DAYA RAMBAN (GRAZING) IKAN BARONANG (SIGANUS GUTTATUS) YANG DIPELIHARA DENGAN RUMPUT LAUT KAPPAPHYCUS ALVAREZII DI PERAIRAN TANJUNG TIRAM, KABUPATEN KONAWE SELATAN

Abstrak: Serangan Hama dan penyakit merupakan penyebab terganggunya pertumbuhan rumput laut sehingga menyebabkan menurunnya produksi rumput laut. Salah satu hama dalam rumput laut adalah ikan baronang (Siganus guttatus). Tujuan dari penelitian ini adalah untuk mengetahui tingkat perambanan S. guttatus terhadap rumput laut K. alvarezii dengan kepadatan berbeda. Penelitian dilakukan pada bulan Januari 2018 di areal budidaya rumput laut perairan Desa Tanjung Tiram, Kabupaten Konawe Selatan. Penelitian dilakukan dengan menggunakan metode rakit jaring apung sebanyak 3 buah dengan ukuran 440 x 100 x 50 cm. Rakit dibagi menjadi empat petak serta diberi label. Dalam masing-masing petak dibudidayakan rumput laut K. alvarezii dengan berat awal 100 g per rumpun dimana masing-masing unit percobaan dibedakan berdasarkan padat tebar 600 g/m2, 900 g/m2 dan 1200 g/m2. Pada setiap pengambilan data grazing (daya ramban) ikan baronang dalam wadah rakit apung ditebar 1 ekor ikan baronang dengan bobot sama sebesar 135 g. Selama 21 hari penelitian, grazing ikan baronang tertinggi berada pada kepadatan 1200 g/m2 sebesar -23,58% dan yang terendah pada kepadatan 600 g/m2sebesar -12,4%.Kata Kunci : Grazing, Ikan Baronang, Rumput laut, Rakit Jaring ApungAbstract: Pest and disease attacks are a cause of disruption of seaweed growth, which causes a decrease in seaweed production. One pest in seaweed is baronang fish (Siganus guttatus). The aim of this study is to determine grazing rate of S. guttatus on K. alvarezii seaweed with different densities. The study was conducted in January 2018 in the area of seaweed cultivation in the waters of Tanjung Tiram village, South Konawe Regency. The total of plots in one unit of floating cage was three plots pieces measuring 440 x 100 x 50 cm. Raft is divided into four plots and labeled. In each plot cultivated K. alvarezii seaweed with an initial weight of 100 g per clump where each experimental unit is differentiated based on stocking density of 600 g / m2, 900 g / m2 and 1200 g / m2. At each grazing data collection (drilling power) Baronang fish in floating raft containers were stocked with one Baronang fish with the same weight of 135 g. During maintenance, Baronang fish is given commercial feed as much as 5% of body weight. As for the 21 days of research, the highest Baronang fish grazing is at a density of 1200 g / m2 at -23.58% and the lowest at a density of 600 g / m2 at -12.4%.Keywords: Grazing, Baronang fish, seaweed, floating cage

Benthic grazing in a eutrophic river: cascading effects of zoobenthivorous fish mask direct effects of herbivorous fish

Benthic grazing strongly controls periphyton biomass. The question therefore arises whether benthic grazing could be used as a tool to reduce excessive growth of periphyton in nutrient-enriched rivers. Although benthic invertebrate grazers reduce the growth of periphyton, this is highly context dependent. Here we assessed whether the only obligate herbivorous fish in European rivers, the common nase (Chondrostoma nasus L.), is able to reduce periphyton biomass in a eutrophic river. We conducted three consecutive in situ experiments at low, intermediate and high densities of nase in the river using standard tiles on the river bottom naturally covered with periphyton that were accessible to fish and tiles that excluded fish foraging with electric exclosures. The biomass of benthic invertebrate grazers was very low relative to nase. We hypothesised that nase would reduce periphyton biomass on accessible tiles and therefore expected higher periphyton biomass on the exclosure tiles, at least at intermediate and high densities of nase in the river. Contrary to our expectation, the impact of fish grazing was low even at high fish density, as judged by the significantly lower chlorophyll a concentration on exclosure tiles even though the ash-free dry mass on accessible and exclosure tiles did not differ. The lower chlorophyll a concentrations on exclosure tiles might be explained by a higher biomass of invertebrate grazers on the exclosure tiles, which would indicate that the effect of invertebrate grazers was stronger than that of herbivorous fish grazers. The high biomass of invertebrate grazers on exclosure tiles likely arose from the exclusion of zoobenthivorous fish, which occur in the river at high densities. The results of our small-scale experiments suggested that cascading top-down effects of zoobenthivorous fish have a higher impact on periphyton biomass than direct effects of herbivorous nase.

Utilization of Periphytic Natural Food as Partial Replacement of Commercial Food in Organic Tilapia Culture - An Overview

ABSTRACT This article summarizes the results obtained during five years of research at the Dor Fish and Aquaculture Research Station on partial replacement of commercial food by periphytic natural food in the culture of organic tilapia (hybrid Oreochromis aureus (Steindachner) x Oreochromis niloticus (L.)). Tilapia culture experiments were conducted in earth ponds with and without substrates, utilizing different substrates. Tilapias of sizes ranging from nursery to market-size fish were tested. Fish were stocked at densities common in organic fish culture, i.e. 5 tilapia/m2 at the nursery stage, 1.2-1.4 tilapia/m2 at the grow-out stage. Substrate experiments were carried out in 1 m3 cages protected from fish grazing to test growth of periphyton on materials with different characteristics. The findings show that the inclusion of substrates in the water body at an amount equivalent to 40-50% of the pond water surface, allows the reduction of commercial food input by 30-40% without significantly hampering fish growth rate. It is recommended to use rough, rigid, white substrates, on which periphyton growth of 2 g dry matter/m2/day has been measured. A figure and a table are provided as a tool to estimate periphyton contribution to the fish food ration, enabling the adjustment of the remaining daily food portion to be supplied as fish biomass increases during the culture period. Applying this technology will save food and money in the culture of organic tilapia, and it can also be appropriate in the conventional pond culture of tilapia as a method to reduce feed costs and increase sustainability.

Feeding preferences of herbivores in a relatively pristine subtropical seagrass ecosystem

Understanding forage choice of herbivores is important for predicting the potential impacts of changes in their abundance. Such studies, however, are rare in ecosystems with intact populations of both megagrazers (sirenians, sea turtles) and fish grazers. We used feeding assays and nutrient analyses of seagrasses to determine whether forage choice of grazers in Shark Bay, Australia, are influenced by the quality of seagrasses. We found significant interspecific variation in removal rates of seagrasses across three habitats (shallow seagrass bank interior, shallow seagrass bank edge, deep), but we did not detect variation in gazing intensity among habitats. In general, grazers were more likely to consume fast-growing species with lower carbon : nitrogen (C : N) and carbon : phosphorus (C : P) ratios, than the slower-growing species that are dominant in the bay. Grazer choices were not, however, correlated with nutrient content within the tropical seagrasses. Slow-growing temperate seagrasses that experienced lower herbivory provide greater habitat value as a refuge for fishes and may facilitate fish grazing on tropical species. Further studies are needed, however, to more fully resolve the factors influencing grazer foraging preferences and the possibility that grazers mediate indirect interactions among seagrass species.

Effects of Urbanization on Stream Ecosystems

<em>Abstract.</em>—Patterns of stream benthic algal assemblages along urbanization gradients were investigated in three metropolitan areas—Boston (BOS), Massachusetts; Birmingham (BIR), Alabama; and Salt Lake City (SLC), Utah. An index of urban intensity derived from socioeconomic, infrastructure, and land-use characteristics was used as a measure of urbanization. Of the various attributes of the algal assemblages, species composition changed along gradients of urban intensity in a more consistent manner than biomass or diversity. In urban streams, the relative abundance of pollutiontolerant species was often higher than in less affected streams. Shifts in assemblage composition were associated primarily with increased levels of conductivity, nutrients, and alterations in physical habitat. Water mineralization and nutrients were the most important determinants of assemblage composition in the BOS and SLC study areas; flow regime and grazers were key factors in the BIR study area. Species composition of algal assemblages differed significantly among geographic regions, and no particular algal taxa were found to be universal indicators of urbanization. Patterns in algal biomass and diversity along urban gradients varied among study areas, depending on local environmental conditions and habitat alteration. Biomass and diversity increased with urbanization in the BOS area, apparently because of increased nutrients, light, and flow stability in urban streams, which often are regulated by dams. Biomass and diversity decreased with urbanization in the BIR study area because of intensive fish grazing and less stable flow regime. In the SLC study area, correlations between algal biomass, diversity, and urban intensity were positive but weak. Thus, algal responses to urbanization differed considerably among the three study areas. We concluded that the wide range of responses of benthic algae to urbanization implied that tools for stream bioassessment must be region specific.