Trawl Gear Research Articles

Commercial feasibility trial for a single-warp deep-water “Maireta” (OTMS) trawl gear

The object of this study was to compare the effectiveness of a single-warp bottom trawl gear and a gear using two warps. The initial design for the gear was an experimental single-warp deep-water Otter trawl Maireta system gear (OTMS) highly suitable for working at considerable depth. From that basic design, two identical but larger gears were constructed for trawling by commercial vessels. The gears were deployed by two vessels with similar power ratings that trawled together in parallel at the same depth. A total of 23 trawls was carried out on fishing grounds on the shelf and slope at depths between 100 and 400 m off Barcelona. The total catch and size composition of the main commercial species in the samples taken by each gear were compared. There were no significant differences ( p<0.01) in catchability (kg/km 2), in the size composition of the catches over the course of 1 week. It was concluded that in the conditions of the experiment, the yields obtained using the single-warp trawl gear were similar to those obtained using the two-warp trawl. The single-warp system required less warp line and shorter bridles and hence represented savings in operating costs.

Comparison of fish and shrimp trawls for sampling deep-water estuarine fish in a large coastal river in eastern Australia

Shrimp and demersal fish trawling gears were compared as methods for sampling the deep-water estuarine fish communities in the Clarence River, a large east Australian coastal river. Three sites at three separate locations within the river were sampled quarterly over a 2-year period with both nets. Of the 68 fish species captured during the sampling program, 47 were caught with both nets, 10 only in the shrimp trawl and 11 only in the fish trawl. Summed over the entire study, more fish and slightly more fish species were captured with the shrimp trawling gear, but estimates of mean CPUEs (combined over species) and fish species diversities were very similar using either net during most seasons and at most sites. However, the nets were highly selective for particular species, with Selenotoca mutifasciata, Mugil cephalus, Liza argentea, Herklotsichthys castlenaui and Acanthopagrus australis caught predominantly with the fish trawling gear and Philypnidon grandiceps, Ambassis jacksoniensis, Ambassis marianus and Arius graeffei caught mainly with the shrimp trawling gear. For some species, shrimp trawling gear caught larger numbers of smaller individuals. Although gear type affected species composition, data obtained from either net were useful in discussing the spatial variation in the estuarine fish communities. Overall, trawling, particularly with the shrimp trawling nets, offered an excellent monitoring tool for the deep-water estuarine fishes, although a combination of methods might be required for a comprehensive description of fish communities.

Limited infaunal response to experimental trawling in previously untrawled areas

There is considerable argument about the effects of bottom trawling on the benthos. Many studies have been done on recently trawled grounds, where community composition has already been modified, and further effects are likely to be minimal. This study tests the effect of trawling on macroinfaunal assemblages in an area where little or no trawling had occurred in the previous 15 years. A spatially replicated Before-After, Control-Impact (BACI) design was used, with adjacent trawl and control corridors. Sampling was done in the same two small sites within each corridor before and after trawling to minimise confounding due to spatial variation. Despite this rigorous design, changes consistent with an effect of trawling were not detected. At only one of the three locations was a potential effect detected. These inconsistent results could be due to different disturbance regimes at each location, influencing the vulnerability of fauna to further disturbance. Given the high levels of variability in infaunal assemblages, however, the changes could also be due to asynchronous natural variation. The combination of high spatial and temporal variability, in association with light trawling gear, means that prawn trawling in South Australia does not have consistent effects on infauna.

Impact of bottom trawling on water turbidity and muddy sediment of an unfished continental shelf

Two experiments were carried out to study the effects of trawling in the muddy prodeltaic deposit of the Llobregat River in the northwestern Mediterranean. Trawling was conducted in two experimental lines, and bottom morphology, sediment texture, and water turbidity were analyzed before trawling and at different time intervals afterward. The tracks of the trawl gears were still observed in sonographs of the bottom 1 yr after the first experiment. The vertical grain size distribution of bottom deposits indicated that the thickness of the sediment removed by the net between the gears was about 2–3 cm on average, though the erosion produced by the gears was deeper. Resuspended aggregates with a high silt content settled during the first hour after trawling, generating a temporary increase in the silt content of the surface sediment. One day after trawling, the surface sediment was mixed and already had a similar grain size distribution to that before trawling. After the beginning of trawling, water turbidity increased first near the bottom for a few hours and later also at shallower levels of the water column within a period of 2–5 d after trawling. At the end of the experiment, about 10% of the sediment affected by trawling was diffused in the water column and the remaining 90% had settled on the bottom. Average turbidity in the water column increased by a factor of up to three for 4–5 d after trawling. This experiment shows that intense and continued trawling on continental shelves has a noticeable effect on water turbidity, which must be considered in addition to natural processes.

The biology and exploitation of John dory, Zeus faber (Linnaeus, 1758) in the waters of England and Wales

John dory have become an important by-catch for trawl gears in the western English Channel (ICES division VIIe). English and Welsh LPUE data indicated the John dory stock was centred to the south and west of the British Isles, but extended north into the Irish Sea and east into the southern North Sea during quarters three and four. The growth and exploitation pattern of John dory in the English Channel are described using length samples of commercial landings, discards, and research vessel samples taken between April 1994 and March 1996. In the English Channel the seasonal peak in landings during quarters three and four coincided with the period of recruitment. Recruitment was at a length of approximately 23 cm, most commercial landings were in the length range 23-39 cm, and the maximum observed length was 59 cm. A seasonal growth model fitted to quarterly length data indicted that recruitment took place at age 1+. The mean length of first maturity was estimated at 26 cm in males and 34.5 cm in females, and suggested that the eastern English Channel (ICES Division VIId) was a seasonal nursery ground for John dory. Some options for stock conservation are discussed.

Steam trawl catches from south-eastern Australia from 1918 to 1957: trends in catch rates and species composition

Haul-by-haul steam trawler catch and effort data for 1918–23, 1937–43 and 1952–57, which cover a large portion of the history of steam trawling in the Australian South East Fishery, were examined in detail for the first time. There were 64371 haul records in total. The catch-rate for all retained catch combined shows a strong decline overall, with a brief recovery during World War II, probably due to increased retention of previously discarded species. The fishing fleet moved to more distant fishing grounds and deeper waters as the catch-rate declined. The catch-rates of the main commercial species followed a similar pattern in a number of regions within the fishery. The catch-rate of the primary target species – tiger flathead (Neoplatycephalus richardsoni) – dropped considerably from the early, very high, catch-rates. Chinaman leatherjacket (Nelusetta ayraudi) and latchet (Pterygotrigla polyommata) – species that were apparently abundant in the early years of the fishery, virtually disappeared from catches in later years. The appearance of greater catches of jackass morwong (Nemadactylus macropterus), redfish (Centroberyx affinis) and shark/skate during the war and afterwards was probably due to increased retention of catches of these species. The disappearance of certain species from the catch may be due to high fishing pressure alone, or to a combination of fishing pressure, changes in the shelf habitat possibly caused by the trawl gear, and environmental fluctuations.

Changes in relative abundance of sharks and rays on Australian South East Fishery trawl grounds after twenty years of fishing

Upper continental slope trawling grounds (200–650 m depth) off New South Wales were surveyed with the same vessel and trawl gear and similar sampling protocols in 1976–77 (during the early years of commercial exploitation) and in 1996–97. The 1996–97 mean catch rate of sharks and rays, pooled for the main 15 species (or species groups), was ~20% of the 1976–77 mean. Individual catch rates were substantially lower in 1996–97 for 13 of the 15 species or species groups. The greatest decline was observed for dogsharks of the genus Centrophorus, which were most abundant in 1976–77 but rarely caught 20 years later. In contrast, 1996–97 catch rates of spiky dogshark (Squalus megalops) and, to a lesser extent, whitefin swell shark (Cephaloscyllium sp. A) were similar to those in 1976–77. Trawling during 1979–81 provided data for nine species, albeit not corrected for larger gear size, and the pooled mean catch rate for sharks and rays in the depth range 300–525 m was ~28% of the mean for 1976–77. The results suggest that the biomass of most species of sharks and rays declined rapidly as the fishery developed and is now at very low levels.

CONSERVATION OF FISH STOCKS THROUGH BYCATCH REDUCTION: A REVIEW

In New England waters, many fish stocks are in precipitous decline while others are already at historically low levels. Bycatch and discard of non-target species is a major contributor to fishing mortality and is a key factor in the observed decline of stocks. In recent years, there have been many initiatives, both globally and locally, to improve selectivity of fishing nets, or more correctly, to reduce the capture and discard of non-target fish. However, it has also become clear that the natural behavior patterns of many species prevent effective selection. For a trawl gear to be truly selective, every fish that enters the net should be tested to insure that those that are small enough to pass through the meshes escape and those that are above the minimum landing size are retained. In general there are two main techniques to aid fish escapement. The first technique is based on making use of species-specific behavior patterns, whereas the second technique involves a process of mechanical sorting on the basis of size. This paper outlines the most widely used bycatch reduction techniques and, where applicable, provides examples within each category where such techniques have been shown to reduce bycatch and discard. The role of bycatch reduction in conservation of fish stocks is discussed.

The impact of otter trawling on mud communities in the northwestern Mediterranean

We report the findings of an experiment conducted in the northwestern Mediterranean to evaluate the short-term effects of otter trawling on a muddy-sediment infaunal community. A commercial fishing vessel, using conventional otter trawling gear, fished two waylines repeatedly until each had been disturbed entirely either once or twice. Infaunal samples were collected from replicate sites located within the fished and adjacent undisturbed control areas. Univariate analyses of species richness and diversity indicated that the infaunal community did not alter during the first 102 h after fishing in the area that was trawled once. However, the abundance of a number of species decreased significantly on the unfished line compared with the fished line 150 h after fishing. No differences were detected on the line that was fished twice, but this part of the experiment was terminated after 72 h. Multivariate analyses revealed that the communities in both fished and unfished areas changed in a similar manner through time and that there was no apparent effect of fishing until 150 h after fishing. These short-term changes are discussed in the light of recent studies of infaunal changes in other habitats.

Differences in orientation and swimming of walleye pollock Theragra chalcogramma in a trawl net under light and dark conditions: concordance between field and laboratory observations

The success of modifications to fishing gear in reducing or eliminating bycatch mortality requires in many cases that escaping fish be able to use vision for volitional guiding out of gear. Light adequate for vision is often not present as fishing operations are commonly conducted at depths or at night when light quantity is below visual capability. Previous observations of walleye pollock behavior in the laboratory indicated that darkness inhibited their ability to respond and orient to trawling gear. The goal of the present study was to test this result in situ by comparing swimming and orientation capabilities of walleye pollock under light and dark conditions in commercial type trawl gear using infrared illumination and video cameras to monitor these behaviors. Under light conditions, fish swam actively and were oriented parallel to the principle axis of the trawl. In contrast, under dark conditions there was little or no active swimming and the fish showed a significant degree of deviation from the principle axis of the trawl. These results showed that vision plays a major role in trawl gear interactions for walleye pollock and points out the importance of measuring light quantity in field studies of bycatch processes and mortality.

Trawl gear selectivity and the effect of mesh size on the deep-water rose shrimp ( Parapenaeus longirostris, Lucas, 1846) fishery off the gulf of Cádiz (SW Spain)

The deep-water rose shrimp ( Parapenaeus longirostris, Lucas, 1846) is a typical shrimp which is one of the main target species in the demersal fishery in the Spanish south Atlantic region. In this paper, the selectivity parameters of this species have been studied in two different selectivity survey designs (research vessels and in commercial fishing vessels). We have found 2 mm difference between both methods in the 50% retention length ( L 50). The selection factors obtained from the oceanographic vessels range from 0.37 to 0.49, while those from the commercial vessels are lower, from 0.32 to 0.39. The effects of mesh size changes in the present exploitation pattern are also considered. The present exploitation pattern is far from to optimal due to an overexploitation of the smaller sizes. The different simulations of changes in mesh size do not show great differences in maximum sustainable yields, but important market variations occur above 30% and 50% over the present obtained values. In reference to the immediate effects which could produce an increase of the mesh size, losses of 36%, 51% and 62% for the referred mesh sizes (50, 55 and 60) using the selectivity parameters obtained from the surveys of research vessels are estimated. If obtained from the surveys of the commercial vessels, these values correspond, respectively, to 27%, 42% and 73%.

The AusTED II, an improved trawl efficiency device 1. Design theories

Improvements to the original AusTED (Australian trawling efficiency device) design were completed in 1994 and the device was renamed AusTED II. New design features, theories and performance of the AusTED II during testing are described. The AusTED II reduced bycatch (including large animals) and slightly reduced byproduct. Variations in prawn catches were dependent on the area being fished. Large fluctuations in net drag precluded any detailed analysis of changes in drag as a result of fitting the AusTED II to standard commercial trawl gear. The AusTED II equipped net required no extra assistance or vigilance from the crew when tested under commercial conditions.

Comparative technical aspects of the &lt;i&gt;Nephrops norvegicus&lt;/i&gt; (L.) fishery in the northern Mediterranean Sea

A comparative study of the technical characteristics of the Norway lobster fishery in the Mediterranean Sea is presented. The different trawl vessels, trawl gears, trawling manoeuvres, and status of the statistics compiled by the administrations are described. The main conclusions are that catchability has increased with technological advances, official landings statistics are flawed in most of the Mediterranean, and fishing methods differed from country to country.

Retained and discarded by-catch from oceanic prawn trawling in New South Wales, Australia

The catches and by-catches of oceanic prawn trawlers were identified and quantified in a stratified, randomized, observer survey of four of the most important fleets in NSW, Australia. For each port, catches were sampled from all tows done during replicate fishing trips in each season from winter 1990 to autumn 1992. Data were examined using analyses of variance of relative abundances of individual species and groups of species, multi-dimensional scaling of the full by-catch dataset, estimates of retained by-catches and discards by the four ports (incorporating estimates of fishing effort) and length–frequency distributions of common discards. The results revealed a latitudinal gradient in by-catch and discarding with larger quantities occurring northwards. However, significant species-specific variations in abundances were detected at all spatial and temporal scales sampled such that the occurrences and abundances of particular by-catch species depended on the location, season and year examined. In catching an estimated 1 579t of prawns during the two-year survey, the oceanic prawn trawlers from the four ports were estimated to have caught approx. 16 435t of by-catch (a by-catch-to-prawn ratio of 10.4 : 1). Of this by-catch, 2 952t were estimated to have been retained for sale and 13 458t were discarded – including several million individuals of commercially and recreationally important species (e.g. snapper, eastern blue-spot flathead, red-spot whiting). We discuss the results in terms of the spatial and temporal variations in these discards and the relative effectiveness of various ways to reduce them, including fixed and flexible closures to trawling and selective trawl gears and fishing practices.

Measuring the brightness contrast of fishing gear, the visual stimulus for fish capture

The brightness contrast of the netting and other components of the trawl gear are examined in different conditions by comparing their luminance to that of their visual background of water, sand cloud, sea bed or self. Luminance of each component was measured directly with a micro-luminance meter in controlled tank experiments and by computer processing of TV images of twines and whole gears recorded in the sea with precise calibration of viewing angle, light intensity and grey levels. The brightness contrast of 3 dimensional components can involve self contrast between adjacent bright and dark zones of the structure. The brightness contrasts of floats, bobbins and otter board can be defined by their self reflectance ratio and this ratio increases with light level and relative viewing angle in both tank and sea observations. Pale coloured twines and their knots appear darker than the water background when viewed from vertically below and they show high values of negative brightness contrast. The same twines are bright against a dark water background when below the viewer and show high positive brightness contrast. There are zenith angles in between these extremes where the same twines are least visible but may still be detected due to self contrast within the knots. These features and the principles involved are examined in detail in order to develop predictive rules and this study shows how the visibility of any part of the gear can be predicted by definable equations. These equations can then form a basis for modelling the relative intensities of the visual stimuli that surround fish swimming at any position within the towed fishing gear.

Modelling the visual stimulus of towed fishing gear

A number of modules are developed for a model with the aim of predicting the strength of the visual stimulus that might initiate fish reactions from any viewing point within a towed fishing gear. The first part of the model defines the properties of the undersea light field through which a trawler might tow a fishing gear including allowances for geographical position, weather, water quality and diurnal changes. A variety of light measurements are made to check day and night light levels used in the model. The underwater luminance of water, sand cloud and sea bed which form the background against which a fish sees the net components is then calculated. An example constructing the space in which a fish may react within a towed trawl gear is developed for the BT130C, North Sea trawl. This part develops an existing net frame model that creates the three-dimensional shape of the towed net from the designers net drawings. Analysis of previous observations of this trawl form the bases for simulation of sand clouds produced by the ground gear and otter board as part of the fish stimulus. Finally the simulated trawl is towed through the predicted light field and methods are developed for calculating the visibility of the net components including netting, ropes, floats, bobbins, otter boards and their sand clouds as viewed from any fish position within the net. An example of the result of these predictions, maps the stimulus strength for one fish position.

分離漁獲装置付き２階式小型底曳網の分離効果

A two-level beam trawl was developed with a square-mesh separator panel in it to separate marketable fish and shrimp from non-commercial crabs and smaller fish. The net-mouth of the lower cod-end in the two-level trawl gear was covered with a square-mesh net panel as a separator. Fishing experiments of the gear were conducted with a chartered trawler off Shimonoseki city in October 1995. Two square-mesh net panels of 40 and 50mm mesh bars were used in the fishing experiments. The net mouth openings of the upper and lower cod-end associated with the efficiency of the separator were measured. The total number of animals caught in 7 hauls was nearly 2, 300. Separation efficiencies defined as the ratio of the catch in the upper cod-end to the total catch of 11 species were compared by two mesh bar sizes. Non-commercial crabs passed through the separator panel and then entered in the lower one, while shrimp were caught in the upper cod-end although its body was enough small to pass through the separator. Separation efficiencies of the main 6 species plotted versus bodysize indicate that the separator has size-selectivity to some extent. Thus, the gear was useful to catch fish of marketable size and shrimp in the upper cod-end.

Cetacean Interactions with Trawls: A Preliminary Review

Cetaceans interact with trawls to an undetermined extent. A preliminary review of global data indicates that individuals of 25 cetacean species (two mysticete, 23 odontocete) have been documented to have died in working trawls or discarded trawling gear. Cetacean interactions with trawls are complex, in part because both fishermen and cetaceans are drawn to areas of high prey density. Furthermore, within such areas, cetaceans are probably often attracted to trawling activities because they make it easier for the animals to exploit a concentrated food source. Individuals of 15 (possibly 16) cetacean species (13 odontocete, and one or even two mysticete) have been reported to feed in association with trawls. Animals follow working nets (feeding on stirred-up organisms or fish gilled in mesh) and also feed on discarded by-catch. Damage to gear as a result of feeding interactions or entanglement has been reported. Such damage results in (1) harm to the animals, (2) creation of negative opinions of cetaceans by fishermen (regardless of whether a cetacean or a shark is, in fact, responsible for the damage in question), and (3) loss of time and money for repair and replacement of gear. The relationships of cetaceans with trawls need to be further studied to determine what effects the trawl fisheries have on the ecology and population status of the whales and dolphins involved.

Analysis of the interaction of fishing gear in Mullus barbatus fisheries of the Western Mediterranean

The exploitation of Mullus barbatus stocks was assessed in four ports; Porto Santo Stefano in Italy, and Vilanova i la Geltrü, Castelló and Santa Pola in Spain. Virtual population analysis (VPA) by size was carried out using the catch equation. The different types of fishing gear exploiting this species in these ports were taken into consideration. In the Italian port the fisheries employ two trawl gear which exploit a wide range of individuals, although concentration is on adult individuals. In the Spanish ports higher concentrations of immature individuals are exploited by trawl gear, while trammel nets exploit a greater number of adults. Conditions of equilibrium were assumed and the influence of parameter variations on the VPA results was also considered. The results show greater sensitivity to variations of natural mortality (M). Simultaneously, an analysis of yield per recruit (Y/R) was carried out, illustrating that the exploitation level of this resource exceeds its optimum. In addition, changes in the exploitation regime, e.g., variations in the mesh size and the effort of trawling gears were simulated to improve the M. barbatus exploitation pattern in the Western Mediterranean.

The ‘MultiSampler’: a system for remotely opening and closing multiple codends on a sampling trawl

A system for remotely opening and closing multiple codends on pelagic sampling trawls has been developed in Norway. The system makes it possible to take several uncontaminated samples during discrete, user-selected periods within a single trawl haul, using conventional large-scale trawl gear. The system has been used routinely to take samples of acoustically detected fish during several hydroacoustic surveys assessing herring in northern Norway. The results obtained with this system show that the length composition of herring throughout the water column is not homogeneous, which was previously assumed when interpreting the catch results obtained with ordinary pelagic trawls. Such new data can be used to improve stock abundance estimates.