Purple scallop

Argopecten purpuratus is a bivalve mollusc of the family Pectinidae, in the order Pectinida ^[1]. In Peru it is called «concha de abanico», and its range runs from Paita, in the north of the country, to Coquimbo, in Chile, over soft bottoms of the infralittoral zone ^[2]. It comes both from natural beds and from farms, and after the whiteleg shrimp it is the most important aquatic species in Peru ^[2]. Its landings rise sharply during strong El Niño events, and the country's main natural bed is in Independencia Bay, south of Pisco ^[3],[4].

Lamarck described it in 1819 as Pecten purpuratus; it is now placed in the genus Argopecten, and WoRMS writes its authorship in parentheses ^[1]. WoRMS also treats Pecten rudis, a name G. B. Sowerby I published in 1846, as a synonym ^[1].

It lives in the Southeast Pacific, between Paita, in Peru, and Coquimbo, in Chile, in the infralittoral zone, on sand, muddy sand or shell debris, down to about 40 m ^[2].

At Lobos de Tierra Island, an IMARPE survey carried out in October and November 2015 found the bed between 6°21.4' and 6°25' south latitude, northeast of Cabo Cruz and off the La Grama pier, at depths of 20.1 to 26.0 m ^[5]. The bottom there was mostly mud and fine sand, with polychaete tubes towards the northeast and rocky and stony ground next to the island ^[5].

The main areas where it is farmed are Sechura Bay, Lobos de Tierra Island, Samanco Bay, Independencia Bay and Paracas Bay ^[2]. The most important natural bed in the country is the one in Independencia Bay ^[4].

It breeds all year round, with the largest spawnings towards the end of summer and in autumn ^[3]. At Lobos de Tierra in 2015, most individuals were maturing (48.55 %) or immature (24.50 %), and larvae at several stages, straight-hinged veliger (D larva), umbonate veliger and pediveliger, were present over the whole sampled area, with a peak of 5161 larvae per cubic metre ^[5].

Its size is given as shell height: in that survey the mean was 37.65 mm, and most individuals measured between 25 and 64 mm, below commercial size ^[5]. With the growth parameters used for Independencia Bay, a theoretical maximum height of 118 mm and a K constant of 0.5 per year, an individual reaches commercial size in about 12 months ^[3].

The studies reviewed by the authors attribute its population increase in warm years to several causes acting together: faster growth, a shorter larval phase with lower mortality, more deaths among its predators, greater gamete output and fewer competitors that also feed by filtering the water ^[3].

During the 1983-84 El Niño, the water over the Pisco beds went from about 16 °C, normal for summer, to almost 25 °C, and the species reoccupied the beds and multiplied ^[3]. Between 2015 and 2017 Independencia Bay had positive temperature anomalies, peaking at +3.0 °C in January 2016, and that warm water again favoured the arrival of spat on the natural beds ^[3].

On the Lobos de Tierra bed it shared the bottom with 22 species of molluscs and 13 of crustaceans, plus polychaetes, echinoderms and other groups; among the most abundant companions were the sea cucumber Neothyone gibber, the polychaete Diopatra rhizoicola and the snail Crossata ventricosa ^[5].

In farmed individuals from Sechura and Samanco bays, collected in 2015, researchers found turbellarians, copepods, cestode larvae, gregarines of the genus Nematopsis and metacercariae of digenean trematodes ^[2]. In histological sections the parasites appeared only in the gonads: cestode larvae in the Sechura scallops and trematode sporocysts in those from Samanco ^[2].

Catches from its natural beds vary greatly from year to year: in Independencia Bay 40,287 t were landed in 1985, after the 1982-83 El Niño, but uncontrolled harvesting cut them to 6,636 t the following year; after the 1997-98 El Niño, 18,287 t were recorded in 1998, 15,815 t in 1999 and 4,175 t in 2000 ^[3].

In the warm period from 2015 to 2017, individuals under 30 mm were taken illegally to be fattened in bottom enclosures, and at the Laguna Grande landing site alone 59.67 t of that spat were landed in 2016 ^[3]. In April 2017 the mean density on the bay's natural beds was only 1.52 individuals per square metre, and the authors blame that harvesting for the low output of the period ^[3].

The minimum harvest size is 65 mm in height ^[3]. An IMARPE report on Lobos de Tierra Island notes that its abundance has fluctuated with a downward trend and that the Ministry of Production therefore issued Ministerial Resolution 293-2006, in force in 2016 ^[5].

In the Paracas National Reserve, SERNANP lists non-compliance with the closed season for the scallop and informal farms among its problems, and works with the fisheries authorities and small-scale fishers to restore its natural beds ^[4].

It is an export product: between January and November 2013 it was sold to 24 markets, led by the United States, with 58.2 million dollars, and France, with 44.4 million ^[2]. In Independencia Bay, where some 35 species of commercial invertebrates are harvested, it is the resource of greatest economic value ^[3].

In the Paracas National Reserve, aquaculture and small-scale fishing are among the main economic activities; the Laguna Grande fishing settlement grew with the exploitation of the scallop during the 1982-83 El Niño, and there are concessions for farming it in the Raspón and El Queso sectors ^[4].

In 2012 and 2014 national production fell markedly, partly because of a shortage of spat and partly because of mortalities attributed to excessive variations in temperature, salinity and oxygen ^[2].

The most important natural bed is in Independencia Bay, inside the Paracas National Reserve, which also has areas of intensive farming ^[4]. At Lobos de Tierra Island, in 2015, the bed lay at 20 to 26 m, off the La Grama pier and northeast of Cabo Cruz, with a core of more than 40 individuals per square metre ^[5]. It is farmed in Sechura and Samanco bays ^[2].