The loss of parasitism in metazoan lineages is often seen as unlikely, but it has occurred in some lineages (e.g., leeches, lampreys). How and why parasitism is lost is aptly addressed by studying lampreys, because extant species include a range of feeding modes and parasitism has been lost repeatedly. An individual‐based model was developed to determine if variations in survival and growth rates in the larval and juvenile stages could favor parasitic or non‐parasitic strategies. A realization of the model for a Lampetra spp. population, a genus which includes parasitic and non‐parasitic animals, indicated that both strategies could be successful. A different model realization of the non‐parasitic species Lethenteron appendix also agreed with expectations, only non‐parasitic strategies were successful. Modeling anadromous Petromyzon marinus produced only parasitic animals, as expected, but suggested two different adult sizes should appear in the population, which has not been reported in the literature. Finally, a realization of an Ichthyomyzon castaneus population, only known to produce parasites, rarely selected for parasitism (~7% of model iterations), possibly because the population used to parameterize the model was unusual for the species. The results suggest that non‐parasitism in lampreys is common because parasitism, while offering better growth, also has lower survival. Additionally, non‐parasitic species may be generated at different rates because growth and survival thresholds in the model favoring parasitism are close to observed estimates in some populations. Loss of parasitism can occur when species have different tradeoffs in growth environments between life stages.

Amazonian fishes employ diverse migratory strategies, but the details of these behaviours remain poorly studied despite numerous environmental threats and heavy commercial exploitation of many species. Otolith microchemistry offers a practical, cost-effective means of studying fish life history in such a system. This study employed a multi-method, multi-elemental approach to elucidate the migrations of five Amazonian fishes: two ‘sedentary’ species (Arapaima sp. and Plagioscion squamosissimus), one ‘floodplain migrant’ (Prochilodus nigricans) and two long-distance migratory catfishes (Brachyplatystoma rousseauxii and B. filamentosum). The Sr : Ca and Zn : Ca patterns in Arapaima were consistent with its previously observed sedentary life history, whereas Sr : Ca and Mn : Ca indicated that Plagioscion may migrate among multiple, chemically distinct environments during different life-history stages. Mn : Ca was found to be potentially useful as a marker for identifying Prochilodus's transition from its nursery habitats into black water. Sr : Ca and Ba : Ca suggested that B. rousseauxii resided in the Amazon estuary for the first 1.5–2 years of life, shown by the simultaneous increase/decrease of otolith Sr : Ca/Ba : Ca, respectively. Our results further suggested that B. filamentosum did not enter the estuary during its life history. These results introduce what should be a productive line of research desperately needed to better understand the migrations of these unique and imperilled fishes.