Influence of grid orientation and time of day on grid sorting in a small-meshed trawl fishery for Norway pout (Trisopterus esmarkii)

Abstract

-A lightweight sorting grid was developed to reduce bycatch in the Danish small-meshed trawl fishery (22 mm full mesh in the cod end) for Norway pout in the North Sea. Experimental fishing with the grid demonstrated the possibility to capture Norway pout with only a minimum of unintended bycatch. Fishing with two different grid orientations, backwards and forwards-leaning, in distinct day and night hauls, resulted in an estimated release of between 88.4 and 100% of the total number of haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) entering the trawl. However, bycatch reductions were not significantly different between day and night or between grid orientations, indicating that the grid rejection of haddock and whiting is not influenced by fish behaviour. The loss of the target species, Norway pout, was low (between 5.6% and 13.7%) in comparison with the bycatch excluded, and clearly length dependent. Consequently, loss of target species would vary with the size structure of the population fished. Although results were not statistically significant, length-based analyses indicated that the grid rejection likelihood for particularly smaller Norway pout (<16 cm) was higher when fishing with the forwards-leaning grid during the night; this might be explained by behavioural and visual aspects of the fish-grid encounter process for Norway pout.

Size selectivity of redfish (Sebastesspp.) in the Northeast Atlantic using grid-based selection systems for trawls

“…The estimation of the 95% confidence limits for the lengthdependent average grid escapement, codend escapement, and combined retention likelihood followed the procedure described in Sistiaga et al (2010), Herrmann et al (2012) and Eigaard et al (2011), based on the applied bootstrapping technique.…”

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

“…To avoid underestimating the confidence limits of the model parameters, which could otherwise occur when estimation is based on pooled data (Fryer 1991), we applied a method that uses a double bootstrapping technique to account for both within-and between-haul variation in selection processes (see Sistiaga et al 2010, for details). This bootstrapping technique also takes into account the additional uncertainty arising from working on subsample data, as described by Eigaard et al (2012). The overall selectivity parameters of the total process (i.e., the towing process followed by the haulback process: L50 total and SR total ) were estimated using the numerical method described in Sistiaga et al (2010).…”

“…The overall selectivity parameters of the total process (i.e., the towing process followed by the haulback process: L50 total and SR total ) were estimated using the numerical method described in Sistiaga et al (2010). Estimation of the 95% confidence limits (Efron 1982) for the average escapement and retention for all length classes was performed by calculating e tow (l), e haul−back (l), and r total (l) according to equation (2) in each of the 10 000 bootstrap resamplings used in the estimation method (Eigaard et al 2012). …”

Effect of lifting the sweeps on bottom trawling catch efficiency: A study based on the Northeast arctic cod (Gadus morhua) trawl fishery

“…All the data analyses carried out in this study were performed using the software SELNET (Sistiaga et al, 2010;Eigaard et al, 2011;Frandsen et al, 2011;Herrmann et al, 2012).…”