Pachyurus bonariensis fish swimming, dam in background

Before the Flood: How Damming Impacts Fish Reproduction

Reese Marlowe in Climate & Environment December 2025 • 3 min read.

"Uncover the hidden story of how dam construction affects the reproductive biology of freshwater fish, with a focus on the Pachyurus bonariensis."

Dams are transforming our rivers, and while they bring benefits like water supply, they also drastically alter the lives of the creatures that call these rivers home. Understanding these impacts is key to protecting freshwater biodiversity.

This article explores the reproductive biology of the Pachyurus bonariensis, a fish species native to South American rivers, before the construction of a dam. By examining this case study, we can gain valuable insights into how such infrastructure projects affect fish populations.

This research, conducted on the Taquarembó stream in southern Brazil, provides a baseline understanding of P. bonariensis reproduction against which future changes can be measured. Let's dive into the pre-dam world of this fascinating fish.

Unveiling the Reproductive Secrets of Pachyurus bonariensis

Pachyurus bonariensis fish swimming, dam in background

Researchers meticulously studied 278 specimens of P. bonariensis over a year to understand their reproductive patterns. This involved analyzing the fish's gonads, tracking environmental factors, and estimating fecundity.

The study revealed a distinct reproductive period for P. bonariensis, stretching from October to February. This period strongly correlated with increasing daylight hours (photoperiod) and water conductivity, suggesting these environmental cues trigger spawning.

Spawning Type: The fish are multiple spawners, releasing eggs in batches.
Fecundity: On average, a female produces around 39,313 eggs.
Maturity: Females mature at approximately 112.6 mm in length, while males mature slightly earlier at 110.7 mm.
Sex Ratio: The overall population exhibits a roughly equal sex ratio.

These findings paint a picture of a fish highly attuned to its environment, with its reproductive cycle closely linked to seasonal changes in light and water conditions.

Dams and Their Ripple Effects on Aquatic Life

The construction of dams drastically alters river ecosystems, impacting water flow, temperature, and habitat availability. These changes can disrupt the delicate balance of fish reproductive cycles, potentially leading to population declines.

This study underscores the importance of conducting thorough environmental impact assessments before undertaking dam construction. Understanding the reproductive needs of fish species like P. bonariensis is crucial for mitigating negative consequences.

By preserving pre-dam baseline data, we are empowered to develop effective conservation strategies, minimizing the environmental footprint of necessary infrastructure projects. Further investigation will be needed to see long term effects.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1590/1678-476620141042216222, Alternate LINK

Title: Biologia Reprodutiva De Pachyurus Bonariensis (Perciformes, Sciaenidae) Na Fase Pré-Represamento Do Arroio Taquarembó, Sul Do Brasil

Subject: Animal Science and Zoology

Journal: Iheringia. Série Zoologia

Publisher: FapUNIFESP (SciELO)

Authors: Giovana I. Lagemann, Clarice B. Fialho

Published: 2014-06-01

Everything You Need To Know

What is known about the reproductive cycle of the Pachyurus bonariensis?

The Pachyurus bonariensis fish, a species native to South American rivers, exhibits a distinct reproductive period from October to February. This period is correlated with increasing daylight hours (photoperiod) and water conductivity. The fish are multiple spawners, releasing eggs in batches, with an average fecundity of around 39,313 eggs. Females mature at approximately 112.6 mm in length, while males mature slightly earlier at 110.7 mm. The population shows a roughly equal sex ratio. This species is highly attuned to its environment, with its reproductive cycle closely linked to seasonal changes in light and water conditions.

What does it mean that the Pachyurus bonariensis are multiple spawners?

Multiple spawning means that the Pachyurus bonariensis does not release all of its eggs at once. Instead, the female fish releases eggs in batches over the reproductive period. This strategy allows the fish to spread their reproductive efforts, potentially increasing the chances of some offspring surviving adverse conditions or predation. This is a critical factor for the population's resilience. By releasing eggs in batches, the fish can extend their spawning period, increasing the likelihood that some of their offspring will survive.

How does water conductivity influence the reproduction of the Pachyurus bonariensis?

Water conductivity, one of the environmental factors, is the ability of water to conduct electricity, which is influenced by the amount of dissolved minerals and salts in the water. In the case of the Pachyurus bonariensis, water conductivity, along with daylight hours (photoperiod), plays a crucial role in triggering the spawning period. Dams alter these water conditions. Changes in water conductivity can disrupt the natural cues that initiate spawning, potentially leading to reproductive failure or reduced reproductive success for the fish, impacting the population's ability to thrive.

How do dams affect fish reproduction?

The impact of dams on fish reproduction is significant. Dams alter river ecosystems by changing water flow, temperature, and habitat availability. These changes can disrupt the delicate balance of fish reproductive cycles. Such disruption can lead to population declines because of the direct impact on spawning behavior, egg development, and larval survival. The Taquarembó stream study of the Pachyurus bonariensis provides a baseline to measure future changes and understand the damage caused by such infrastructure projects.

Why is studying the reproductive biology of the Pachyurus bonariensis before dam construction important?

Understanding the reproductive biology of the Pachyurus bonariensis before dam construction provides a critical baseline for assessing the impact of dams on freshwater fish populations. Researchers meticulously studied the fish, tracking environmental factors, and estimating fecundity to create a picture of the fish's reproductive patterns. This pre-dam data allows scientists to compare the fish's reproductive behavior before and after the construction of dams. It highlights the importance of studying and monitoring fish species and how human activity impacts the natural world.