Penelitian tentang distribusi ukuran kepiting bakau (Scylla serrata) di Perairan Delta Sesayap Kabupaten Bulungan Provinsi Kalimantan Utara telah dilakukan pada bulan November sampai dengan Desember 2016, yang bertujuan untuk mengetahui penyebaran kepiting bakau. Pengambilan sampel dilakukan dengan menggunakan alat tangkap ambau, yang dilakukan dalam 6 trip pengambilan sampel dilokasi penelitian. Hasil penelitian menunjukkan bahwa keberhasilan penangkapan kepiting dengan menggunakan alat ambau pancang tidak terlepas dari cara pengoprasian alat ambau pancang. Beberapa faktor yang sangat mempengaruhi hasil tangkapan kepiting bakau (laju tangkap, efektifitas alat ambau, komposisi ukuran, nisbah kelamin). dipengaruhi oleh penyebaran kepiting yang ada diperairan, kualitas air, waktu pemasangan, tempat pemasangan dan teknik operasinal pemasangan ambau, penarikan ambau pancang, dan umpan yang digunakan. Distribusi kepiting dipengaruhi oleh periode pasang surut, kepiting yang ditangkap didominasi oleh kepiting juana dengan ukuran lebar karapas < 15 cm. Hasil analisis jumlah nisbah kelamin juga menemukan kepiting jantan 133 ekor dan kepiting betina 110 ekor hal ini menandakan bahwa di perairan tersebut nisbah kelamin yang terbanyak adalah kepiting jantan dibandingkan kepiting betina, dengan rasio adalah rasio 1.0 : 0.70.
Kata Kunci: Kepiting (Scylla serrata), Distribusi ukuran, Laju tangkap, Komposisi ukuran, Nisbah kelamin

This research was about distribution of mangrove crabs size (Scylla serrata) in Waters of Delta Sesayap, Regency of Bulungan, Province of North Kalimantan. This research was conducted in November through December, 2016, aimed to know the distribution Scylla serrata. Sample intake was done by using catching appliance of stake trap as much brought by the fisherman, which conducted in 6 phases of sampel intake in research location. Result of research indicated that the efficacy of crabs arresting by using catching appliance of Trap stake is not separated of the way of appliance of Trap stake operation. Some factors were very influencing to the catching result (catching rate, evectiveness of appliance Trap, size composition, sex ratio) that is crabs spreading in waters, water quality, setting time, setting place, operational technique of Trap, pulling of Trap, and bait used. mangrove crabs influenced by the crabs spreading in waters, water quality, setting time, setting place, and operational technique of ambau setting, pulling of ambau stake, and bait used. Distribution was of crabs influenced by tide period, the catched crabs were dominated by young crabs with the size carapace was < 15 cm. the analisis result showed that the number of sex ratio also find the masculine crabs were 133 and femele crabs were 110, these indicated that in that waters, the most sex ratio was the masculine crabs compared by famale crabs, with the ratio 1.0 : 0.70. Key words: crabs (Scylla serrata), distribution, catching rate, size composition, sex ratio