# Composition of fish egg assemblages varies with depth on the West Florida Shelf

## TL;DR

Fish eggs collected from different depths show varied species composition, challenging the assumption that surface sampling captures all spawning activity.

## Contribution

This study experimentally demonstrates that fish egg assemblages vary with depth, challenging the assumption that surface sampling captures all spawning species.

## Key findings

- Most fish egg taxa were found in only one or two depth bins per station.
- Only 19 of 44 identified taxa were observed in the upper 20 m of the water column.
- Egg assemblage composition was heterogeneous throughout the water column.

## Abstract

Genetic barcoding of fish eggs has furthered our knowledge of fish spawning patterns and locations, providing valuable insights for conservation and management efforts. Since fish eggs tend to behave as buoyant, passive particles, most studies collect them from surface waters and assume that this method captures eggs from all the species that have recently spawned throughout the water column. To experimentally test this assumption, we used a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) to collect fish eggs from six depth bins within the upper 130 m of the water column at five stations on the West Florida Shelf. We used DNA barcoding to identify fish eggs collected within each depth bin to determine if the diversity of eggs recovered was consistent throughout the water column. Fish egg assemblage composition was heterogeneous throughout the water column, with most taxa only detected at one or two distinct depth bins per station, only a few taxa found at more than half the depth bins at any given station, and only a single taxon found at all depths within a single station. Disproving the hypothesis that all eggs present throughout the water column would be detected at the surface, only 19 of the 44 taxa identified in this study were observed in the samples collected from the upper 20 m. These findings suggest that exclusively sampling at the surface provides an incomplete picture of the fish assemblage spawning at a given station, which is difficult to predict due to variability in the rates of egg rise through the water column and further complicated by potential mismatches in the time of spawning relative to when collections are made, encounters with subsurface currents while rising to the surface, and the potential for denser eggs to reach neutral buoyancy at deeper isopycnals.