Terje Jørgensen

Life history and ecology of the gadoid resources of the Barents Sea

Abstract Knowledge of the distributional patterns, reproduction, early life history, recruitment, growth, stock-size fluctuations and ecology of the main gadoids of the Barents Sea and neighbouring coastal areas is reviewed. Today, approximately one-third of the annual fish catch of ∼ 3 million tonnes in these waters is gadoid fish, primarily Atlantic cod, haddock and saithe. A comparative analysis of the still limited information on the biology of the large gadoids indicates that differences in life histories and ecological strategies have evolved which tend to minimize inter-gadoid interactions. Intra-specific interactions may be more significant, although the available data do not allow assessment of their regulatory power. The understanding of the gadoids as elements of the Barents Sea food web is also highly limited. Accordingly, it is suggested that future research on gadoids should primarily focus on their relationships with other co-occurring species and on intra-specific regulatory mechanisms. Since selective exploitation by man affects important characteristics of the ecosystem (e.g., diversity and age/size structure of populations), such research may prove essential for a sound management of the resources.

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.

Variable swimming speeds in individual Atlantic cod (Gadus morhua L.) determined by high-resolution acoustic tracking

Abstract Although several studies have determined swimming speeds for Atlantic cod (Gadus morhua L.) using acoustic telemetry, none have reported high-resolution tracking (high sampling frequency) for individual fish in order to obtain more accurate estimates of swimming speeds and to quantify within- and between-individual variation. In this study, in situ swimming speed of acoustically tagged Atlantic cod were recorded with high resolution (position fixing every 17 s) throughout the diel cycle during summer in a fjord in northern Norway using a stationary positioning system. The frequency distributions of swimming speeds are presented for individual cod (~30–60 cm). The high-resolution tracking technique revealed higher swimming speeds than previously reported for cod. Swimming speeds below 1.0 body lengths (bl) s−1 were most common (>70%), with only 4–7% of the recordings above the sustained swimming speed for this species. The observation that cod employed a blend of swimming speeds could reflect that searching for and capturing different prey types require a mix of swimming speeds.

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.

Walking speed and area utilization of red king crab (Paralithodes camtschaticus) introduced to the Barents Sea coastal ecosystem

The red king crab (Paralithodes camtschaticus) was introduced in the Barents Sea in the 1960s and soon established a viable population. Proper management and exploitation of the Barents Sea king crab stock require better understanding of the spatial dynamics at different scales. This study examines the small-scale movement patterns of seven adult male crabs tracked for a period of up to one month from mid July to mid August at 150 m depth in a semi-enclosed fjord on the Russo-Norwegian border. The crabs were tagged with acoustic transmitters and their movements monitored with an acoustic positioning system. Low walking speeds (<0.01 m s−1) were most frequent but the crabs could move at a maximum speed of 0.15 m s−1 and walk an actual distance of up to 270 m over a period of one hour. However, the crabs usually moved within a relatively restricted area with mean hourly longest rectilinear distance varying from 26 to 64 m. The crabs alternated between periods of low and high activity, which could reflect feeding in and movements between food patches. The lack of a diel activity rhythm may be due to high light levels during the polar summer night, or a chemically mediated food search strategy.

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.

Horizontal and Vertical Odor Plume Trapping of Red King Crabs Explains the Different Efficiency of Top- and Side-Entrance Pot Designs

Abstract Interactions between the food search behavior of the red king crab Paralithodes camtschaticus and pot design and the consequences for entry success were studied in situ with a square pot with two funnels on opposite sides and a conical pot with one vertical funnel at the top. Red king crabs that approached the pots upcurrent were chemically stimulated and appeared to be locked onto the odor plume, whereas those that approached the pots across-current showed more flexible search behavior. The location of the funnels meant that entry also requireed a vertical search phase. Forty percent of the red king crabs encountering the pots performed vertical searches on each type of pot, but the probability of entry once a vertical search had commenced was 20 times as high for the square pot as for the conical pot. Chemically stimulated red king crabs limited their vertical search to the bait plume. The location of the bait relative to the entrance may have caused chemically stimulated rheotaxis to lead red ...

Species-specific residence times in the aft part of a pelagic survey trawl: implications for inference of pre-capture spatial distribution using the Deep Vision system

Species-specific residence times in the aft part of a pelagic survey trawl: implications for inference of pre-capture spatial distribution using the Deep Vision system Melanie J. Underwood*, Shale Rosen, Arill Engås, Terje Jørgensen, and Anders Fernö Department of Biology, University of Bergen, PO Box 7800, NO-5020 Bergen, Norway Institute of Marine Research, PO Box 1870, Nordnes, NO-5817 Bergen, Norway *Corresponding author: tel: þ47 941 27 085; e-mail: melanie.underwood@imr.no.