In situ determination of bottom trawl ground gear contact

Exact definition of towed distance with proper ground gear contact is an essential parameter in bottom trawl swept area estimates of fish abundance. Errors in this key parameter may lead to a first order bias in abundance estimates of demersal fish. The paper describes the results obtained with a new sensor for measuring the exact timing of trawl ground gear bottom contact and departure, as well as monitoring of the ground gear contact during the haul. Measured towed duration with the new system is compared with the traditional survey measures of this parameter on a number of hauls, and the results are discussed with respect to survey bias

Avoidance reactions of ultrasonic tagged cod during bottom trawling in shallow water

Introduction: Observations made by echo sounder (Ona 1988) have shown that cod (Gadus morhua L.) may avoid an approaching survey vessel horizontally and vertically. However, there has been a lack of information on the escapement behaviour of the individual fish. This short note discribes an experiment carried out in a shallow fjord in Northern Norway, where the reactions of single cod towards a small survey vessel during bottom trawling were studied, using ultrasonic tagging technique

Catch comparison between rockhoppers and bobbins ground gear on the Norwegian bottom sampling trawl

In a study aimed at improving the reliability of the Norwegian bottom sampling trawl, comparative fishing between standard bobbins gear and rockhopper ground gear were carried out in the Barents Sea. The catch ratios bobbins/ rockhopper (B/R) for cod and haddock show a higher efficiency for rockhopper than for bobbins, especially for small fish. The catch ratio B/R for cod varied by time of day. Rockhopper seemed to have a higher efficiency compared to bobbins during daytime. The same trend was not found for haddock. These experiments confirm earlier experiments that rockhoppper is more effective than bobbins gear in catching fish close to the bottom. This is particulary true for small cod

Behavior-dependent selectivity of yellowtail flounder (Limanda ferruginea) in the mouth of a commercial bottom trawl

-To improve the efficiency of a commercial bottom trawl for catching yellowtail flounder (Limanda ferruginea), we studied the behavior of individuals in the middle of the trawl mouth. Observations were conducted with a high-definition camera attached at the center of the headline of a trawl, during the brightest time of day in June 2010 off eastern Newfoundland. Behavioral responses were quantified and analyzed to evaluate predictions related to fish behavior, orientation, and capture. Individuals showed 3 different initial responses independent of fish size, gait, and fish density: they swam close to (75%), were herded away from (19%), or moved vertically away from (6%) the seabed. Individuals primarily swam in the direction of initial orientation. No fish were oriented against the trawling direction. Fish in the center of the trawl mouth tended to swim along the bottom in the trawling direction. Only individuals that were stimulated to leave the bottom were caught. Individuals in peripheral locations within the trawl mouth more often swam inward and upward. Fish that swam inward were twice as likely to be caught. Fish size, gait, and fish density did not influence the probability of capture. A trawl that stimulates yellowtail flounder to orient inward and leave the bottom would increase the efficiency of a trawl

Day / night variation in fish directivity in the trawl opening

Still photographs of fish in the mouth area of a bottom trawl were taken by a downwards-oriented automated strobe camera mounted near the headrope. Fish angles relative to the towing direction were measured. Fish were significantly less polarized by night than by day, and in the daytime photographs less polarization was seen at low fish densities than at higher. The results are discussed with regard to fish herding patterns and potential escapement beneath the fishing line of a trawl

Collecting size-selectivity data for Antarctic krill (Euphausia superba) with a trawl independent towing rig

For the development of efficient trawls to minimize catch loss, escape mortality and potential negative ecosystem impacts from the fishery, the understanding about trawl selectivity processes are crucial. Small crustaceans are regarded as being less motile than most fish species. Crustaceans also display low levels of active avoidance from trawl netting, which in turn may cause direct contact with netting on multiple occasions on their passage towards the codend increasing the probability for escapement. Full-scaled experiments to estimate gear selectivity are highly resource demanding and are highly technically challenging for several types of fisheries. In this study, we developed and tested a trawl-independent towed-rig construction designed to investigate size selectivity of Antarctic krill (Euphausia superba). The results indicate that valid selectivity estimates can be obtained using this method, but due to the small sample size, results are inconclusive. However, the findings of the current study show a potential for developing easier and more cost-effective ways of investigating and estimating size selectivity of Antarctic krill and other small crustacean species in trawls

Quantifying the escape mortality of trawl caught Antarctic krill (Euphausia superba)

<div><p>Antarctic krill (<i>Euphausia superba</i>) is an abundant fishery resource, the harvest levels of which are expected to increase. However, many of the length classes of krill can escape through commonly used commercial trawl mesh sizes. A vital component of the overall management of a fishery is to estimate the total fishing mortality and quantify the mortality rate of individuals that escape from fishing gear. The methods for determining fishing mortality in krill are still poorly developed. We used a covered codend sampling technique followed by onboard observations made in holding tanks to monitor mortality rates of escaped krill. Haul duration, hydrological conditions, maximum fishing depth and catch composition all had no significant effect on mortality of krill escaping 16 mm mesh size nets, nor was any further mortality associated with the holding tank conditions. A non- parametric Kaplan-Meier analysis was used to model the relationship between mortality rates of escapees and time. There was a weak tendency, though not significant, for smaller individuals to suffer higher mortality than larger individuals. The mortality of krill escaping the trawl nets in our study was 4.4 ± 4.4%, suggesting that krill are fairly tolerant of the capture-and-escape process in trawls.</p></div