new synonymy:
Plagioscion ternetzi Boulenger, 1895
is a senior synonym of
Plagioscion macdonaghi Daneri, 1954

published in:
Casatti, L. (2005):
Revision of the South American freshwater genus Plagioscion (Teleostei, Perciformes, Sciaenidae).
Zootaxa 1080: 39–64

abstract (from publication):
The South American freshwater sciaenid genus Plagioscion is reviewed. Five of the 15 nominal species assigned to the genus are considered to be valid: Plagioscion squamosissimus, widely distributed in Atlantic drainages east of the Andes; P. auratus from the Río Orinoco and Río Amazonas basins; P. magdalenae from the Río Magdalena and Río Amazonas basins; P. ternetzi from the lower Río Paraná, Río Paraguay and Río Uruguay basins; and P. montei from the Río Amazonas basin.

remarks (from publication):
Daneri (1954) described P. macdonaghi from the Río de La Plata, based on a juvenile specimen (113.0 mm). The diagnostic characters emphasized by Daneri (depth of body, interorbital width, horizontal orbit diameter, and number of gill rakers) all fall within the range of those values for P. ternetzi. This justifies the recognition of P. macdonaghi as a junior synonym of the latter species.

further remarks:
This synonymy has been proposed earlier by Braga and Azpelicueta (1982), but their papers were ignored by later authors as López et al. (1987, 2003), and probably unknown to Casatti (2005).

additional references:
Azpelicueta, M. & L. Braga (1982):
Estudio sobre Plagioscion ternetzi Boulenger, 1895 (Perciformes, Sciaenidae). Parte I.
Neotrópica 28 (79): 7-18

Braga, L. & M. Azpelicuieta (1982):
Estudio sobre Plagioscion ternetzi Boulenger, 1895 (Perciformes, Sciaenidae). Parte II.
Neotrópica 28 (80): 103-116