Environmental correlates of circannual breeding periodicity in a multi-species assemblage of Amazonian electric fishes

Abstract

We quantified patterns of circannual reproductive breeding periodicity in a multi-species assemblage of the gymnotiform knifefish genus Brachyhypopomus from Amazonian floodplain and terra firme stream habitats. We applied linear mixed-effect models and model selection to a dataset of 3205 fish from the five most abundant species to identify environmental variables predictive of gonadosomatic index (GSI) – including rainfall, river level, water temperature, conductivity, dissolved oxygen (DO), and pH. A single common floodplain species exhibited a well-demarcated ca. 6-month breeding season corresponding to the rising-water and high-water period. In this species the strongest predictors of GSI in both sexes were river level and pH. Conductivity, temperature, and DO all exhibited substantial circannual variation in the floodplain but were uncorrelated to GSI. Four common terra firme stream-dwelling species exhibited longer (ca. 7–9-month) and less distinct breeding seasons that were approximately coincident with the late dry/early rainy season. The stream species exhibited a congruent species-wide response in which GSI was predicted in both sexes by rainfall, but not by physico-chemical water properties (all of which exhibited relatively minor circannual variation). Five of the most abundant species in this study have a semelparous one-year life history, but one (B. beebei) has an iteroparous two-year life history. Consistent with a pattern of terminal reproductive investment, we observed a more pronounced elevation of GSI (especially in females) in B. beebei’s final second breeding season than in its first.