# Identifying Recruitment Sources Across Trophic Levels in a Large River Food Web

## TL;DR

This study shows how fish in the Upper Mississippi River use tributaries for recruitment, linking predator and prey populations across different areas and time.

## Contribution

The study demonstrates the coupling of predator and prey recruitment sources using trace element analysis of otoliths in a large river system.

## Key findings

- More prey (44%) than predators (17%) recruited from tributaries to the mainstem river.
- Tributaries consistently contributed recruits to both prey and predator fishes, showing spatial and temporal consistency.
- Predators used tributaries directly and indirectly through consumed prey for recruitment and persistence.

## Abstract

Assemblages are connected through the movement of physical and biological resources including recruits. Identifying recruitment sources for predators and their prey could help us understand how assemblages use connectivity across multiple trophic levels and whether predator and prey recruitment is coupled. Recruitment sources of organisms across multiple trophic levels can be quantified by trace element analysis of stomach contents. We used trace element analysis of otoliths to determine recruitment contributions from tributaries of predatory largemouth bass (
Micropterus salmoides
) and bowfin (
Amia calva
) and their consumed prey collected from Pools 4, 8, and 13 of the Upper Mississippi River. We used laser ablation inductively coupled mass spectrometry to quantify strontium:calcium of the core of each otolith and classified each fish to a natal origin (i.e., tributary or potential resident). We compared patterns of natal origin across study reaches, collection years, and species and with previously published origins of independently sampled prey fish. Predator and prey assemblages across all study reaches recruited from tributaries. More prey (44%) than predators (17%) recruited from tributaries. Of fishes originating from tributaries, individuals recruited from various rivers including the large Minnesota and Wisconsin Rivers and several small tributaries. Patterns in natal origin were similar among predators and prey families and among reaches, across sampling years, and between consumed prey and independently sampled prey. Tributaries consistently contributed recruits to both prey and predator fishes, leading to a coupling of predator and prey recruitment sources across space and time. Predators directly and indirectly used tributaries for recruitment and persistence through their own and their prey's recruitment. We further highlighted the utility of using consumed prey to simultaneously study the ecology of prey and predator assemblages, thereby reducing research sampling needs.

We quantified the recruitment sources of predators and their consumed prey across space and time in the Upper Mississippi River. Prey recruited more often than predators from tributaries to the mainstem river, indicating a direct (predators themselves) and indirect (consumed prey) use of tributaries to sustain predator populations. Use of tributaries indicated predator and prey coupling among recruitment sources and was consistent spatially and temporally, suggesting processes leading to these recruitment patterns lead to persistence of species in multiple trophic levels.