Catch quality of Northeast Atlantic cod (Gadus morhua) caught by bottom trawl and gillnet – Effects of changes in gear design and fishing practices

Abstract

<p>Cod caught with bottom trawls and gillnets have often been associated with poor and variable quality, and little is known about how changes in gear design and fishing practices may influence the quality of fish caught by these two fishing gears. It is difficult, if not impossible, to improve catch quality if fish are damaged during the capture process. Therefore, preventing the deterioration of the catch during the capture process is of utmost importance in improving the quality of caught cod. This thesis studied the effects of changes in gear design and practice in bottom trawl and gillnet fisheries on the catch quality of Northeast Atlantic cod (<i>Gadus morhua</i>). <p>Paper I evaluated the improvement in catch quality of cod caught using a dual sequential codend compared with a conventional codend by evaluating residual blood levels in cod fillets as measured by hemoglobin concentration. No significant differences were found in the residual blood levels of fish caught by the two different codends. Whether the use of a dual sequential codend mitigated the effect of postponed bleeding compared with a conventional codend was also investigated. It was found that postponed bleeding influenced the residual blood levels in caught cod, and that this effect was similar whether the fish were caught using the conventional or sequential codends. <p>Another solution that potentially could improve catch quality is to turn the direction of the codend netting 90 degrees (T90) perpendicular to the towing direction. Paper II compared the amount and severity of external catch-related damage and the residual blood levels in muscle between cod caught using a conventional codend configuration (a sorting grid followed by a diamond-mesh (T0) codend) and a T90-codend without a grid. No significant differences were found between fish caught by the two different trawl configurations. <p>Paper III compared the color and amounts of residual blood in cod loin captured by trawling both with and without buffer towing. Further investigation looked at whether buffer towing affected the shelf life of fish, measured as total volatile basic nitrogen (TVB-N) levels of thawed cod. The results showed that cod exposed to buffer towing had slightly, but non-significant increased redness and hemoglobin concentrations in the loin. No significant differences in TVB-N levels during chilled storage were found. <p>In Paper IV, an experimental gillnet study was conducted to investigate the effect of soaking time on fish survival and residual blood levels, measured as muscle hemoglobin concentration. The results showed that longer soaking times led to lower survival rates and increased fillet redness. <p>Previous information on the blood distribution in cod fillets is scarce. This issue was therefore addressed in both Papers I and IV, which clearly demonstrated that belly had significantly higher levels of residual blood than the loin and tail.