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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

Study of the Mussel Aulacomya ater,
Molina, 1782 (Bivalvia: Mytilidae),
near Santa Rosa Island, Independence
Bay, Peru, during the El Niño
Phenomenon 1997- 98

Alex Gamarra, Oswaldo Cornejo

Laboratorio de Ecología. Facultad de Ciencias
Biológicas. Universidad Nacional Mayor de San
Marcos, A.A.:1898, Lima 100,


The aim of the study is to evaluate some of the aspects of the population dynamics of Aulacomya ater in relation to the El Niño phenomenon in 1997-98. The assumption is that no biological responses indicating changes in the normal life of this species will be present.

The following objectives were proposed for this purpose:

• Investigate the response of the mussel - Aulacomya ater - population located near Santa Rosa Island during the El Niño event 1997-98.

• Investigate the oceanographic parameters in the area under study and observe the changes produced in them during a warm event.

Results & Discussion

Oceanographic Conditions

The results show that in 1997, the temperature of the ocean floor, between May and September, was above the standard average (15.2 to 19.4 °C), reaching its maximum value in January 1998 (22.8 °C). A decrease in temperature was observed afterwards, from March to June. From July to December recorded temperatures fell within the standard average (14.7 to 13.5 °C). The dissolved oxygen concentration at the ocean floor showed values between 1.80 and 4.35 ml/l during the period under study. Values above 4 ml/l matched high temperature values.

Aulacomya ater Population

21 samples, taken almost monthly in 1997 and 1998, were analyzed. A total of 8127 specimens was collected. The length frequency shows a bimodal structure, in 1997, during the months of January to August (0-10 and 60-70 mm length = LT). Between August and December, only one mode was found, varying from 70-80 mm LT. In January-April 1998, we once more observed the bimodal structure (0-10 mm and 70-80 mm LT). The following months were characterized by a unimodal population (70-80 mm LT).

The average monthly density ranged from 608 to 704 spec/m2 in the first months of 1997, increasing to a maximum of 4429 spec/m2 in June 1997, decreasing in the months that followed. In 1998, density peaked during January at 1379 spec/m2, which decreased afterwards and maintained an average of 800 specimens until the end of the period of study.

In terms of sexual maturity, mature, spawning and spawned specimens appeared during nearly the entire period of study. The largest proportion of spawning and spawned specimens appeared during Spring and Summer 1997 and 1998, which is the usual response of mussels. The same occurred with the other gamete maturation stages.

In terms of the weight variation of specimens, the most evident factor was the theoretical dry visceral weight of 70 mm specimens, in which a scaled behavior of values was observed. An important peak was found in Spring 1997 (2.19 g), with the lowest value (1.24 g) in Summer 1998. This value recovered gradually, with another spawning in May, a recovery period in Spring 1998 (2 g). This trend in values would explain the stages of spawning and recovery in the specimens.

Thus, according to the above mentioned results, it may be observed that the response of mussels was expressed by decrease of population density, affecting 1997 and 1998 recruitment, because, although there were significant quantities of youth under 1 cm between April and August 97 and then January and April 98, these disappeared. As a consequence, unimodality was observed during the remaining months. In conclusion, the organisms were `partially' affected by this EN 97-98 event, which is an important difference considering the high mortality rates that occurred during EN 82-83 (Soennens, 1985). Thus, this response demonstrates a degree of resistance and elasticity of the species studied.

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