Ecological monitoring of trawling grounds

The development of mechanized fishing in India is traced to point out the harmful effects of trawling by way of disturbing the ecological balance of the ecosystem, of which the fish stocks are a part. It may cause the disappearance of some demersal species from the catches as brought out poignantly in the case of a marine catfish namely, Arius tenuispinis. History of trawling has been the same all over the world showing that depletion of demersal fish stocks is not only due to excessive harvesting but also due to the damage done to the ecological balance of the trawling grounds. The need for ecological monitoring of the trawling grounds is pointed out.

Trawling pull exerted by a trawler: a method of estimation from propeller dimensions and its comparison with sea measurements

The proper matching of the pull exerted by a trawler and the size of trawl is important for maximizing the catching efficiency. The available pull is more dependent on the propeller and its working conditions than the installed engine power. The normal practice is to directly connect net size to the installed power in the boat by formulae without reference to the prope1ler dimensions or the available trawling pull and this is not adequate to find out the optimum combination. By the method outlined in this paper, the accurate calculation of trawling pull is possible by taking into account only the propeller diameter, pitch and r. p. m. The predictions by the method are compared for trawlers with powers between 30 and 60 hp and agreement is found to be within + 5%. The power absorbed by the propeller in trawling condition can also be calculated by this method for checking whether the engine is being overloaded.

On the relative catch efficiency of different shaped otter boards in bottom trawling

Three different types of otter boards varying in shape were tried to find out their relative catch efficiency. They were operated with two types of nets. The results indicated that the curved otter boards gave more catch per unit effort with all the combinations of nets and riggings tried.

Impacts of Thai silver barb (Puntius gonionotus Bleeker) inclusion in the polyculture of carps

The impact of inclusion of Thai silver barb, Puntius gonionotus (Bleeker) in the polyculture with two major Indian carps viz., Labeo rohita, Catla catla and common carp Cyprinus carpio has been studied in seasonal ponds for 115 days. The presence of silver barb decreased the growth of Indian carps while increased that of common carp. A significantly higher (P<0.05) fish yield (1793.65 Kg/ha/yr) was observed in the four species polyculture system containing silver barb when the combined yield of all species was compared.

The design and construction of flat rectangular otter boards for bottom trawling: a review

The design and construction of the otter board is a subject of great importance for economy in trawling. This review incorporates a historical resume tracing the change and development• of otter boards. The size of the otter board and its relationship with the horse power of the engine and size of the net and the methods of rigging are dealt with. The factors influencing the horizontal spread are discussed. The effect of the angles of attack, heel and tilt and the ground reaction on the force coefficients have been reviewed and discussed with particular reference to flat rectangular otter boards used for bottom trawling. A short account of other designs of otter boards used for improved efficiency is given. Suggestion for improving the efficacy of otter boards based on the work hitherto done has been made. The contributions relating to the various aspects of design and performance of trawl boards carried out till 1969 have been considered.

Wadge Bank trawl fishery studies. Pt. 1. The effect of the 1928 to 1935 commercial trawling on the demersal stock

The demersal fish stock of Wadge Bank is one of the important fish resources for both Sri Lanka and India. Sivalingam and Medcof (1957) have given an account of its history, general features and relative productivity. According to records the total fishing effort on the bank had been fluctuating and very recently the number of boats operating on the bank has suddenly increased, and there is a possibility that still more will begin operating on the bank in the near future (Mendis, 1965). The increased fishing effort with the possibility of still further increase calls for proper management practices by those concerned, in order to obtain the maximum sustained yield from the demersal stock. For this purpose a detailed study of the past performance of the fishery is essential. With this in view all records of commercial operations up to 1960 are being analysed by the present author and are to be published in a series. This is the first paper in the series and gives a detailed analysis of the first commercial trawling operations from 1928 to 1935. Since there had been a major break of about 10 years between this and the present fishery this data is being analysed separately.

Wadge Bank trawl fishery studies. Pt. 2. The effect of trawling on the catch per hour from 1945 to 1960

The object of this series of papers has been given in Part 1 (see Additional informations for details) which deals with the first known commercial fishery on the Wadge Bank from 1928 to 1935. There is no recorded trawling on the Bank between 1936 and 1944. This paper deals with the changes in the total catch (i.e. all species combined) per hour of trawling in relation to the changes in fishing intensity from 1945, when the present trawling activities started, to 1960. The effect of trawling on individual categories or varieties will be presented later.

Small mesh trawling

Experimental trawling operations for bottom fish were first begun in 1920 and consisted of surveys of the continental shelf around Ceylon and the west and east coasts of India. These preliminary surveys showed that the continental shelf of Ceylon was either very poor in bottom fish or that the ground was so rough that trawling has proved to be economically not sustainable (Malpas, 1926). Although a considerable amount of trawling has been carried out using small-meshed trawls, there is no record of the details of the gear used or any account of the relative efficiency of the different designs of trawls used in the operations. Experimental operations were, therefore, carried out off 80 h.p. boats such as m.f.v. Canadian and North Star, to select the most effective gear for these boats. In more recent times experiments were also conducted to select the most effective trawling gear for the 11-Ton boats of the Ceylon Fisheries Corporation. The results are reported in this publication.

Investigations on the bottom conditions and the possibilities for marine prawn and fish trawling on the north and east coast of Ceylon

The execution of this survey was decided upon by Swedevelop in agreement with the Sri Lanka Fisheries Corporation and was a part of the investigations and studies for a fishery project at Trincomalee which was carried out by Scandia Consult. The main purpose was to investigate the following aspects: bottom conditions, especially prawn trawling conditions; size of the potential prawn trawling catches; size and quality of the potential fish trawling catches; suitable sizes and types of boats and trawling gear; duration of trawl fishing season.

Migration of fishermen and its impacts on fisheries management on Lake Victoria, Uganda

The purpose of the study was to investigate migratory movements of fishermen on lake victoria.To identify the cuases of fishrmen migration with a view to establishing the paterns of migration;determining season of migration and assessing the impacts of migration on fisheries management and development.

Shifting rice farming to fish culture in some selected areas of Mymensingh, Bangladesh: the process, conflicts and impacts

The evolutionary process of converting low-lying paddy fields into fish farms and its impact on agrarian communities in some selected areas of Mymensingh district were studied. This study was conducted through participatory rural appraisal (PRA) covering 12 villages from each of selected upazillas viz. Fulpur and Haluaghat of Mymensing [sic] district. A total of 12 PRA sessions were conducted where 90 farmers participated during 29 July to 26 August 2004. It is seen that the use of low-lying paddy fields was mostly confined to Broadcast Aman (B. Aman) rice production until 1960s. With the introduction of modern rice farming technology, the farmers started to produce Boro rice in Rabi season and B. Aman rice in Kharif season. With the passage of time, aquaculture technologies have been evolved and the farmers realized that fish farming is more profitable than rice cultivation, and then they started to utilize their paddy fields for alternate rice-fish farming and rice-cum-fish farming. Now a days, aquaculture based crop production system is in practice in more than 25% of the low-lying paddy fields. Conversion of rice fields in to fish ponds has brought up a change in the livelihood patterns of the rural farmers. The areas where the farmers involved themselves in the new production systems were fingerling collection, transportation and marketing of fry and fingerlings. During 1960s to 1970s, a few people used to culture fish in the permanent ponds for their own consumption, the species produced were rohu, catla, mrigal, ghainna, long whiskered catfish, freshwater shark (boal), snake head (shol) etc. Small fishes like climbing perch, stinging catfish, walking catfish, barb, minnows etc. were available in the rice fields during monsoon season. In 1980s to mid 1990s, some rice fields were converted into fish ponds and the people started to produce fish for commercial purposes. When rice-fish farming became profitable, a large number of people started converting their rice fields in to rice-fish culture ponds. Culture of some exotic fishes like silver carp, tilapia, grass carp, silver barb etc. also started in the paddy fields. Higher income from fish farming contributed positively in improving the housing, sanitation and education system in the study areas. It is seen that the medium and medium high lands were only used for alternate rice fish farming. The net income was high in any fish based cropping system that motivated the farmers to introduce fish based cropping system in the low-lying inland areas. As a result, the regional as well as communal income disparities occurred. However, the extraction of ground water became common during the dry period as the water was used for both rice and fish farming. Mass conversion of paddy fields into rice-fish culture ponds caused water logging in the study areas. In most cases, the participated farmers mentioned that they could be easily benefited by producing fish with T. Aman or only fish during the monsoon season. They agreed that this was an impressive technology to them and they could generate employment opportunities throughout the year. Finally, the social, economic and technical problems which are acting as constraints to rapid expansion of fish production system were reported from the interviewee.

A review of experimental trawling from small and medium sized mechanized vessels off Kanikada

The most suitable otter trawl for small boats was found to be a 10.9 to 15m 2-seam trawl with 100cm x 50cm x 35kg horizontally curved otter boards together with long single sweep line. For operation from medium sized trawls, 18.26m 2-seam 18.3m 4-seam and 29.26m long wing trawl were found suitable. An 18.3m 4-seam trawl was netting a considerable quantity of off-bottom fishes. Shrimps predominated in the catches of the 29.26m trawl. Productive grounds for Cynagris species, Psenus species and Decapterus species within 50 to 100m depth ranges off Kakinada were available for profitable exploitation.

Development of trawling in India

Of the three main methods of capturing fish, namely luring, attacking and snaring, the last mentioned has hitherto proved to be relatively more effective and apparently yielded better returns. Trawling falls under this method. The trawl is an economic gear, in terms of investment and yield, even in spite of the fact that the total trawl catches come only second to that of the seines. Commercial trawling in Indian seas is of recent origin and seems to have been initially introduced for the purpose of ground resources survey.

On the resources of demersal fishes for bottom trawling in inshore waters off Kakinada by small mechanised boats

An attempt is made to assess the available resources of demersal fishes for bottom trawling off Kakinada, in inshore waters. From the experimental fishing operation during 1964-66, the average catch per hour was 52.79 kg for 9.13 m (30') OAL mechanised boat. The catch composition was dominated by prawns and sciaenids forming 45% of the total catch. The average catch per trawling hour was more during the quarter April - June. An assessment on productive depth range has indicated that catch rate is increasing with increase in the depth of fishing.

Double-rig shrimp trawling, its rigging, comparative efficiency and economics

A radical and revolutionary change from the hitherto employed shrimp trawling is reported for better productivity and economy in deep sea trawling with medium and large vessels. Studies carried out off Veraval with a 13.7 m. four seam shrimp trawl as conventional single-rig and two 6.8 m four seam shrimp trawls as double-rig from a 15.2 m fishing vessel showed an increase of 98% shrimp catch by double-rig over single-rig at 86.3 % of the power utilised by the latter. It has also been worked out, theoretically, how better this available power can be utilised for further improvement of catch. An increase of 15% head rope length in case of double-rig over the optimum single-rig gear of any vessel is recommended.