The impact of trophic cascades on the interspecific relationship between two omnivorous stream fish, ayu (Plecoglossus altivelis altivelis) and Japanese dace (Tribolodon hakonensis), were analyzed in experimental pools. Benthic algae grew on the pool beds, and macroinvertebrates colonized the pools. Ayu predominantly grazed the algae, occasionally forming territories. Japanese dace fed on both benthic algae and invertebrates when ayu were absent, and predominantly on invertebrates when ayu were present. The presence of Japanese dace reduced the number of invertebrates and increased algal biomass (chlorophyll a) through trophic cascades, but the effects of respiration and excretion of dace did not notably increase algal biomass probably because the water continuously pumped into the pools was already nutrient rich. The growth rate of ayu was enhanced in the presence of Japanese dace due to increased algal biomass. In contrast, as the number of ayu increased, the growth rate of Japanese dace was reduced because of the exploitation of benthic algae and occasional attacks by ayu on dace. The relationship between ayu and Japanese dace is not merely competitive, but partly indirect contramensalism through trophic cascades. The occasional attacks by ayu on Japanese dace were mainly directed at individuals moving near the pool beds and therefore might function to increase benthic algal resources by limiting grazing by dace.
ABSTRAK Ikan kakap merah (L. gibbus) adalah jenis ikan demersal dari famili Lutjanidae yang bernilai ekonomis penting dan banyak tertangkap di Indonesia. Informasi tentang kebiasaan makan dan aspek reproduksi ikan kakap merah di Indonesia masih relatif sedikit. Selain itu, telah terjadi penurunan stok ikan kakap merah di Selatan Banten selama 6 tahun terakhir (2008-2013). Penelitian ini bertujuan untuk mengkaji hubungan panjang-berat, kebiasaan makan dan reproduksi ikan kakap merah.. Penelitian dilakukan selama 3 tahun (2013, 2015 dan 2016). Ikan contoh diambil dari hasil penangkapan ikan oleh para nelayan dengan alat tangkap pancing rawai dasar dan pancing ulur dengan mata pancing no 7-10 yang didaratkan di Binuangeun-Banten. Analisis fekunditas dilakukan di Laboratorium dengan metode gravimetrik. Hasil penelitian menunjukkan bahwa ikan kakap merah jantan memiliki ukuran lebih panjang dibandingkan ikan betina, pola pertumbuhannya bersifat isometrik. Kebiasaan makan ikan kakap merah tergolong ikan karnivora dimana makanan utamanya adalah ikan dan kepiting (Portunidae). Nisbah kelamin jantan dan betina tidak seimbang yaitu 1: 1.53. Fekunditas berkisar 14.050-596.243 butir dengan rata-rata 170 869 butir, diameter telur berkisar 0,03-1,02 mm dan pola pemijahannya bersifat salin sebagian (partial spawner). Kata Kunci: Pertumbuhan; kebiasaan makan; reproduksi; kakap merah; Banten ABSTRACT The humpback red snapper (Lutjanus gibbus) is the family of lutjanidae which has important economic value in Indonesian capture fisheries. In addition, there has been a sharp decline on the population of humpback red snapper in the Southern part of Banten during the last 6 years (2008-2013). This study aims to examine the growth function, food habits and reproductive biology of L. gibbus in the Southern part of Banten Waters. Fish samples were collected for 3 years (2013, 2015 and 2016) both from fishing ground and landing places in Binuangeun-Banten, caught by handline and bottom longline. The fecundity analysis was performed in Laboratory by gravimetric method. The results showed that the average size of males of humpback red snapper was longer than females with the growth pattern was isometric. The food habits of humpback red snapper was classified as carnivorous fish in which the main food item consist of fish and crab (Portunidae). Sex ratio of males and females were unbalance by 1: 1.53. The fecundity ranges from 14.050-596.243 eggs with an average of 170.869 eggs. The humpback red snapper found as partial spawner which eggs diameter ranged from 0,03 to 1,02 mm.
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