Fulanda trawl duration and abundance estimations
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Experimental trawl surveys were conducted in Kagoshima Bay, Japan to determine effective tow durations for estimating fish and shellfish abundance. A total of 304 tows were conducted with preset tow durations of 10 and 20 minutes. Loggers attached to the trawl net recorded actual tow durations, which were on average 51% longer than presets. Using preset durations rather than effective durations led to overestimates of 10-73% in abundance indices for the 10 minute tows and 14-52% for the 20 minute tows. Water depth differences among stations accounted for most of the variation in effective tow durations.
![Determination of Effective Tow Duration for Swept-Area Abundance Estimations of Fish and Shellfish in Deeper-water Bottom Trawl Surveys Bernerd FULANDA United Graduate School of Agricultural Sciences, Faculty of Fisheries, Kagoshima University, Japan Kagoshima Bay is located in southern, Japan, sandwiched between two sinuate Peninsulas; Satsuma and Osumi. The bay opens to the south facing the Pacific Ocean and the East China Sea, measuring ~ 80 km long and 25 km wide. Water depths reach upto 230 m (Ohtomi & Irieda, 1997). The bay is influenced by the warm (15-28 o C) Kuroshio Current of the South Pacific Ocean and the Sakurajima volcano towers out of the bay joined to the northern part of Osumi peninsula. Kagoshima Bay fishery is a resource with complex exploitation regimes. The bay is utilized by fisheries cooperatives, small-scale bottom seiners, recreational fishers as well as shipping. Its is home to an estimating 7800 fishing vessels from 139 fishing ports. A large number of commercially important species of fish and decapod-crustaceans occur in the bay, and are target species of the small-scale bottom seine fishery [Ohtomi, 2001]. Including the deep-water mud shrimp Solenocera melantho , Plesionika semilaevis and Trachysalambria curvirostris, stomatopod crustaceans such as Squilloides leptosquilla. The grenadier, Coelorinchus jordani is one of the dominant fish species in the bottom seine fishery bycatch. Experimental trawl surveys were conducted onboard Nansei Maru , 175 t, training vessel of the Faculty of Fisheries, Kagoshima University, Japan. The vessel is equipped with onboard echo sounder (KFC-3000) and global positioning system (GPS). The survey employed a simple trawl net measuring 23.5 m long with a 12 m head rope and mesh size at body and codend was 37.9 and 20.2 mm respectively. The wings of the net are fanned by 1 m 2 canvas kites. The net was attached with depth data logger (ATD-HR, ALEC, Japan) at head rope for monitoring net depth. Wingspread and vertical mouth opening were pre-determined as 8 m and 2 m at towing velocities of 2 kt. Trawl duration The trawl durations were as estimated from logger and echo sounder depth plots and the swept areas calculated using simple Euclidean distances. Swept distance:- Experimental trawl surveys were conducted to determine effective tow duration for swept-area abundance estimation of fish an shellfish in deeper-water bottom trawl surveys. A total of 304 hauls were conducted in Kagoshima Bay, southern Japan, using a simple trawl net attached with submersible loggers. Towing was conducted at 10 and 20 min preset tow durations at 2 kt velocity. The effect of tow durations on catch per unit swept area (CPUE) and biomass estimation of grenadier Coelorinchus jordani, a dominant species in the bay was further investigated.. Shooting, towing and lift-off times were determined from the trawl net contact with sea bottom using depth data from onboard echo-sounder. Effective tow durations were computed from plots of logger and echo sounder depth and compared with preset tow durations registered at bridge. Trawl distances and swept areas were calculated using Euclidean distance method. Effective trawl durations varied from preset tow durations and were on average 51 % longer: 20-37 min. for the 20 minutes presets respectively. Consequently, indices computed from the preset durations would overestimate abundance by 10-73 % and 14-52 % in the 10 and 20 min. preset durations. Moreover, the errors were be more pronounced in shorter tow durations in deeper water stations. Computation of effective tow durations using the logger method therefore presents a simple corrective approach to inadvertent estimations of effective tow durations, swept area and abundance indices. ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],CONCLUSIONS ABSTRACT INTRODUCTION Bottom trawl surveys provide important estimates of abundance of demersal fish and shellfish stocks and the relative frequency of various population characteristics such as distribution, reproduction and growth [Godo, 1994; Engaas et. al 2001]. Well designed experimental survey-based assessments provide more accurate prognosis of the stock status than catch-based assessments [Korsbrekke et al. 2001]. The surveys provide a wider coverage than commercial data [Stratoudakis et al. 1998] uncertainties associated with the estimations can be studied and quantified [Engaas et. al., 2001]. Moreover, it is easy to standardize fishing gear and procedures in experimental bottom trawl surveys thus reducing variations in catch per unit effort (CPUE) attributable to variations in trawl efficiency. Research shows that variability in tow velocity speed and effective tow duration (actual fishing time) are the main sources of uncertainty in swept area estimations for abundance [Engaas & Ona, 1993; Engaas et. al., 2001]. MATERIALS AND METHODS ,[object Object],[object Object],[object Object],[object Object],[object Object],Net haul Where lat 1 , lat 2 and lon 1 , lon 2 are the initial and final GPS positions of the net during start (time t 1 ) and end ( t 2 ) of towing respectively. The total tow distance from t 0 to t X was computed as d 1 + d 2 +d 3 +d 4 …….d X Effective tow distance = RESULTS AND DISCUSSION Table 1. Mean water depth, warp length and no. of hauls at each. Net lag was calculated assuming straight-line warps between trawl net and vessel Fig. 1. Variations between preset tow and computed effective tow durations in the present study Fig. 2. Expected variations between abundance estimates computed from the preset and effective tow durations. On average, bias on estimated biomass from the 10 min preset tow durations were up to 90 % while in the 20 min. presets, the bias was lower at 85% Source: http://www-odp.tamu.edu/publications/186_SR/110/110_f1.htm = Swept dist. x Width(Ws) = D * Ws hr = 12 m D Sampling Identification & sorting Sample Net Lag](https://image.slidesharecdn.com/fulandatrawldurationandabundanceestimations-111010004139-phpapp02/85/Fulanda-trawl-duration-and-abundance-estimations-1-320.jpg)
