Chronic and severe hypoxic conditions in Paracas Bay, Pisco, Peru: Consequences on scallop growth, reproduction, and survival

The response of Argopecten purpuratus (Peruvian scallop) to environmental conditions was studied in the up-welling-influenced Bay of Paracas (Peru). Scallops growth and reproduction were monitored weekly over a 7-month period under two conditions (culture depths): on the bottom and 2 m above (suspended culture). At the same time, a high-frequency environmental monitoring was realized at the aforementioned depths. The results indicate that environmental conditions in the Bay are highly variable, especially during the summer: temperature fluctuations up to 8 degrees C were recorded in less than one day while oxic conditions ranged from oversaturation to anoxia. Milky-turquoise waters discoloration events (sulphide presence suspected) were simultaneously observed together with anoxic conditions and sudden temperature drops. Increased stratification and upwelling favorable winds during the summer might be partly responsible for the observed environmental variability. Higher somatic and gonadal growth were observed in scallops growing in suspended culture compare with bottom culture. This pattern might be related to different environmental conditions at culture depth: turbidity was lower in suspended culture and hypoxic conditions occurred only 18% of the whole study period versus 48% on the bottom. The Gonadosomatic index varied according to an approx. 30-day cycle between winter and late spring and it was synchronous between the different scallop size groups. During summer, when milky waters and anoxic events occurred frequently, cessation of reproduction, somatic weight losses and increased scallop mortality were observed. Exposure to severe hypoxia and related environmental conditions clearly affected scallop bioenergetics and subsequently growth and reproduction. In addition, the occurrence of milky waters harmed the survival of the Peruvian scallop in culture. These results are discussed in terms of management strategies for sustainable scallop aquaculture in the Bay.