Species co-occurrence and population dynamics in annual fish assemblages in the lower Río Uruguay basin

Abstract

Austrolebias is a genus of annual killifishes from subtropical and temperate grasslands of South America. They coexist in assemblages of up to five species in pools with variable connection to permanent water bodies and complete their entire life cycle within 1 year, terminated by summer desiccation of their habitat. To quantify community and population characteristics of Austrolebias assemblages, we sampled a set of 18 pools in the lowlands of the confluence of Río Negro and Río Uruguay in western Uruguay, with 16 sampling trips conducted over the entire seasonal cycle in 2015. Austrolebias bellottii was a ubiquitous and generalist species, A. nigripinnis was more common in pools influenced by active floodplain, A. elongatus (a large predatory species) was rare overall, and four A. alexandri was found only in two pools. Unexpectedly, many pools desiccated in winter (mid season) and some of them supported a second cohort after their re-inundation. Fish abundance and fish density declined steadily prior to pool desiccations. There were indications that adult A. bellottii can survive a short period of habitat desiccation in wet mud, leading to coexistence of an older A. bellottii cohort with a younger A. nigripinnis cohort. Males disappeared from populations at a higher rate, especially towards the end of the season. The overall lifespan of both common species was up to 7 months in pools that did not experience mid-season desiccation. This study provides fundamental demographic parameters for the annual fish in this region.