Determination of effective tow duration for estimation of fish and shellfish abundance in deeper-water bottom trawl surveys

Abstract

Experimental trawl surveys were conducted to determine effective tow durations for estimating fish and shellfish abundances in deeper-water bottom trawl surveys. A total of 304 hauls were conducted in Kagoshima Bay, southern Japan, for preset tow durations of 10 and 20 min, using a simple trawl net fitted with depth loggers. Towing durations were recorded at the ship's bridge based on the shooting, towing and hauling times of the trawl net, using pre-established warp lengths for different water depths. Effective tow durations were computed by plotting depth logger and echo sounder data against tow time, and compared with preset tow durations. Trawled distances and swept areas were calculated using the Euclidean distance method. Effective tow durations ranged from 9 to 23 and from 19 to 37 for the 10 and 20 min preset tow durations, respectively. Preset tow duration estimations for abundance were highly biased: -10 to 130% and -5 to 85% biases relative to the effective tow duration estimations, respectively. Moreover, the biases were more pronounced in the shorter tow durations in deeper-water trawl hauls. The computation of effective tow durations using depth loggers and the ship's onboard instrumentation presents a simple approach for correcting inadvertent estimations of effective tow duration, swept area and abundance indices.

Effect of tow duration on estimations of CPUE and abundance of the grenadier Coelorinchus jordani (Gadiformes, Macrouridae)

“…The preset durations were based on towing for 10 or 20 min using pre-established warp lengths for various water depths. Effective tow durations were determined from the initial and final times of net contact with the sea bottom, deduced from the trawl depth profiles from the compact-TD loggers (Fulanda and Ohtomi, 2011). Warp lengths were measured to 0.1-m accuracy using a calibrated in-line counter on the vessel.…”

“…Both warp length and tension were monitored using in-line tension recorders to maintain trawl net symmetry during towing. The mouth opening height of the trawl net was determined during field trials using dual compact-TD loggers attached to the headrope and footrope, while wingspread was measured using a trawl monitoring system (SIMRAD PI-32, Norway: range, 1-350 m; accuracy, 0.1 m; precision, ±0.25% FS) (Fuwa et al, 2010). The mouth opening height and wingspread were estimated at 2.5 ± 0.3 m (mean ± SD) and 6.7 m, respectively (Fuwa et al, 2010).…”

“…Nominal warp lengths were determined using ratios between the water depth and warp length for each of the established sampling station in Kagoshima Bay (Table 1). Thereafter, sampling procedures were maintained throughout the study to ensure minimal variation in the established trawl net geometry (Fulanda and Ohtomi, 2011). However, because of variations in the trawl net shooting and retrieval process and the influence of oceanographic and bathymetric factors, preset tow durations that were recorded at the vessel's bridge often varied from the actual tow duration, which was the time that the net was in contact with the sea bottom.…”

“…Therefore, the actual (effective) tow durations were computed from the trawl depth profiles derived from the compact-TD loggers that were attached to the net. Precise estimations of the effective tow durations were achieved by plotting the trawl depth profiles and echo-sounder depth data against the tow time to determine the actual duration of time that the trawl net was in contact with the sea bottom, according to Fulanda and Ohtomi (2011). The effective tow duration, i.e., the actual duration that the net was in contact with the sea bottom, was computed in the following manner.…”

“…Trawl net depth profiles, echo-sounder bottom depth recordings, and GPS data were used to establish the footrope's initial latitude (Lat t ) and longitude (Lon t ) and final latitude (Lat t+1 ) and longitude (Lon t+1 ). The effective tow durations were then estimated by synchronizing the trawl depth profiles with bottom depth data, according to Fulanda and Ohtomi (2011). The corresponding tow distances and swept areas were then calculated directly using the Euclidean distance method.…”