I know that it's during spawning season that fish go back upstream... what I don't know is if there's any other more "neutral" term to refer to it.

Yo no lo traduciría. La subienda es la migración de peces de agua dulce de las zonas bajas de un río, generalmente un sistema de ciénagas, hacia los arroyos tributarios del mismo río. Estos movimeintos siguen patrones complicados en el tiempo y geográficamente.

The "Subienda" migratory process is a particular migratory process with its own pattern, different to the Salmon pattern and different to that of other species/regions, and it is particular to colombia. Carlos Vergara is right, due to it's specificity one should keep the "Subienda" term, but I would prefer to acompany it with a surname "migration" or it's derivatives.

Este proceso genera que algunas especies de peces como el bocachico, nicuro, bagre rayado y blanquillo que se encuentran en las ciénagas inicien su migración tendiente al desove en las partes altas del canal principal del río, facilitando su captura.
http://www.agronet.gov.co/www/docs_agronet/2009511154849_Bol...

species from the family Anastomidae move upstream in the main channel, and are easily detected by local fishermen, especially in rapids. These movements are referred to as subienda, occurring primarily during the high-water season from June-September.
http://www.eepsea.org/en/ev-58883-201-1-DO_TOPIC.html

Riverine migrant species which ascend the main course of the river and its tributaries during low water are an exception to the more general behaviour pattern and form the basis of such fisheries as those of the “Subienda”....
http://www.fao.org/docrep/003/T0537E/T0537E07.htm

The river fisheries of Latin America have concentrated mainly on the low water “Piracema” or “Subienda” migrations.
http://www.fao.org/docrep/003/T0537E/T0537E07.htm

If there can be said to be a general pattern for reproductive migrations, it is an upstream spawning migration (piracema or subienda), followed by a downstream dispersion of eggs, larvae and spent adults into floodplain areas.
http://www.idrc.org/fr/ev-58864-201-1-DO_TOPIC.html