Harnessing the potential for otolith microchemistry to foster the conservation of Amazonian fishes

Abstract

Freshwater environments host roughly half of the world’s fish diversity, much of which is concentrated in large, tropical river systems such as the Amazon. Fishes are critical to ecosystem functioning in the Amazon River basin but face increasing human threats. The basic biology of these species, and particularly migratory behaviour, remains poorly studied, in part owing to the difficulty associated with conducting tagging studies in remote tropical regions. Otolith microchemistry can circumvent logistical issues and is an increasingly important tool for studying fish life histories. However, this approach is still new in the Amazon, and its potential and limitations to inform fish conservation strategies remain unclear. Here, otolith microchemistry studies in the Amazon are reviewed, highlighting current possibilities, and several key factors that limit its use as a conservation tool in the Amazon are discussed. These include the dearth of spatiotemporal elemental data, poor understanding of environment–fish–otolith pathways, and insufficient funding, facilities, and equipment. A research initiative is proposed to harness the potential of this technique to support conservation in the Amazon. Key aspects of the proposal include recommendations for internal and external funding, which are critical to acquiring and maintaining technical staff, cutting‐edge equipment, and facilities, as well as fostering regular scientific meetings and working groups. Meetings can facilitate a systematic approach to investigating environment–otolith pathways, broadening the chemical baseline for most Amazonian tributaries, and exploring potential valuable elements. These outcomes are urgently needed to conserve biodiversity and ecosystem functioning in the Amazon, especially given threats such as widespread hydroelectric damming. The initiative proposed here could make otolith microchemistry an important, cost‐effective tool to inform and foster conservation in the Amazon, and act as a template for other imperilled tropical river basins, such as the Mekong and the Congo.

Reading the biomineralized book of life: expanding otolith biogeochemical research and applications for fisheries and ecosystem-based management

“…Sr:Ca or Ba:Ca) (e.g., Elsdon et al 2008;Gillanders et al 2015;Walther & Limburg 2012). Migrations that occur solely within freshwater require a different approach, and changes in otolith 87 Sr/ 86 Sr ratios can represent movements of individuals between isotopically distinct regions (Barnett- Johnson et al 2008;Hegg et al 2013), including within and among rivers (Brennan et al 2019a;Hermann et al 2021;Humston et al 2017;Tran et al 2021;Walther & Limburg 2012). This latter approach has recently revealed exceptionally long-distance migrations (> 8000 km) by goliath catfish (Brachyplatystoma rousseauxii) in the Amazon Basin, including natal homing (Duponchelle et al 2016).…”

Otolith microchemistry highlights the importance of extensive connectivity for conservation of an iconic migratory fish in a large tropical river basin

“…Thus, a suite of fish otolith elemental ratios may be used to infer differential patterns in habitat use and associated environmental variation (Patterson, Taylor & McBride, 2005; Ley & Rolls, 2018). For migratory fishes in large tropical rivers, otolith microchemistry can provide a means to document movement patterns where tagging studies would be logistically prohibitive (Hermann et al, 2021).…”

A synthesis of the diversity of freshwater fish migrations in the Amazon basin

“…Fortunately, novel technologies such as telemetry (Hahn, Martins, et al., 2019) and otolith (i.e. ear bones) microchemistry (reviewed by Hermann et al., 2021) have unveiled fine temporal and spatial information about the movement journeys of individual fishes for a few species. Considering the intrinsic limitations of deployed methods (e.g.…”