Pollution of river waters is most often the result of human activities in managing agricultural land, industry, and transportation. The entry of pollutants such as heavy metal mercury (Hg) into the waters can interfere with the survival of fish waters. Red Bader Fish (Barbonymus altus) can be used as a biomarker in analyzing aquatic environmental conditions in the Brantas River, Jombang, East Java. This study aimed to analyze the relationship between metallothionein and heavy metal mercury in the gill tissue of Barbonymus altus. Sampling was conducted at three stations with three replications from February to March 2022. Data analysis used regression and correlation methods to determine the relationship between metallothionein and metal mercury in gill tissue. The density and intensity of metallothionein in the gill tissue of Barbonymus altus were analyzed using the immunohistochemical method. The mercury (Hg) content in the gill tissue of Barbonymus altus was analyzed using Atomic Absorption Spectroscopy (AAS). The results showed that the metallothionein density at each station included station 1

The Brantas River is the longest in East Java. One of the areas included in the flow of the Brantas River is the Jombang Regency. The Jombang Brantas River is used by the community to meet their daily needs in agriculture, plantations, industry, and fishing grounds. The input of waste generated from several community activities will result in the survival of fish in the waters. Fish as a biomarker agent is quite significant in estimating the level of heavy metal pollution. Red Bader Fish (Barbonymus altus) is often found and lives around the Jombang Brantas River. This study aims to determine the condition of the aquatic environment by observing the health of the fish through hematological observations of the Red Bader Fish (B. altus). Sampling was carried out at three stations with three replications every two weeks, which was carried out in February – March 2022 in the Jombang Brantas River. Data analysis used the Canonical Correspondence Analysis (CCA) method as an analytical method to determine the correlation between the dependent variable (hematological profile) and the independent variable (water quality conditions). In this study, the results showed that the water quality at Station 1 and Station 2 was classified as normal, while at Station 3, it was classified as polluted. Data analysis showed that erythrocytes, hemoglobin, and hematocrit had a relationship with temperature, pH, DO, BOD, ammonia, TSS, and TDS in low concentrations. Meanwhile, leukocytes and micronuclei showed a relationship with high concentrations of ammonia, BOD, TDS, and TSS.