Selectivity experiments with window codends fished in the Baltic Sea cod (Gadus morhua) fishery

Madsen, Niels; Moth-Poulsen, Thomas; Lowry, Nicholas

doi:10.1016/s0165-7836(98)00085-x

Cited by 21 publications

(21 citation statements)

References 9 publications

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“…a and b, which are the Rev Fish Biol Fisheries (2007) 17:517-544 519 two parameters to be estimated, represent the intercept and slope, respectively, after a logit transformation. The logistic curve has been found to fit codend selection curves for Baltic cod very well for recently conducted experiments (Madsen et al 1998Tschernij and Holst 1999). The selection curve will however normally be described by the selectivity parameters L50 and SR where (Wileman et al 1996:…”

Section: Modelling Size Selectivitymentioning

confidence: 60%

“…However, argues that chafing gear is often used to protect the underside of the codend that will obstruct the meshes at the lower panel and hence reduce the selectivity anyway. Codend covers which completely enclose the codend have been used in most experiments up to today (Strzyzewski et al 1973;Bagge 1978;Olofsson and Otterlind 1978;Bohl , 1981Treschev and Shevtsov 1976;Shevtsov 1981;Lowry et al 1995;Tschernij et al 1996;Tschernij and Holst 1999;Madsen et al 1998. A serious problem using the covered codend method is when the cover gets in physical contact with the codend, blocking the codend mesh openings and hence bias results.…”

Section: Conducting Trawl Selectivity Experimentsmentioning

confidence: 99%

“…The ICES manual of methods for measuring the selectivity of towed fishing gears strongly recommends the use of hoops when using a codend cover (Wileman et al 1996). Hoops were used in Baltic selectivity experiments conducted in the 1990s Tschernij et al 1996;Moderhak 2000b;Madsen et al 1998). …”

Section: Conducting Trawl Selectivity Experimentsmentioning

confidence: 99%

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Selectivity of fishing gears used in the Baltic Sea cod fishery

Madsen

2007

Rev Fish Biol Fisheries

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The Baltic cod (Gadus morhua) plays a very significant role in the Baltic Sea ecosystem being the major fish top predator and the most important commercial species for the Baltic Sea fishing industry. Consequently the management and understanding of the exploitation pattern of the stock is of major importance. Technical regulations, improving the selectivity of cod, have been a major management strategy and the Baltic Sea is likely to be the area where most fishing gear selectivity studies, focussing on size selectivity, have been conducted over time. The methodology for conducting and analysing selectivity data has been significantly improved in recent years. The subject is reviewed since the choice of methodology can have a significant effect on the interpretation of the outcome of selectivity experiments. Factors affecting the selectivity of trawl and gill nets are considered. Alternative ways to improve the size and species selectivity of trawls using selective devices are reviewed. Selectivity parameters from available literature are listed and the correlations of selectivity parameters to the mesh size for different gears are estimated. The historical legislation on selectivity is reviewed and the expected selectivity for trawls is estimated. Management considerations concerning the mortality of escaping and discarded fish and wider management impacts have to be considered if improving selectivity. The review is ended by conclusions including reflections on needed research in the future.

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Section: Modelling Size Selectivitymentioning

confidence: 60%

Section: Conducting Trawl Selectivity Experimentsmentioning

confidence: 99%

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Selectivity of fishing gears used in the Baltic Sea cod fishery

Madsen

2007

Rev Fish Biol Fisheries

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“…Stiffer twine might open the mesh more and the size of the knot relative to the mesh size will decrease with decreasing twine thickness; these factors might reduce the T 90 effect. This scenario is opposite to the expected effect of standard meshes, where the selectivity will decrease with increasing twine thickness (Lowry and Robertson 1995;Sala et al 2007) and stiffness (Madsen et al 1998;Tokac et al 2004). These issues clearly need to be investigated because twine thickness often is included in technical regulations because it is related to the expected effect of standard meshes.…”

Section: Discussionmentioning

confidence: 96%

Comparing selectivity of a standard and turned mesh T90 codend during towing and haul-back

Madsen¹,

Herrmann²,

Frandsen³

et al. 2012

Aquat. Living Resour.

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-In this study, we compared the size selectivity of a T 90 codend (netting turned by 90 degrees) with that of a standard codend made of similar netting. Sea trials were conducted in a Norway lobster directed fishery in the KattegatSkagerrak area, where there is a need for improved selectivity because of a severe discard problem. The codends were tested by fishing simultaneously with them in a twin trawl rig. Codend covers mounted with Minisamplers were used, which made it possible to catch individuals escaping during towing and haul-back separately. Herein we proposed a model to assess the sequential selection during towing and haul back. This model takes into account the parameter C tow , which can be interpreted as the proportion of fish that comes into contact with the codend meshes during towing and, thereby, has a chance of escape. Compared to the standard codend, the T 90 codend retained fewer Norway lobster both below and above the legal minimum landing size (40 mm, cephalothorax length), thereby causing a reduction of commercial catch. The difference was mainly due to a significantly higher escape rate during towing for the T 90 codend. For plaice below minimum landing size (27 cm), the retention was slightly but significantly higher for the T 90 codend compared to the standard codend. A model developed for both codends showed that not all plaice are able to attempt escapement during the towing process. For cod, the results indicated an increased L50 (the length at which 50% of this species is caught) for the T 90 codend, but the effect was not statistically significant, probably due to the limited number of cod retained during the sea trials. The results demonstrated that, for all three species, a significant proportion did escape during haul-back in both codends.

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“…However, others have obtained the opposite result (O'Neill & Kynoch, 1996;Dahm et al, 2002), while emphasising that selectivity tends to decrease when the catch size is very high. On the other hand, the studies of Madsen et al (1998), O'Neill et al (2006) and Grimaldo et al (2008) yielded inconclusive results or found only a weak effect of the catch variable.…”

Section: Discussionmentioning

confidence: 98%

The effects of subsampling and between haul variation on the size selectivity estimation of Chilean hake (Merluccius gayi gayi)

Queirolo

Ahumada

Hurtado

et al. 2012

lajar

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ABSTRACT. Using the data collected in a size selectivity experiment on Chilean hake (Merluccius gayi gayi) carried out in 2000, the selectivity parameters for four codend mesh sizes (100, 110, 130, and 140 mm of mesh size opening) were estimated and modelled by the SELECT model. These analyses included considerations of the sampling proportions of the catch in the codend and cover. Furthermore, the analyses took into account between-haul variation. The l 50 values were 30.8, 29.9, 30.0, and 41.2 cm of total length, respectively, values lower than the estimates obtained from previous studies. The contribution of explanatory variables to the selectivity model was also tested in order to determine the role of mesh size, catch size (in number), and towing speed. Increases in catch size and in towing speed were accompanied by decreases in the l 50 estimates. These results demonstrate how incorporation of subsampling effect and explanatory variables to model between-haul variation can improve selectivity estimates and management of a valuable resource. Keywords: size selectivity, mesh size, subsampling effect, between-haul variation, Merluccius gayi gayi, Chile.Los efectos de submuestreo y variación entre lances en la estimación de la selectividad a la talla de la merluza común (Merluccius gayi gayi)RESUMEN. Usando los datos recolectados en un experimento de selectividad a la talla de merluza común (Merluccius gayi gayi) realizado en el año 2000, se estimaron y modelaron los parámetros de selectividad para copos de cuatro tamaños de malla (100, 110, 130 y 140 mm de tamaño de malla interno) mediante el modelo SELECT. Los análisis incluyeron consideraciones de las proporciones de muestreo de la captura en el copo y en el cubrecopo. Además, los análisis tuvieron en cuenta la variación entre lances. Los valores de l 50 fueron 30,8; 29,9; 30,0 y 41,2 cm longitud total respectivamente, valores menores que los obtenidos en estudios previos. Se probó también la contribución de variables explicatorias al modelo de selectividad, para determinar el aporte del tamaño de malla, el volumen de captura (en número) y la velocidad de arrastre. Los incrementos en el volumen de captura y en la velocidad de arrastre produjeron una disminución en los estimados de l 50 . Estos resultados demuestran cómo, a partir de la incorporación del efecto de submuestreo y de variables explicatorias al modelo con variación entre lances, es posible mejorar los estimados de selectividad y manejar un valioso recurso. Palabras clave: selectividad a la talla, tamaño de malla, efecto de submuestreo, variación entre lances, Merluccius gayi gayi, Chile.

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Selectivity experiments with window codends fished in the Baltic Sea cod (Gadus morhua) fishery

Cited by 21 publications

References 9 publications

Selectivity of fishing gears used in the Baltic Sea cod fishery

Selectivity of fishing gears used in the Baltic Sea cod fishery

Comparing selectivity of a standard and turned mesh T90 codend during towing and haul-back

The effects of subsampling and between haul variation on the size selectivity estimation of Chilean hake (Merluccius gayi gayi)

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