Evaluating the Role of Boat Electrofishing in Fish Monitoring of the Sacramento–San Joaquin Delta

Brian Mahardja,Ryan McKenzie

doi:10.15447/sfews.2021v19iss1art4

Abstract

The San Francisco Estuary is an incredibly diverse ecosystem with a mosaic of aquatic habitats inhabited by a number of economically, culturally, and ecologically important fish species. To monitor the temporal and spatial trends of this rich fish community, long-term fish monitoring programs within the estuary use a variety of gear types to capture fish species across life stages and habitats. However, concerns have been raised that current sampling gears may fail to detect certain species—or life stages—that inhabit areas that are not accessible by current gear types (e.g., riprap banks, shallow vegetated areas). Boat electrofishing is one sampling method that has been proposed to supplement current long-term fish monitoring in the upper estuary. In this study, we used fish catch data from past boat electrofishing studies, a long-term beach seine survey, and a couple of long-running trawl surveys to compare the relative probability of detecting various fishes across these sampling gears. Overall, we found that boat electrofishing led to notable improvements in the detection rates for many native and non-native fishes we examined. Boat electrofishing gear was better at detecting the majority of species in the spring (20 out of 38 species, 53%) and fall-winter (24 out of 34 species, 70%) sampling periods. Based on these findings, we recommend that resource managers consider the implementation of a long-term boat electrofishing survey to help them in their long-term conservation planning for fishes within the upper estuary.

Highlights

Aquatic ecosystems worldwide have been rapidly degraded as a result of anthropogenic effects, leading to the development of ecological monitoring programs in order to understand and manage these changes (Radinger et al 2019)
We evaluated the relative differences in the probability of species being detected among boat electrofishing, beach seine, mid-water trawl, and Kodiak trawl fishing gears deployed within the Sacramento-San Joaquin River Delta (Delta)
Using an occupancy modeling framework, we set out to answer two questions: (1) How does the probability of species detection compare across boat electrofishing and long-term net-based monitoring gears? (2) Would we see a significant improvement in species detection with the use of boat electrofishing, and if so, which species? Our goal was to highlight the strengths and weaknesses of long-term fish monitoring gears, and to provide information on how boat electrofishing may improve the monitoring of fish communities within the estuary

Summary

Introduction

Aquatic ecosystems worldwide have been rapidly degraded as a result of anthropogenic effects, leading to the development of ecological monitoring programs in order to understand and manage these changes (Radinger et al 2019). Fish monitoring programs currently use a combination of trawls and beach seines to sample pelagic (mid-channel) and littoral (nearshore) habitats for fishes across multiple life stages Sampling gears for these programs were selected based on their ability to capture select fishes of concern to management at the time programs were started (Honey et al 2004). Net-based monitoring gears are not able to sample structured habitats that dominate large portions of the estuary—e.g., riprap banks and shallow vegetated areas This gear limitation has raised concerns that some fish species and ecological interactions may not be adequately captured by fish monitoring efforts, limiting the effectiveness of resource management and conservation plans for the estuary (Hart and Hunter 2004; IEP 2013; Hestir et al 2016)

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