Selectivity Of Codends Research Articles

Simulation-based investigation of the paired-gear method in cod-end selectivity studies

In this paper, the paired-gear and covered cod-end methods for estimating the selectivity of trawl cod-ends are compared. A modified version of the cod-end selectivity simulator PRESEMO is used to simulate the data that would be collected from a paired-gear experiment where the test cod-end also had a small mesh cover. Thus, estimates of the selectivity parameters of the test cod-end can be made using both the paired-gear method and the covered cod-end method. These estimates are compared and, as it is assumed that the covered cod-end method is objective, we conclude that the paired-gear method is biased. We demonstrate that extreme parameter estimates as well as discrepancies between the paired-gear and covered cod-end experiments do not necessarily reflect physical or biological mechanisms. We believe that this phenomenon may help explain cases in the literature where the covered cod-end and paired-gear methods produce different estimates of cod-end selectivity.

Selectivity of large mesh trawl codends in the Gulf of Maine: I. Comparison of square and diamond mesh

Selectivity properties of five codend types were evaluated using the covered codend method. The codends tested were 152, 165 and 178 mm diamond mesh codends, and 165 and 178 mm square mesh codends. A new hydrodynamic codend cover, which was expanded by 12 water-borne kites, was developed, tested and used during the codend mesh selectivity studies. Eighty-six successful tows were completed for the five codends. This paper describes results for two species of roundfish – Atlantic cod ( Gadus morhua) and haddock ( Melanogrammus aeglefinus) and four species of flounders – American plaice ( Hippoglossoides platessoides), yellowtail flounder ( Limanda ferruginea), witch flounder ( Glyptocephalus cynoglossus) and winter flounder ( Pseudopleuronectes americanus). The results indicate that larger codend mesh sizes retained larger fish for all species for both diamond and square mesh codends. The mesh shape (diamond or square) has no significant effect on the 50% retention length ( L 50) for roundfish ( P > 0.1) while the square mesh codends have significantly smaller L 50s for flounders ( P < 0.001). The square mesh codend has a narrower selection range (SR) than the corresponding diamond mesh codend for both cod and haddock.

Selectivity of PE and PA material codends for rose shrimp ( Parapenaus longirostris) in Turkish twin rigged beam trawl fishery

This study investigated the differences in the selective properties of 32 mm nominal polyethylene (PE) and 32, 36, 40 and 56 mm nominal polyamide (PA) codends in beam trawl fisheries targeting rose shrimp, Parapenaeus longirostris, in the Sea of Marmara, Turkey. Data were collected in spring 2005 onboard a commercial trawler, on commercial fishing grounds. Selectivity data were collected using the covered codend method in two twin beam trawls in every haul. Using this method, four selection curves could be obtained in one haul. Selectivity parameters were calculated by fitting a logistic function using maximum likelihood. The L 50 values of 32, 36 and 40 mm PA codends were found to be 9.1, 10.2 and 12.6 cm, respectively. Due to retention of only a few specimens, realistic selectivity parameters could not be calculated for the 56 mm PA codend. Mean L 50 decreased with a change in mesh material from PA to PE. The lowest L 50 (8.0 cm) was found for the 32 mm PE material codend. This study has shown that for rose shrimp the 32 mm PA diamond mesh codend is not appropriate for the first maturity size (FMS) of 10 cm for rose shrimp in the Turkish twin rigged beam trawl fishery. In addition, the PE netting material provides a significantly lower L 50 value than the PA netting material.

Effect of catch size and shape on the selectivity of diamond mesh cod-ends

Abstract An individual-based model that simulates fish selection processes in diamond mesh cod-ends of towed fishing gears is outlined. The model is implemented in a computer program called PRESEMO. A typical simulation can be carried out within a few minutes on a personal computer. Up to four different populations of fish entering the cod-end during a tow can be accounted for. Each fish is assigned a weight, girth, width and height according to its length, and is assumed to have an elliptical cross-section. Fish are allocated a period of travel time down the cod-end, a period for swimming in the cod-end without being exhausted, a period between escape attempts and a packing density for those swimming ahead of the catch. An escape attempt is deemed successful if a fish can pass through the mesh opening at the position in the cod-end where the escape attempt takes place. The mesh opening value is obtained from information on the shape of the cod-end, which depends on the catch weight. The cod-end shape is updated dynamically as the catch builds up during the tow. During a simulation the selection process is continually visualized, that is, the entry, movement and escape attempts of individual fish are shown as well as the changes in the cod-end geometry. At the end of a simulation, a logistic function is automatically fitted to the selection data to obtain estimates of the 50% retention length and the selection range.

Effect of catch size and shape on the selectivity of diamond mesh cod-ends

Abstract A series of computer simulations were carried out to predict how the selectivity of haddock in a diamond mesh cod-end varies according to total catch, by-catch, entry time of by-catch and the shape of the catch build-up in the cod-end. Results were compared to those from sea trials. I also investigated the predicted selectivity on the assumption that the shape of the cod-end does not change as the catch accumulates. The latter investigations were carried out for various shapes of the cod-end. The simulations indicated that the 50% retention length (L50) increases with both the total catch and the by-catch weight. They also indicated that the entry time of the by-catch may have an important influence on both the 50% retention length and selection range (SR). The shape of the catch in the cod-end had a similar effect, for the cases analyzed. Different cod-end shapes showed very large differences in the values for L50 as well as more narrow SRs, assuming that the shape of the cod-end remained constant during the tow. The simulations strongly indicate that the change of shape of the diamond mesh cod-end as the catch builds up during towing is a major contribution to the SR.

The kite cover: a new concept for covered codend selectivity studies

The covered codend method is that most commonly used for estimating the size selectivity of codends for towed fishing gears. A codend cover held open by kites has been developed to improve the performance and handling properties compared to the hooped covers normally used. The new cover can be wound directly onto the net drum as the catch is hauled. The development and testing of the cover in a flume tank, where it was compared to a hooped cover, are described as well as the practical experience obtained when using the cover at sea.

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

The declining cod stock in the Baltic Sea has highlighted a need for the use of more selective fishing gears. From 1995, the use of 105-mm `Danish' and `Swedish' designed escape windows was adopted into legislation in the Baltic Sea, as alternatives to a mesh size increase from 105 to 120 mm. As the legislation was based on very few experiments, one objective of this work was to collect more selectivity data on a `Danish' window codend. The second objective was to test if black netting sections could be used as contrasting colours to manipulate the cod behaviour, and further improve the escape through the window. Experiments were conducted from a commercial vessel. The selectivity of window codends with contrasting colours and conventional green netting, were measured simultaneously using hooped codend covers and a twin trawl rig. Comparative fishing experiments were then conducted using uncovered codends. No significant improvement in the escape of cod was found for the three different types of codends with contrasting colours tested in the experiments. The overall selection factor for cod estimated by combining 25 hauls for a codend with a 115-mm square mesh `Danish' window was 2.84. This is lower than estimates from other experiments with window codends in the Baltic Sea. A significant, but relatively weak, codend catch size effect on the selectivity parameters was found.

Size selectivity of diamond and square mesh cod-ends for Nephrops norvegicus in the Aegean Sea

Trawl cod-end mesh selection for Nephrops norvegicus was studied in two experiments in the western Aegean Sea using the covered cod-end method (cover: diamond-shaped, 10 mm). The following cod-end mesh sizes were used: 14 mm (knot-to-knot) diamond-shaped (14D), presently operated in the Hellenic commercial trawl fishery; 20 mm square-shaped (20S); 20 mm diamond-shaped (20D), this being the mesh size proposed by the European Union (EU) for the entire Mediterranean. Selectivity estimates were derived from pooling the data over all hauls and sampling dates. The results showed that the proportion of the population retained was higher for 14D than for 20D and 20S (0.99, 0.79 and 0.84, respectively). The direct estimation of the 50% retention length for 14D was not possible because of the very small number of specimens escaping through that cod-end. The results also suggested that 20S has a higher and sharper selectivity than 20D (higher 50% retention length and selection factor, lower selection range) and retains fewer undersized individuals than 20D. Yet, the 50% retention lengths for 20D and 20S were both lower than the length at 50% maturity, a fact indicating that even the use of 20S and 20D may provide little opportunity for reproduction.

Size selectivity of diamond and square mesh codends for four commercial Mediterranean fish species

In the present study, we compared the size selectivity of the 14 mm (knot-to-knot) diamond-shaped codend (14D), presently used in Hellenic waters, with those of 20 mm square-shaped (20S) and 20 mm diamond-shaped (20D) codends for four commercially important demersal species (Merluccius merluccius, Micromesistius poutassou, Trisopterus minutus capelanus and Lepidorhombus boscii). Codend selectivity was estimated using the covered codend method. Our results suggested that 14D was not selective for any of the target species and the proportion retained by 14D ranged between 0.88 and 1. In addition, only in the case of M. merluccius did the results suggest that 20S is significantly more selective and retains fewer under-sized fish than 20D. With the exception of M. merluccius, the direct estimation of the 14D L50 values was not possible, because of the small number of specimens retained by the cover codend of 14D. The L50 value of M. merluccius for 14D was lower than its currently enforced minimum landing size (MLS) and length at 50% maturity. In contrast, for 20S and/or 20D codends, even though the estimated L50values were generally close to, or higher than, the species’ MLS, the L50 value of 20S for M. merluccius and that of 20D for L. boscii, were much smaller than their lengths at 50% maturity, a fact indicating that even the use of 20S and 20D, respectively, provides small opportunities for reproduction. Hence, the appropriate mesh sizes for these two species must be greater than 20 mm and their MLS must be raised. In contrast, forT. m. capelanusand M. poutassou, the L50values of 20D (but not of 14D) were close to their lengths at 50% maturity, a fact indicating that 20D is appropriate for these two species. The above mentioned facts clearly showed that: (a) 14D is harmful, in terms of size of landed fish, for the Hellenic demersal stocks and hence its use should be discontinued and (b) the use of a single appropriate mesh size for the Mediterranean trawl fishery as a whole is impossible because of its multispecies nature and the diVerent body sizes of species involved. Since the majority of the individuals escaping through the meshes of 20D or 20S are immature and under-sized (i.e. their length is smaller than MLS), the replacement of 14D by either 20D or 20S, the former being the codend mesh size proposed by EU for the entire Mediterranean, will be accompanied by increased yield per recruit. ? 1997 International Council for the Exploration of the Sea

Nonparametric maximum likelihood estimation of mesh size selectivity

A nonparametric model for estimating mesh size selectivity is presented based on a multinomial likelihood for sampling from a mixture of discrete size classes. No functional form for the selectivity curve is assumed when comparing the relative selectivity of standard and experimental mesh codends. Auxiliary information from the size distribution of fish gilled in the experimental mesh codend is used to identify when the selectivity approaches 1. Analysis is illustrated for Dover sole ( Microstomus pacificus) caught in an alternate haul study. Parametric methods of estimating selective curves based on logistic functions showed tendencies for systematic bias in the tail regions as well as possible errors in the mid-range (approximately 50%) of selectivity.

Size Selectivity of Diamond and Square Mesh Codends in Pelagic Herring Trawls: Only Small Herring Will Notice the Difference

A twin codend trawl was fished in the northern Baltic to study the size selectivity of square mesh and diamond mesh codends of 36-mm nominal mesh size. For each codend, 15 hauls were completed with a small mesh (20 mm) codend deployed on the other side of the trawl. The relative size of the catches in the two sides of the trawl varied considerably from haul to haul (the separator section was not operating properly) and selection curves were estimated from each individual haul using a method that incorporated the differences in catching efficiency of the two sides. The length of 50% retention decreased with increased catch for both the diamond and square mesh codends, although in neither case was this relationship statistically significant. Selection curves fitted to the combined haul data were asymmetric. The square mesh codend retained significantly less small herring than the diamond mesh codend, and for larger herring the two codends had similar selectivity. In both codends, most escapes occurred at the front of the catch bulge, from the upper side of the codend. At high catch rates, mesh blockage was observed for several metres ahead of the catch bulge during the later part of the tow.

Selectivity of square mesh codends of pelagic trawls for Atlantic mackerel (Scomber scombrus L.)

Abstract To assess possible management strategies for conserving small, immature Atlantic mackerel ( Scomber scombrus L.), selectivity trials were undertaken in the western English Channel aboard F.V. Marbella (H99) during the period 24 January to 6 February 1990. An experimental codend of 60 mm polypropylene knotless square netting was attached to a 3700 Ijmuiden stores midwater trawl (Type No. P.101.B) used with superkrub doors. This experimental gear was fished against a similar trawl fitted with a codend constructed from 40 mm knotted nylon mesh rigged in the conventional diamond configuration. A total of 14 successful tows were made, nine using the trawl fitted with the experimental codend and five using the standard trawl. An estimated total catch of some 470 tonnes of mackerel was taken using both gears, and mackerel were caught on all tows except one. The size composition of the mackerel caught ranged from 18 to 37 cm, well within the expected selection range for the gear under test. A comparison of the length-frequency distributions indicated that there was no difference in the size composition, and hence selection, of fish taken by the two gears.

Selectivity experiments on sole with diamond and square mesh codends in the Belgian coastal beam trawl fishery

The selectivity of diamond and square mesh codends in the Belgian coastal beam trawl fishery for sole were compared. Two experiments, involving two different codend lengths were carried out. Contrary to the results obtained with roundfish, no influence of the mesh shape on the codend selectivity could be shown for the flatfish species investigated. It is suggested that the application of square meshes may reduce the catch of small roundfish while maintaining the catch level of the target flatfish species. The length of the codend did not influence the selectivity for sole, but a reduction in benthos by-catch was observed. The selectivity for sole in the coastal beam trawl fishery seems to be lower than the selectivity of the beam trawls used by larger vessels operating in the open sea. This is in agreement with the results of similar experiments, but there is no evidence that the selection factor depends on the amount of bycatch.

A comparison of size selection of haddock and whiting by square and diamond mesh codends

When loaded with fish the codend of a fishing net is pulled into a bulbous shape. Fish escape mainly through the relatively small area of open meshes at the front of the bulb, and forward of this most of the meshes are stretched and closed. If the codend netting is hung so that the meshes are square they remain open during towing. Comparative fishing experiments withtrawls and seines to investigate the relative size selectivity of square and conventional diamond mesh codends for haddock and whiting are described. Several mesh sizes of each type were tested. It was found that the 50 % fish retention lengths for square mesh codends were larger than those for diamond mesh codends of the same nominal mesh size. For haddock, the selection ranges of square mesh codends on trawls were narrower than those of diamond mesh codends; on seines the ranges were similar. For whiting, the selection ranges of square and diamond mesh codends on trawls were similar but wider for square mesh codends on seines

Selectivity of Four Trawl Cod Ends toward Smelt

Three short-term experiments on selectivity of cod ends with various mesh sizes for smelt, Osmerus mordax (Mitchill), were conducted in Lakes Ontario and Erie during 1961. In each experiment the paired tow method was used. Four cod ends were attached alternately to a “Western bottom trawl” fished by the MV Erie I. Estimates of percent retention within 0.3-inch length strata were obtained from a comparison of length-frequency distributions in catches made by the different cod ends fishing approximately the same population of smelt on the same date. Additional information was gained from the weight of catches by different cod ends. Under the conditions of the experiment, cod end mesh sizes of 1 3/16 to 1 3/8 inches appeared most suitable for smelt trawling given market preferences as they existed in October 1961.