The affect of speed through the water on footrope contact of a survey trawl

Somerton, David A.; Weinberg, Kenneth L.

doi:10.1016/s0165-7836(00)00272-1

Cited by 43 publications

(38 citation statements)

References 5 publications

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“…Swept area Swept area for each haul was estimated as the average net width from data collected with a Marport 5 acoustic net mensuration system (Marport Stout Inc., Snohomish, WA), multiplied by the length of the tow path, derived from GPS data of vessel locations at first and last contact of the footrope with the seabed; seabed contact was determined with a bottom contact sensor (Somerton and Weinberg, 2001). Outlier measurements of net width were removed by using a sequential outlier rejection algorithm, and the remaining data were fitted with a smoothed spline from which the average net width was calculated for each tow (Kotwicki et al, 2011).…”

Section: Discussionmentioning

confidence: 99%

“…If an assessment model is not well informed by the data, there will be uncertainty about whether the shape of the estimated function accurately reflects the survey sampling processes or whether it reflects parameter confounding in the model (Maunder and Punt, 2013). The difference between interpretations of the shape of the estimated function with regard to these 2 types of uncertainty may have a pronounced effect on the determination of stock size and recommended harvest rates.Field studies designed to describe survey-gear efficiency and stock availability provide a source of "direct" evidence and can be useful in the fitting of the selectivity function (Cadrin et al, 1999;Weinberg et al, 2004; Clark and Kaimmer, 2006; Nichol et al, 2007;Somerton et al, 2007;Somerton et al, 2013). We present the results from a new study and review results from previous works to determine whether direct evidence from field studies corroborates the dome-shaped survey selectivity function estimated by the current assessment model used for Pacific cod.…”

mentioning

confidence: 99%

“…Field studies designed to describe survey-gear efficiency and stock availability provide a source of "direct" evidence and can be useful in the fitting of the selectivity function (Cadrin et al, 1999;Weinberg et al, 2004; Clark and Kaimmer, 2006; Nichol et al, 2007;Somerton et al, 2007;Somerton et al, 2013). We present the results from a new study and review results from previous works to determine whether direct evidence from field studies corroborates the dome-shaped survey selectivity function estimated by the current assessment model used for Pacific cod.…”

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confidence: 99%

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Is the survey selectivity curve for Pacific cod (Gadus macrocephalus) dome-shaped? Direct evidence from trawl studies

Weinberg¹,

Yeung²,

Somerton³

et al. 2016

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Abstract-Survey selectivity can be viewed as a function of the availability of the stock to the sampling gear and the sampling efficiency of the gear. A dome-shaped survey selectivity function is one in which survey selectivity decreases with larger and older fish. Such a function is estimated for eastern Bering Sea (EBS) Pacific cod (Gadus macrocephalus) in the NOAA National Marine Fisheries Service stock assessment model, which would be appropriate if large (≥55 cm in fork length) Pacific cod avoid capture by the EBS survey bottom trawl. To test this assumption, a field study was conducted to determine whether large Pacific cod escape capture by either outswimming the survey trawl or by swimming above the trawl. Our results show that large Pacific cod do not outswim the trawl because catches did not increase when we increased towing speed. Additionally, large Pacific cod do not routinely swim above the trawl because analysis of acoustic backscatter collected concurrently with trawl hauls indicated that only 4% of the acoustic backscatter attributed to Pacific cod occurred at heights above the headrope. We found no evidence that survey-gear efficiency decreased with increasing fish length either because large fish outswam the trawl or because they tend to occur further from the bottom. Therefore the results of our experiment do not support the use of a dome-shaped survey selectivity function in the EBS Pacific cod assessment model.Fisheries stock assessment surveys are intended to produce an index of relative stock abundance that varies over time in constant proportion to the true stock abundance. In stock assessment models, the scaler that relates modeled abundance to a survey index is often considered a product of a constant catchability and of a fish age-or length-dependent survey selectivity function (which, hereafter, for reasons of simplicity, we refer to as length-dependent functions, but the same concept applies to agedependent functions). Both catchability and selectivity are typically estimated when a stock assessment model is fitted to data (Maunder and Piner, 2015), although, in some cases, the catchability coefficient is fixed a priori (Thompson 1,2,3 importance of availability (e.g., Do small fish occur at depths shallower than those of surveys?) and sampling efficiency (e.g., Do small fish pass through trawl mesh?) in determining the shape of a selectivity function is difficult to determine without additional information. The shape of the survey selectivity function is at issue for the model used for stock assessment of Pacific cod (Gadus macrocephalus; Thompson 1,2,3 ) in the eastern Bering Sea (EBS). The assessment model, conducted with the Stock Synthesis package, vers. 3.24q (Methot and Wetzel, 2013), is fitted to commercial catch data dating back to 1977, as well as to fisheriesindependent data from the National Marine Fisheries Service annual bottom trawl survey of demersal fishes in the EBS (hereafter referred to as the survey). The survey provides estimates of relative abundance and len...

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Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Is the survey selectivity curve for Pacific cod (Gadus macrocephalus) dome-shaped? Direct evidence from trawl studies

Weinberg¹,

Yeung²,

Somerton³

et al. 2016

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show abstract

“…This method has been incorporated into the standard procedures used in the ongoing experimental trawl surveys in Kagoshima Bay, and can be applied to similar fisheries where the use of sophisticated trawl net monitoring equipment is practically untenable. However, the use of both headrope and footrope compact TD loggers as well as bottom contact recorders [19][20][21] would be more useful, especially in fisheries where the bottom topography is highly rugged. Consequently, data and information from the surveys should be analyzed continuously to account for any variations in abundance estimations from the computed effective tow durations.…”

Section: Discussionmentioning

confidence: 99%

“…Further, all tows were made along the axis of the dominant current direction, based on the characteristics of the water currents and the circulation in Kagoshima Bay. Similar sampling procedures have been employed in other fishery surveys with positive results [16,[18][19][20]. (Fig.…”

Section: Methodsmentioning

confidence: 99%

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

Fulanda

Ohtomi

2011

Fish Sci

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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.

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A review of bycatch reduction in demersal fish trawls

Kennelly

Broadhurst

2021

Rev Fish Biol Fisheries

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The affect of speed through the water on footrope contact of a survey trawl

Cited by 43 publications

References 5 publications

Is the survey selectivity curve for Pacific cod (Gadus macrocephalus) dome-shaped? Direct evidence from trawl studies

Is the survey selectivity curve for Pacific cod (Gadus macrocephalus) dome-shaped? Direct evidence from trawl studies

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

A review of bycatch reduction in demersal fish trawls

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