Donax obesulus

Donax obesulus is a bivalve of the family Donacidae, accepted as a species with the authority Reeve, 1854 ^[1]. It lives in exposed sandy beaches of the south-eastern Pacific and, on the Lambayeque coast, is harvested (the resolution writes Donax sp.) under permits regulated by the Regional Government ^[2]. At Sarapampa beach, south of Lima, a 2005-2006 study estimated its annual turnover rate (P/B) at 1.20 ^[3].

Two co-occurring forms in the Humboldt Current system were treated as separate species, Donax marincovichi and Donax obesulus. A study sampled them across their shared range: shell proportions differed significantly within each area, but the two forms were often inconsistent for key morphometric parameters; in addition, there was no significant mitochondrial genetic differentiation and sperm ultrastructure was indistinguishable. Its authors concluded that they are not distinct species and should be grouped under Donax obesulus ^[4].

As size references: at Sarapampa, between October 2005 and August 2006, 9943 individuals were measured, with a shell length of 5.4 to 31.0 mm, and the estimated asymptotic length was 33.7 mm ^[3]. The IMARPE report cited in the Lambayeque resolution takes individuals of more than 22 mm of valve length as the reference for catches ^[2].

According to the introduction by Aguirre and Mendo, citing other authors, the species ranges from Canoa, in Manabí (Ecuador), to La Rinconada, in Bahía Moreno at Antofagasta (Chile); in Peru those authors highlight banks in Tumbes, Piura, Lambayeque, Áncash and Lima ^[3].

Sarapampa is a wide sandy beach with a gentle slope and heavy surf; there the species' distribution is bounded by rocky areas to the north and cliffs to the south, over 5520 m. It was concentrated mostly in the centre and south of the beach, where the sandy stratum predominates, and sampling was done between 0.20 and 1.5 m depth ^[3]. The vertices of the El Negro production area, in Mórrope (Lambayeque), run from 06°20' to 06°31' south latitude ^[2].

At Sarapampa the fitted growth curve gave a curvature parameter K of 0.991 and an asymptotic length of 33.7 mm; 3 cohorts were observed, and the recruitment pattern indicated one recruitment per year, most intense in April and May ^[3].

According to the introduction by Aguirre and Mendo, members of the family live in exposed sandy beaches and feed mainly on phytoplankton and suspended organic detritus; this is a generalisation for the family, not an observation from that study ^[3].

At Sarapampa, where the species was not harvested at the time, mean biomass was 54.1 g of wet soft tissue per m² and production was 64.85 g per m² per year; total mortality was estimated at 3.28 and taken as natural mortality ^[3]. Individuals of 22 and 23 mm contributed most of the annual production, 10.27 % ^[3].

On the Lambayeque coast, the IMARPE executive report of February 2023 recommended, for the March to August 2023 season, that total catch not exceed 420 t of individuals larger than 22 mm, or that the season end once that figure was reached ^[2]. The regional resolution also cites IMARPE reports according to which several resources, among them Donax obesulus, would be fully exploited ^[2].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

The sanitary standard for live bivalve molluscs requires that production areas be delimited, evaluated, classified and kept under a health surveillance programme. In Lambayeque, SANIPES classified the El Negro area, in Mórrope, as Approved or type A for the resource Donax spp. through Directoral Resolution 009-2023-SANIPES/DFS, and the regional authority grants access to harvest them only in that area of the coast ^[2].

Aguirre and Mendo note that the banks of southern Peru have gained importance through harvesting for export, mainly to European markets, and that according to the Ministry of Production 2650 t of the species were exported nationwide in 2005 ^[3].

In June 2023 the Lambayeque Regional Government approved the extension of a non-vessel fishing permit, requested on behalf of an association of artisanal fishers of Mórrope, to harvest 500 kg per month of Donax sp. at El Negro ^[2].

In Lambayeque, the El Negro production area, in Mórrope, was in 2023 the only one where the regional authority gave access to harvest Donax spp., with an approved sanitary classification; it is a fishing area, not a beach set up for visitors ^[2].

Further south, the Sarapampa study was carried out in the district of Asia, near kilometre 109 of the Panamericana Sur, on a beach with heavy surf; the data are from 2005-2006, with 6 sampling stations along the beach ^[3].