Waduk Ria Rio merupakan perairan tawar di Jakarta Timur yang dimanfaatkan masyarakat setempat untuk menangkap ikan. Jenis ikan yang paling sering ditangkap adalah ikan nila (Oreochromis niloticus). Penelitian ini bertujuan untuk mengetahui rasio kelamin, tingkat dan indeks kematangan gonad, fekunditas, dan diameter telur ikan nila di Waduk Ria Rio. Pengambilan sampel dilakukan pada bulan Februari 2023. Metode purposive sampling digunakan di sembilan stasiun untuk pengambilan sampel air, sedangkan pengambilan sampel ikan berdasarkan lokasi ikan sering ditemukan menggunakan jala tebar. Pengukuran panjang dan penimbangan berat ikan dilakukan secara langsung di lapangan. Gonad diawetkan dalam alkohol 70% dan diamati di Laboratorium Ekologi Departemen Biologi FMIPA UI. Hasil penelitian menunjukkan bahwa rasio kelamin jantan dan betina ikan nila tidak seimbang dengan perbandingan 1 : 2,04 yang didominasi oleh betina. Ikan nila betina yang diperoleh sebanyak 49 ekor, sedangkan ikan jantan sebanyak 24 ekor. Tingkat kematangan gonad bervariasi yang menunjukkan ikan nila dapat memijah sepanjang tahun. Ikan nila yang didapatkan sedang memasuki masa pemijahan dilihat dari banyaknya jumlah ikan yang telah matang gonad (TKG III dan IV). Nilai indeks kematangan gonad kurang dari 20% dengan kisaran 0,01—3,78%. Fekunditas ikan nila berkisar 258—1190 butir. Ukuran diameter panjang telur berkisar 0,86—2,75 mm dan diameter lebar telur berkisar 0,64—2,39 mm. Berdasarkan sebaran diameter telur, diketahui ikan nila memiliki pola pemijahan partial spawner.

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Ria Rio Reservoir is a freshwater ecosystem in East Jakarta that is used by local people to catch fish. The most commonly caught fish is nile tilapia (Oreochromis niloticus). The aims of this research were to determine sex ratio, gonad maturity stage, gonadal-somatic index, fecundity, and egg diameter of nile tilapia in Ria Rio Reservoir. Sampling was carried out in February 2023. Purposive sampling method was used in nine stations for water sampling, while fish sampling based on fish location is often found using cast net. Measurement of the length and weighing of fish is carried out in the field. The gonads were preserved in 70% alcohol and observed in the Ecology Laboratory of Departement of Biology FMIPA UI. The results showed that sex ratio of male and female was unbalanced with a ratio of 1 : 2,04 and was dominated by females. The female caught were 49 fish, while the male were 24 fish. Gonad maturity stage was varies, indicating that nile tilapia can spawn throughout the year. The nile tilapia obtained was entering the spawning period seen from the large number of fish that have matured (stage III and IV). Gonadal-somatic index was less than 20% with a ranged of 0,01—3,78%. Fecundity of fish ranged from 258—1190 eggs. Egg length diameter ranged from 0,86—2,75 mm and egg width diameter ranged from 0,64—2,39 mm. Based on distribution of egg diameter, nile tilapia is a partial spawner.