Effect of tow duration on length composition of trawl catches

Godø, Olav Rune; Pennington, Michael; Vølstad, Jon Helge

doi:10.1016/0165-7836(90)90062-z

Cited by 60 publications

(60 citation statements)

References 8 publications

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“…These mechanisms imply that the temperature and size-dependent endurance swimming ability of cod should bias the swept area estimates downwards for small fish in the cold eastern parts of the Barents Sea and for large fish in the warmer southwestern parts. However, the selection of large fish is questioned by Godø et al (1990). They state that fish greater than 50 cm will also get caught due to what they call ''catching by surprise''.…”

Section: Discussionmentioning

confidence: 96%

Ambient temperature and distribution of north-east Arctic cod

Ottersen

1998

ICES Journal of Marine Science

114

103

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Many studies on the effects of temperature on fish distribution and population parameters have considered the temperature and its variability at fixed stations or sections rather than the ambient temperature actually surrounding the fish. In the present paper ambient winter temperature was estimated for 1-7 year-old north-east Arctic cod in the period 1988-1995 from spatial distributions of fish density and temperature. Four different estimates were calculated for each age and year based on fish density observations from acoustic and bottom-trawl surveys and temperature recordings at the bottom, as well as averaged from 100 m depth to bottom. The estimates of ambient temperature were compared with each other and with temperature series in fixed areas and a standard section, the Kola meridian. The inter-annual variability in ambient winter temperature was found to be larger than in the Kola section series. Older fish were found at higher temperatures. For the younger age groups the range extended eastwards when numbers were high. This could explain the observed decrease of mean ambient temperatures in which cod were found during a relatively warm period in the Barents Sea. The mean ambient temperatures are also compared with the temperatures used for calculations of consumption rate by cod in the ICES Working groups.1998 International Council for the Exploration of the Sea

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Section: Discussionmentioning

confidence: 96%

Ambient temperature and distribution of north-east Arctic cod

Ottersen

1998

ICES Journal of Marine Science

114

103

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“…Since the sampling unit is the trawl and not the individual fish ( Godø et al, 1990;Vølstad, 1991, 1994), RMS length of the population (L) as estimated from data from a set of trawl stations is…”

Section: Discussionmentioning

confidence: 99%

Effects of trawl sampling variability on precision of acoustic abundance estimates of gadoids from the Barents Sea and the Gulf of Alaska

Godø

Karp

Totland

1998

ICES Journal of Marine Science

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. 1998. Effects of trawl sampling variability on precision of acoustic abundance estimates of gadoids from the Barents Sea and the Gulf of Alaska. -ICES Journal of Marine Science, 55: 86-94.The quality of acoustic estimates of fish abundance is dependent on the quality of the biological information that provides the basis for determining target size, species composition and acoustic target strength distribution. Information on biological characteristics of acoustic targets is usually obtained from trawl samples. In this paper we analyse the effects of trawl sampling variability on estimates of gadoid stock abundance in two areas where behavioural characeristics of the target species differ. The analyses demonstrate the sensitivity of abundance estimates to trawl sampling. Improved procedures for allocation of trawl sample information to acoustic survey results are discussed.1998 International Council for the Exploration of the Sea

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“…Systematic variations in trawl catchability over time due to changes in trawl geometry have been shown to influence the average catchability with changes in tow duration [25,26]. Other factors, such as escapement under the footrope, have also been shown to influence catchability and abundance estimations [25,26].…”

Section: Discussionmentioning

confidence: 97%

“…Other factors, such as escapement under the footrope, have also been shown to influence catchability and abundance estimations [25,26]. However, since escapement is a strongly size-selective process [25][26][27], future surveys to estimate abundances should incorporate studies on the length-frequency distributions of species in order to assess the escapement factor and its influence on the estimation of abundance indices for different species.…”

Section: Discussionmentioning

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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Effect of tow duration on length composition of trawl catches

Cited by 60 publications

References 8 publications

Ambient temperature and distribution of north-east Arctic cod

Ambient temperature and distribution of north-east Arctic cod

Effects of trawl sampling variability on precision of acoustic abundance estimates of gadoids from the Barents Sea and the Gulf of Alaska

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

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