Impact of a ring net fishery in the inshore marine waters of Kilifi on the reproductive biology of six pelagic fish species

Ring nets are modified purse seines used for pelagic fishing along the Kenya coast. This method is however associated with potential negative environmental implications in inshore and shallow fishing grounds. Biological impacts on selected pelagic species caught in the marine waters of Kilifi were investigated during the Southeast Monsoon season (SEM) in April, May and September 2014, and the Northeast Monsoon season (NEM) in October and November 2014, and March 2015. Amblygaster sirm, Rastrelliger brachysoma, Sardinella gibbosa, Rastrelliger kanagurta, Hemiramphus far and Sphyraena obtusata were the most abundant species caught by the ring nets and comprised 68.9 % of fish catch composition. Analysis of ring net impacts on the fish sizes and maturity status were conducted for the NEM and SEM seasons. The catch composition for A. sirm in October comprised of 53.3 % juveniles and 53.4 % juveniles for S. obtusata in November. High percentages of H. far (78.3%) and S. obtusata (58.9 %) were captured in September. Incidences of undersize fish for A. sirm, S. gibbosa, R. kanagurta and R. brachysoma were observed across the seasons. Sex ratios for A. sirm, R. brachysoma, R. kanagurta and S. obtusata in the NEM season deviated from a normal ratio of 1:1 with more males observed. Females were dominant in the catch for H. far (1: 1.28) but the ratio was not different for S. gibbosa (χ² = 5.564, df = 1, p = 0.21). Males were dominant for A. sirm (1: 0.8) and S. gibbosa (1: 0.7) in the SEM season but the ratios were not different for H. far, S. obtusata, R. brachysoma and R. kanagurta. Differences in sex ratios were attributed to fish migrations and reproductive processes. Ring nets fished relatively offshore during the NEM season and targeted mainly spawning aggregations in stage IV. Immature fish in stage I and II which comprised of juveniles were harvested within sheltered inshore waters in the SEM season. Use of ring nets to target juveniles and spawning aggregations may disrupt recruitment processes. To enhance sustainable management of the ring net fishery, there is a need to develop harvesting strategies based on the information on stock status of the target fish.

PEMANFAATAN ALAT TANGKAP IKAN TRADISIONAL OLEH NELAYAN PULAU BAWEAN KABUPATEN GRESIK

Bawean Island is located in the middle of the Java Sea, about 150 km north of Gresik. The name Bawean was taken from the Sanskrit which means there is sunlight. The Bawean people are acculturation from several ethnic groups of Java, Madura, Kalimantan, Sulawesi and Sumatera including their culture and language. The majority of Bawean residents work as fishermen. Bawean Island fishermen utilize marine resources to meet the needs of their family by using knowledge that has been obtained from previous generations. The purpose of this study was to determine the use of traditional fishing gear and fishing strategies used by fishing communities in Bawean Island. The research method used was direct observation, and sampling using Purposive Sampling and Snowball Sampling. The results showed that the fishing gears used by Bawean Island fishermen were kareket (waring), pajeng (payang), jhering cursor (ring net), rompon (bubu), jhering pemhir (drift gill net), payimbek rentengan (drag fishing), gerany Panyimbek (drag fishing line), panceng (fishing line). These fishing gears are very environmentally friendly because the usage does not damage the seabed ecosystem. Each fishing gear has its own characteristics, so that different fishing gear will result in different fishing catch

Preliminary findings on the food and feeding dynamics of the anchovy Stolephorus commersonnii (Lacepède, 1803) and the Indian mackerel Rastrelliger kanagurta (Cuvier, 1817) from Tanga Region, Tanzania

Small pelagic fishes play an important role in the ecosystem by linking planktonic production and higher trophic level predators, and provide a livelihood to both the small-scale and commercial fisher communities. This study analyzed the food and feeding habits of Stolephorus commersonnii (Lacepède, 1803) and Rastrelliger kanagurta (Cuvier, 1817) from the ring-net fishery in Tanga, Tanzania. A total of 1 434 and 320 stomachs of S. commersonnii and R. kanagurta respectively were examined for gut contents using the relative volumetric method. S. commersonnii was found to be a planktivorous carnivore, feeding principally on planktonic penaeid shrimps (48.6%), fish larvae (33.2%) and zooplankton (12.3%). R. kanagurta was found to be carnivorous, feeding predominantly on fish (60.6%), mainly S. commersonnii, while penaeid shrimps, juvenile fish, and juvenile stages of squids formed 26.5% of the total number of food items in R. kanagurta guts. Both S. commersonnii and R. kanagurta exhibited ontogenic diet shifts, where they fed exclusively on small prey as juveniles and consumed larger food items as they grew. The index of vacuity was higher in S. commersonnii (46.1%) than in R. kanagurta (16.6%). This study revealed the importance of penaeid shrimps as food for S. commersonnii, that in turn formed the main food for R. kanagurta. This implied that the two species were able to coexist in the same niche by avoiding interspecific competition for food.

A Study on the Development of Ring-Net using 2,040 MPa High Strength Steel Wire for Rockfall and Debris-Flow Barriers

In this study, research was conducted on the development of Ring-Net, a major component of the Ring-Net System for the localization of a high-level debris-flow barrier, which is fully applied with foreign technology. Ring-Net was constructed using the 2,040 MPa class high-strength steel wire with a 2.8 mm diameter,. Its performance was verified through simulation and a 3-Ring Test (ETAG 027). As a result, the 3-Ring breaking load of 7-19/2.8/300 Ring-Net was analyzed to be 79.8-329.0 kN. The 3-Ring energy dissipating capacity was 2.9-8.7 kJ, which is equivalent to the performance of the Ring-Net made of 1,770 MPa class wires with a 3.0 mm diameter. However, since it can be manufactured with a 13% weight reduction compared to the existing technology, it is believed to be advantageous for lightening the high-level Rockfall & Debris Barrier and improving its construction capability.

Development of Energy-Dissipating Device for Rockfall and Debris-Flow Barriers

In this study on the development of energy-dissipating devices, a significant component of the ring-net system was investigated for the localization of a high-performance rockfall fence and debris flow barriers. The energy-dissipating device was developed as a structure that dissipated the resistance and frictional forces generated by the pipe passing through two steel bars, and the tensile force was transmitted by utilizing the pipe deformation. The performance of the developed energy-dissipating device was verified through simulation analysis and tensile tests. It was confirmed that the most effective dissipating device was made of a D60.5-3.2t pipe subjected to a rolling interval of 40 mm, and the device exhibited an energy-dissipating performance of 52.8-60.2 kJ/m.