Effect of the Human Utilization of Northern Snakehead (Channa argus Cantor, 1842) on the Settlement of Exotic Fish and Cladoceran Community Structure

Empirical studies suggest that changes in the density of top predators, such as carnivorous fish, in freshwater food webs, strongly affect not only fish communities but also various primary and secondary consumers. Based on these findings, we explored how differences in the utilization of carnivorous fish (i.e., Northern Snakehead, Channa argus) by humans affected the fish and cladoceran community structure as well as the settlement of exotic fish species (i.e., Lepomis macrochirus and Micropterus salmoides) in 30 wetlands located in the upper and lower reaches of the Nakdong River. Our results show that in the mid–lower reaches of the Nakdong River, the density of C. argus was low, while high densities of L. macrochirus and M. salmoides were observed. Exotic fish species are frequently consumed by C. argus, leading to a low density of L. macrochirus and M. salmoides in the upper reaches, which supported a high density of C. argus. However, in the mid–lower reaches, the density of L. macrochirus was high because of the frequent collection of C. argus by fishing activities. The dominance of L. macrochirus significantly changed the structure of cladoceran communities. L. macrochirus mainly feeds on pelagic species, increasing the density of epiphytic species in the mid–lower reaches. The continued utilization of C. argus by humans induced a stable settlement of exotic fish species and strongly affected the community structures of primary consumers in the 30 wetlands. The frequency of C. argus collection has to be reduced to secure biodiversity in the mid–lower reaches of the Nakdong River, which will reduce the proportion of exotic fish species and increase the conservation of native fish.

Effects of Aquatic Macrophytes on Spatial Distribution and Feeding Habits of Exotic Fish Species Lepomis macrochirus and Micropterus salmoides in Shallow Reservoirs in South Korea

Aquatic macrophytes determine the physical complexity of aquatic environments and may influence the distribution and feeding habits of fish species. We explored the influence of different microhabitats, including vegetated beds (VB), edges of vegetated beds (EVB), and open water zones (OW), on two exotic fish species (Lepomis macrochirus and Micropterus salmoides) in shallow reservoirs. Lepomis macrochirus was more abundant in VB than in other zones and M. salmoides was mainly distributed in EVB. In VB, L. macrochirus mainly consumed branchiopods and isopods, while M. salmoides in EVB relied on relatively larger food items, such as dipterans, odonatans, and young fish. The consumption of young fish by M. salmoides, including L. macrochirus, showed little difference between winter and summer. Based on these findings, we suggest that young L. macrochirus (~20 cm) utilize VB as a refuge to avoid predation by M. salmoides. Meanwhile, M. salmoides mainly occupied in areas surrounding VB, preying on animals at these edges. As such, the presence of aquatic macrophytes appears to plays a key role in the survival and population growth of L. macrochirus. Proper management of aquatic macrophytes can help reduce populations of exotic fish and support native fish species.

A demographic study of the water-rat (Hydromys chrysogaster) on the River Torrens in Adelaide, South Australia

The population of water-rats (Hydromys chrysogaster) on the River Torrens in Adelaide was monitored over a three-year period. Initially, the population density was high but it became greatly reduced after the water level was lowered and did not recover again until over a year after the high water level had been restored. Males tended to have a larger home range than did females although they did not appear to exclude other individuals from their territory. Breeding generally took place in spring and summer although a few young appeared to be produced in most months of the year. This high population of water rats may, to some extent, be due to the abundance of the exotic fish species that occur in the River Torrens.

Comparative performances of exotic and indigenous fish species with commercial culture systems of Mymensingh district in Bangladesh

An experiment was conducted to assess the relative performances of indigenous and exotic fish species under commercial culture system at Muktagacha and Trishal upazila in Mymensingh district. Six exotic fish species namely Thai koi (Anabas testudineus), Thai pangus (Pangusius hypophthalmus), Silver carp (Hypophthalmichthys molitrix), Vietnam koi (Anabas testudineus), Big head carp (Aristichthys nobilis) and Tilapia (Oreochromis niloticus) and nine indigenous fish species namely Rui (Labeo rohita), Shing (Heteropneustes fossilis), Sar punti (Puntius sarana), Gulsha (Mystus bleekeri), Koi (Anabas testudineus), Mrigal (Cirrhinus cirrhosus), Kalibaush (Labeo calbasu), Catla (Catla catla) and Magur (Clarias batrachus) were selected in Muktagacha while exotic Big head carp and indigenous Gulsha, Koi and Magur were lacking for observation in Trishal upazila. Results revealed that the production of exotic Thai pangus was comparatively higher in Muktagacha (5810.81 kg/acre) than Trishal (4521.74 kg/acre) upazila. In contrast, silver carp and tilapia production were 3063.04 kg/acre and 1008.26 kg/acre in Trishal, respectively which were higher than Muktagacha. Average selling price of tilapia, Vietnam koi and silver carp was also comparatively higher in Muktagacha than Trishal upazila. Similar observation was also obtained in respect of indigenous fish species. On the other hand, average selling price of shing was found highest (BDT. 333.33 per kg) in Muktagacha than Trishal while other fish species were sold at similar price. Gross income was also significantly higher with the exotic fishes in Muktagacha accounted for BDT. 858,716/acre/cycle while the highest gross income obtained with the indigenous fishes was in Trishal upazila, valued BDT. 121,5987/acre/cycle. It can be concluded that, both indigenous and exotic fish farming has spread in the area because of easy to adopt culture system, availability of fry, feed, good profit and high demand in the market. Some limitations of the ongoing practices have been identified that need to be addressed properly by the concerned authority. Progressive Agriculture 29 (4): 371-379, 2018