. variaron desde niveles menores a 400 hasta 3 977 y 15 468 unidades ratón/100 g, respectivamente. Los niveles más altos y persistentes se registraron en los bancos de ostras y mejillones de la zona oeste del país. El Ministerio de Salud y Asistencia Social atendio 41 intoxicaciones de leves a moderadas atribuidas al consumo de moluscos bivalvos.



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Armero-Guardado, J.A. 2002. Brote de intoxicaciones paralíticas por mariscos, El Salvador, Agosto 2001- Enero 2002. Ministerio de Salud Pública y Asistencia Social, Unidad Nacional de Epidemiología. 6 p.        [ Links ]

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