# Acute Effect of Short-Term Benzocaine Anesthesia on the Skin Mucus Microbiome of Atlantic salmon (Salmo salar)

## TL;DR

This study shows that using benzocaine anesthesia in Atlantic salmon disrupts the balance of bacteria on their skin, which could increase disease risk.

## Contribution

This is the first study to evaluate the impact of benzocaine anesthesia on the skin mucus microbiome of Atlantic salmon.

## Key findings

- Benzocaine significantly reduced bacterial richness and diversity in salmon skin mucus.
- Benzocaine exposure altered the bacterial community structure, potentially leading to dysbiosis.
- Gammaproteobacteria and Betaproteobacteria were the dominant bacterial classes in both treated and untreated groups.

## Abstract

Handling fish in aquaculture disrupts the natural balance of skin bacteria, which play a crucial role in disease prevention. Anesthetic baths, used to reduce stress during handling, may further disturb this balance. This study revealed that benzocaine, used as an anesthetic bath, alters the skin mucus bacterial community structure of Atlantic salmon, potentially leading to dysbiosis (an imbalance in bacterial composition). Such disruptions could impair host–microbe interactions and increase susceptibility to opportunistic pathogens. These findings highlight the importance of understanding the impacts of anesthetics on fish microbiomes to improve aquaculture practices, promote fish health, and support more sustainable farming systems.

Routine aquaculture practices such as capture, transportation, and handling can disrupt the relationship between commensal and opportunistic bacteria in the fish skin microbiome. Anesthetic baths are a common welfare practice in aquaculture to reduce stress during handling. However, to date, no studies assessed the effect of anesthetics on bacterial communities in fish skin mucus. This study is the first to evaluate the influence of benzocaine, a widely used anesthetic, on the skin mucus bacterial microbiome of Atlantic salmon reared in a recirculating aquaculture system (RAS). Using Illumina high-throughput 16S rRNA gene sequencing, we found that bacterial richness and diversity were significantly reduced in skin mucus samples from fish with anesthesia (ANE) when compared with those without anesthesia (CTR). The predominant bacterial classes in both groups were Gammaproteobacteria (54.1–62.6%) and Betaproteobacteria (22.6–22.9%). However, significant dissimilarities in beta diversity were observed between the bacterial community structure of salmon skin mucus samples from ANE and CTR. These findings demonstrate that benzocaine exposure alters skin mucus microbiome of Atlantic salmon potentially leading to dysbiosis. This study also provides baseline information on the bacterial communities of Atlantic salmon skin mucus microbiome in an RAS. As no temporal resampling was performed, the duration and persistence of these changes remain unknown and warrant further investigation.