Sungai Mentarang Hulu di Kabupaten Malinau merupakan habitat yang masih alami, dengan berbagai vegetasi di sepanjang tepian serta minimnya aktivitas manusia. Sungai ini memiliki lebar antara 60–70 meter, aliran air yang deras, dan terdapat banyak riffle, merupakan kondisi alami sungai serta karakteristik unik dalam mendukung kehidupan berbagai spesies ikan. Namun, informasi mengenai keberadaan ikan di Sungai Mentarang Hulu masih sangat terbatas dan belum banyak dikaji secara ilmiah. Penelitian ini bertujuan untuk mengidentifikasi ikan di Sungai Mentarang Hulu serta kelimpahan relatif setiap jenisnya agar diketahui komposisi dan dominasi jenis yang ada. Sampel dikumpulkan dari hasil tangkapan nelayan dengan menggunakan dua alat tangkap jala dan gillnet. Identifikasi ikan dilakukan dengan mengacu pada buku identifikasi. Identifikasi ikan dilakukan dengan melihat bentuk morfologi seperti bentuk mulut, kepala, sirip, ekor, serta warna dan pola tubuh. Penelitian dilakukan selama 2 bulan dari Januari hingga Februari 2025. Sampel yang dikumpulkan Sebanyak 570 yang terdistribusi kedalam 1 filum, 1 kelas, 3 ordo, 4 famili, dan 13 genus. Hasil analisis menunjukkan bahwa spesies ikan Nilem (Osteochilus spp.) memiliki kelimpahan tertinggi sebesar 31,75%, diikuti oleh ikan Lalang (Parachela spp.) sebesar 20,18%, dan ikan Pelian (Neolissochilus spp.) sebesar 16,67%. Kelimpahan terendah terdapat pada jenis ikan jenis ikan Benteur (Barbodes spp.) 0,18%, ikan Garra (Garra borneensis) 0,53%, ikan Tilan (Mastacembelus spp.) 0,53%.

Kata Kunci: Ikan, Keanekaragaman, Sungai Mentarang Hulu

Mentarang Hulu River in Malinau Regency is a natural habitat with various vegetation along the river banks and it still has minimum human activity. This river has a width of 60-70 meters, fast-flowing water, and many riffles, which are natural conditions of the river as well as its unique characteristics that support the life of various fish species. However, information regarding the presence of fish in Mentarang Hulu River is still very limited and has not been extensively studied scientifically. This research aimed to identify fish in Mentarang Hulu River and the relative abundance of each species in order to determine the composition and dominance of existing types. Samples were collected from the catch of fishermen using two fishing gear types: nets and gillnets. Fish identification was performed by referring to identification books. Fish identification is carried out by observing morphological shapes such as the shape of the mouth, head, fins, tail, as well as the color and pattern of the body. The study was conducted over 2 months from January to February 2025. A total of 570 samples were collected, distributed into 1 phylum, 1 class, 3 orders, 4 families, and 13 genera. The analysis results showed that the Niem fish species (Osteochilus spp.) has the highest abundance at 31.75%, followed by Lalang fish (Parachela spp.) at 20.18%, and Pelian fish (Neolissochilus spp.) at 16.67%. The lowest abundances were found in Benteur fish (Barbodes spp.) at 0.18%, Garra fish (Garra borneensis) at 0.53%, and Tilan fish (Mastacembelus spp.) at 0.53%. Keywords: Fish, Diversity, Mentarang Hulu River