Arill Engås

The ‘MultiSampler’: a system for remotely opening and closing multiple codends on a sampling trawl

Abstract A system for remotely opening and closing multiple codends on pelagic sampling trawls has been developed in Norway. The system makes it possible to take several uncontaminated samples during discrete, user-selected periods within a single trawl haul, using conventional large-scale trawl gear. The system has been used routinely to take samples of acoustically detected fish during several hydroacoustic surveys assessing herring in northern Norway. The results obtained with this system show that the length composition of herring throughout the water column is not homogeneous, which was previously assumed when interpreting the catch results obtained with ordinary pelagic trawls. Such new data can be used to improve stock abundance estimates.

Reactions of penned herring and cod to playback of original, frequency-filtered and time-smoothed vessel sound

Abstract The behavioural responses of penned cod ( Gadus morhua L.) and herring ( Clupea harengus L.) to playback of original, frequency-filtered and time-smoothed sound recordings from a trawler were tested. Avoidance reactions of both cod and herring were observed during playback of the original, 60–300 Hz and 300–3000 Hz spectra, but hardly to the 20–60 Hz spectrum. The duration of responses of cod was longer to original than to time-smoothed sound. The conclusion is that the main determinant for triggering avoidance reactions of cod and herring is vessel sound level within the most sensitive frequency region, although other sound characteristics, such as temporal structure, also seem to be of importance.

EFFECTS OF SEISMIC SHOOTING AND VESSEL-GENERATED NOISE ON FISH BEHAVIOUR AND CATCH RATES

Carlson, T.J. & Popper, A.N. (1997). Using Sound to Modify Fish Behaviour at PowerProduction and Water-Control Facilities: A Workshop. Phase II Final Report. DOE/ BP 62611-11, U.S. Department of Energy, Bonneville Power Administration. Portland, OR. Coutant, C.C. (2001a) Behavioural Technologies for Fish Guidance. American Fisheries Society, Bethesda, MD. Coutant, C.C. (2001b) Turbulent attraction flows for guiding juvenile salmonids at dams. In Behavioural Technologies for Fish Guidance (C. C. Coutant, ed.). American Fisheries Society, Bethesda, MD. Kynard, B. (1984) Studies of downrunning adult alosids in the Holyoke Dam canal system 1983. Final Report to Northeast Utilities Service Company, Berlin, CT. Mann, D.A., Higgs, D.M., Tavolga, W.N., Souza, M.J. & Popper, A.N. (2001) Ultrasound detection by clupeiform fish. J. Acoust. Soc. Am. 109, 3048-3054. Odeh, M. (1999) Innovations in Fish Passage Technology. American Fisheries Society, Bethesda, MD. Office of Technology Assessment (1995) Fish Passage Technologies: Protection at Hydropower Facilities. U.S. Congress, Office of Technology Assessment Report OTA-ENV-641, U.S. Government Printing Office, Washington, D.C. Ploskey, G.R., Schilt, C.R., Hanks, M.E., Skalski, J.R., Nagy, W.T., Patterson, D.S. & Kim, J. (In preparation). Hydroacoustic evaluation of fish passage at Bonneville Dam in 2000. Technical Report EL of the U.S. Army Research and Development Center, Waterways Experiment Station, Vicksburg, MS. Popper, A.N. (1995) Fish Sensory Responses: Prospects for Developing Behavioural Guidance Technologies. U.S. Congress, Office of Technology Assessment Report OTA-ENV-641, U.S. Government Printing Office, Washington, D.C. Popper, A.N. & Carlson, T.J. (1998). Application of sound and other stimuli to control fish behavior. Trans. Am. Fish. Soc. 127, 673-707.

Baiting gill nets—how is fish behaviour affected?

Abstract Movements of acoustically tagged cod were tracked in a fjord in northern Norway using a stationary telemetric positioning system. Having observed the basic movement patterns under the prevailing natural conditions, an experimental fleet of gill nets having baited and non-baited nets was introduced. The nets were fished for 12 nights and swimming behaviour of the tagged fish towards the baited and non-baited nets compared. Baiting resulted in increased number of encounters with the gear, but this did not significantly increase catch rates. The majority of fish swam slower in the vicinity of baited as compared to non-baited nets, but did not stay for a longer time at baited than at non-baited nets following an encounter. The mesh size and texture of the bait bags may be crucial for releasing strong responses leading to netting.

Size Selection of Antarctic Krill (Euphausia superba) in Trawls

Trawlers involved in the Antarctic krill (Euphausia superba) fishery use different trawl designs, and very little is known about the size selectivity of the various gears. Size selectivity quantifies a given trawls ability to catch different sizes of a harvested entity, and this information is crucial for the management of a sustainable fishery. We established a morphological description of krill and used it in a mathematical model (FISHSELECT) to predict the selective potential of diamond meshes measuring 5–40 mm with mesh opening angles (oa) ranging from 10 to 90°. We expected the majority of krill to encounter the trawl netting in random orientations due to high towing speeds and the assumed swimming capabilities of krill. However, our results indicated that size selectivity of krill is a well-defined process in which individuals encounter meshes at an optimal orientation for escapement. The simulation-based results were supported by data from experimental trawl hauls and underwater video images of the mesh geometry during fishing. Herein we present predictions for the size selectivity of a range of netting configurations relevant to the krill fishery. The methods developed and results described are important tools for selecting optimal trawl designs for krill fishing.

A comparison between vessel and trawl tracks as observed by the ITI trawl instrumentation

Instrumentation measuring the tracks of a bottom trawl and a towed underwater vehicle relative to the track of the towing vessel was tested under varying towing conditions during a research survey in the Barents Sea in 1998. The measured mean distance between vessel and trawl track (centre of headline) varied from 27 to 213 m during the 15 hauls for which measurements were obtained. The recorded maximum distance between the tracks was 336 m. The deviance appeared to depend on vessel heading relative to wind and current and on warp length. The magnitude of the measured distance between vessel and trawl tracks may strongly affect abundance estimates based on data from trawl catches. Depending on the avoidance reactions of different species and length groups to the approaching vessel, the track of the trawl relative to that of the vessel might influence both species and size composition of the catch. Experiments quantifying the relationship between bottom trawl catches and acoustic recordings under the vessel will be particularly exposed to errors when trawl and vessel paths diverge. The magnitude of escape responses behind a vessel during trawling has been reported to vary within limited periods and over-restricted geographical areas. These escape responses have been documented using a towed underwater vehicle without knowing the exact position of the vehicle with regards to vessel and trawl. Measurements of the positions of the vehicle with regard to the position of the vessel and trawl during this experiment showed substantial variation both between and within hauls, indicating that at least part of this variability in fish behaviour could be due to the variation in position of the vehicle.

Effects of hanging ratio and fishing depth on the catch rates of drifting tuna gillnets in Sri Lankan waters

Abstract Comparative fishing trials for tuna with drifting gillnets in Sri Lankan waters tested whether a reduction of the hanging ratio ( E ) from the currently used 0.6 to 0.5 would increase catch rates. The effect of varying fishing depth was also studied, using three different pendent line lengths (1, 6 and 8 m). Tuna (frigate, skipjack and yellowfin) and sharks made up approximately 95% by number and 80% by weight of the catches. Nets with E = 0.5 resulted in 40% higher overall catch by weight per unit netting area than nets with E = 0.6. The higher catch rates were mainly due to more large skipjack and yellowfin tunas being caught by tangling. Even when catches per net were compared, the E = 0.5 nets gave 25% higher catches by weight, despite the net area being 10% less due to the reduced hanging ratio. The fishing trials further indicated that pendent lines exceeding 6 m will lower catch rates compared with shorter pendent lines. This is caused by declining skipjack catches as fishing depth increases.

Effects on catch rates of baiting gillnets

In this paper we investigate whether the use of bait on gillnets increases catch rates. Experiments were conducted during ordinary fishing in Norwegian waters for three different species; cod, ling and Greenland halibut. Each third gillnet of a fleet consisting of 24 nets was baited with mackerel. Catch rate of this experimental fleet was compared with a similar fleet without bait. The baited nets gave a significantly higher catch for all three species, with an increase of 61%, 23% and 36% for cod, ling and Greenland halibut, respectively. On the baited fleet, the catch rates were highest on the baited nets. For cod and saithe the non-baited nets on a baited fleet had higher catch rates than the (non-baited) nets of the control fleet.

Deep Vision: An In-Trawl Stereo Camera Makes a Step Forward in Monitoring the Pelagic Community

Ecosystem surveys are carried out annually in the Barents Sea by Russia and Norway to monitor the spatial distribution of ecosystem components and to study population dynamics. One component of the survey is mapping the upper pelagic zone using a trawl towed at several depths. However, the current technique with a single codend does not provide fine-scale spatial data needed to directly study species overlaps. An in-trawl camera system, Deep Vision, was mounted in front of the codend in order to acquire continuous images of all organisms passing. It was possible to identify and quantify of most young-of-the-year fish (e.g. Gadus morhua, Boreogadus saida and Reinhardtius hippoglossoides) and zooplankton, including Ctenophora, which are usually damaged in the codend. The system showed potential for measuring the length of small organisms and also recorded the vertical and horizontal positions where individuals were imaged. Young-of-the-year fish were difficult to identify when passing the camera at maximum range and to quantify during high densities. In addition, a large number of fish with damaged opercula were observed passing the Deep Vision camera during heaving; suggesting individuals had become entangled in meshes farther forward in the trawl. This indicates that unknown numbers of fish are probably lost in forward sections of the trawl and that the heaving procedure may influence the number of fish entering the codend, with implications for abundance indices and understanding population dynamics. This study suggests modifications to the Deep Vision and the trawl to increase our understanding of the population dynamics.

Quantifying the Escape Mortality of Trawl Caught Antarctic Krill (Euphausia superba)

Antarctic krill (Euphausia superba) 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.