1
Determination of Effective Tow Duration for Determination of Effective Tow Duration for Swept-Area Abundance Estimations of Fish and Swept-Area Abundance Estimations of Fish and Shellfish Shellfish in Deeper-water Bottom Trawl Surveys 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. Source: http://www-odp.tamu.edu/publications/186_SR/110/110_f1.htm Results showed that on most, the trawl net bottom contact was established approximately 3-5 min. before the start of recording of the preset tow durations at the vessel’s bridge. Trawl net contact with sea bottom extended for several minutes after start of trawl net retrieval which was recorded as the end of the effective tow duration, thus increasing bias in the estimation of fishing duration. Effective tow durations were significantly different (p<0.05) among stations and were on average two minutes longer than the corresponding 10 and 20 min. presets Generally, the preset tow durations were an underestimation of the effective tow durations with higher bias in the shorter 10 min. presets. In the 10 min. preset tow durations, the error calculated from use of preset tow durations for abundance estimations reflected -10 to 90 % bias suggesting both an under and overestimation of the abundance indices. In the 20 min. preset tow durations, similar errors in underestimation of the effective tow durations would result in inadvertent overestimation of the abundance indices by up to 85 %. Analysis of variance components showed that differences in water depth among sampling stations accounted for 76.1 % of the variations in effective tow durations (sampling error accounted for 19.6 %). Comparison between expected bias in estimations of abundance indices from the preset and effective tow durations showed that errors in definition of effective tow duration would have a highly significant influence on these indices including CPUE and biomass (p < 0.01). On most of the tows, plots of trawl net depth against echo sounder depth showed that bottom contact was established approximately 3-5 min. before the start of recording of the preset tow durations at the vessel’s bridge. The use of submersible depth loggers attached to trawl nets presents a simple method for detecting the non-normal behaviour of the trawl along the sea bottom and can be corrected for by adjustment of the warp or tow velocity based on observations on the logged trawl net depth data. The effective to durations were estimated precisely from the data logger depth -echo sounder water depth plots against tow time and fishing durations varied significantly from the preset durations. Use of the preset tow-durations for estimation of abundance indices results in up to 90 % overestimation of the biomass. The present study established the use of submersible loggers to monitor trawl net depth for precise computation of the effective tow durations by calibration with echo sounder depth data. The method should be incorporated as part of standard procedures in the ongoing experimental trawl surveys in Kagoshima Bay and similar fisheries where use of sophisticated trawl net monitoring equipment is practically untenable CONCLUSIONS CONCLUSIONS ABSTRACT ABSTRACT INTRODUCTION 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 MATERIALS AND METHODS • A stratified experimental design based on bathymetric features was used. • Bay sub-divided into: Bay head - St. 1 and 2 established; Channel area - St. 3; Central basin – St. 4, 5, 6 and 7 and Bay mouth – one station St. 8 • 306 hauls conducted with tow duration preset at 10 and 20 min (Table 1.). • Trawl geometry was monitored from the onboard instrumentation for warp length and tension at vessel bridge and bottom depth was recorded from onboard echo sounder • Vessel positions were logged at 1-sec intervals on GPS and the time of net shooting and start and end of netting, and liftoff were registered for the preset tow durations Sw ept distance D =V x t Velocity,V Width 8m Sw eptarea = Swept dist. x Width(Ws) = D * Ws hr = 12 m D Net haul Sampli ng Identification & sorting Sample 2 2 1 1 2 2 ( - ) ( - ) i d lat lon lat lon 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 = 1 2 2 1 . - n i i i eff D x y RESULTS AND DISCUSSION RESULTS AND DISCUSSION 0 30 60 90 120 0 20 40 60 W aterdepth (m ) Tow tim e (m in.) Logger(traw lnet) Echo-sounder … E ffective tow duration t 0 t X Net Lag Station M ean w ater depth (m) W arp length (m) N et lag relative to vessel(m ) N um ber ofhauls 1 136 620 605 18 2 143 650 634 41 3 128 600 586 49 4 179 700 677 34 5 229 900 871 59 6 79 430 423 33 7 136 600 586 46 8 99 500 490 24 Total 304 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% 80 100 120 140 160 180 1 2 3 4 5 6 7 8 Preset 10 m in. Abundance estim ation (% ) E ffe ctive tow du ratio n (M ean SD) Effective tow 14.62.1 13.61.7 13.71.7 15.52.1 13.92.1 14.61.3 14.91.3 14.02.0 Station 80 100 120 140 160 180 1 2 3 4 5 6 7 8 Preset 20 m in. Abundance estim ation (relative % ) Effective tow 23.12.7 24.12.9 23.64.0 26.45.4 23.24.2 24.83.6 23.53.8 ----- E ffe ctive tow d u ratio n (M ean SD) Station
