Estimating codend selectivity and fish escapement from a covernet of an insufficiently small mesh size

Abstract

SUMMARY: This paper describes a method of evaluating virtual codend selectivity with covernet selectivity when the mesh size of the covernet is not small enough to retain all the fish that enter the covernet. If the covernet mesh size is not small enough, small fish can escape through the covernet mesh after passing through the codend, and then their body length proportion retained in the codend will be overestimated. We developed a model in which covernet selectivity can be estimated at the same time as accurate codend selectivity. The validity of the method was examined with data from a covered‐codend experiment on the Patagonian grenadier Macruronus magellanicus collected in the southwest Atlantic. The model fits the data well even though the plots of proportion retained were U‐shaped. For both the codend and covernet selectivity estimated in the model, fish of length close to 100% selectivity had a girth close to mesh perimeter. This supported that both the codend and covernet selectivity estimated in the model were reasonable. The analysis proposed in the present study would be useful to obtain accurate estimates of codend selectivity by evaluating the effects of covernet selectivity on the data.

An actively stimulating net panel and rope array inside a model cod-end to increase juvenile red seabream escapement

“…As found in previous studies, the retention rate of juvenile seabream varied by each factor of the mesh size (O'Neill and Kynoch, 1996;Mituhasi et al, 2000) or light level (Ryer and Olla, 2000). Furthermore, the results of the present study are the first to show that the retention rate by mesh size was dependent on light level, because an interaction between the two factors was identified by ANOVA.…”

“…However, their method for counting the numbers of fish escaping was restricted to the field of view of a video camera and the experiments used quite different marine species. The escape rate as a factor of fish number or catch determined by methods using cod-end covering nets have shown wide variations of up to 90% by factors such as fishing ground, species, and cod-end design (O'Neill and Kynoch, 1996;Lök et al, 1997;Mituhasi et al, 2000;Madsen et al, 2002). Observations of the pulsing motion of the cod-end by O'Neill et al (2003) or of square window netting held slack and fluttering by Grimaldo et al (2007) make no mention of measured escape rate values.…”

Statistical estimation in cover-escape model for covered-codend experiments

“…This assumption is appropriate if the mesh size of the covernet is small enough. Conversely, if the covernet mesh size is not sufficiently small enough and allows the fish escape, then the retention probability of the codend will be overestimated 2 . It is recommended that the mesh size of the covernet should be smaller than half that of the codend 1 .…”

Estimated total abundance and numbers at age of longtail hake (<i>Macruronus magellanicus</i>) in the Southwest Atlantic during the years 1987-2000

“…Fish >30 cm TL were clearly vulnerable to the 20-mm-mesh codends used during the surveys, but smaller fish were caught only very exceptionally. In an analysis of commercially fished longtail hake, Mituhasi et al (2000) discovered that length at 50% retention was 68.4 cm TL with a codend mesh size of 120 mm and 34.0 cm TL with a mesh size of 60 mm. If winter and spring are the spawning seasons (Inada, 1983;Bezzi, 1984;Janusz, 1987;Giussi, 1993), the youngest fish would have been 3-7 months old at the time the surveys were carried out, making their capture rather unlikely.…”