Torfinn Lindem

The Arctic Charr Salvelinus alpinus in Thingvallavatn

The coexistence of four morphs of arctic charr in Thingvallavatn, Iceland, was demonstrated by investigations of morphology, habitat use, diet, endoparasitic fauna, life history, time and place of spawning, early ontogeny, and population genetics. Head morphology in embryos, juveniles and adults revealed the presence of two charr morphotypes, each including two morphs: (1) a benthic morphotype including large benthivorous (LB-) and small benthivorous (SB-) charr, featuring overshot mouth and relatively large pectoral fins, and (2) a pelagic morphotype including planktivorous (PL-) and piscivorous (PI-) charr, featuring termimal mouth and relatively small pectoral fins

Design of a multi-needle Langmuir probe system

The main goal of this work was to develop a Langmuir probe instrument for sounding rockets capable of performing high-speed absolute electron density measurements, and thereby be able to detect sub-meter ionospheric plasma density structures. The system comprises four cylindrical probes with a diameter of 0.51 mm and a length of 25 mm, each operated at a different fixed bias voltage in the electron saturation region. The probe diameter was chosen significantly less than the Debye shielding length to avoid complex sheath effects but large enough to ensure a probe area sufficiently large to accurately measure the electron currents drawn by the probes (in the range 1 nA to 1 µA). The crucial feature of the University of Oslos multi-needle Langmuir probe (m-NLP) is that it is possible to determine the electron density without the need to know the spacecraft potential and the electron temperature Te. The m-NLP instrument covers a density range from ne = 109 m−3 to 1012 m−3, with sampling rates up to 9 kHz. The m-NLP instrument was successfully tested on the ICI-2 (Investigation of Cusp Irregularities) sounding rocket flight from Svalbard on 5 December 2008.

Acoustic assessment of the number of pelagic fish in Lake Kinneret, Israel

The pelagic fish stock (mostly lavnun, Mirogrex terraesanctae) of Lake Kinneret was assessed using HADAS, a newly developed echo-counting analysis system. Total fish numbers (during Mar–Apr) were estimated to be 61 million in 1987, 218 million in 1988, 120 million in 1989, and 69 million in April 1990. The total fish number for March 1981 was recalculated from published data and estimated at 170 million. The decline in total numbers between 1988 and 1990 resulted primarily from a decrease in the relative abundance of the smallest size classes of fish. Since fish abundance apparently declined between 1981 and 1990, the decline in zooplankton standing stocks documented during these years cannot be attributed to higher fish predation caused by larger fish stocks. The HADAS estimates of lavnun population size are large enough to account for the known catch of lavnun.

Density and in situ target strength of herring and sprat: a comparison between two methods of analyzing single-beam sonar data

Abstract We analyzed sonar data obtained with a 70-kHz single-beam echosounder with two methods for removing the beam pattern effect: one based on the Craig and Forbes logarithm; the other based on a z -transformation and deconvolution operation. The sonar data were collected from 4 July to 23 October 1985, in a coastal area of the northern Baltic proper where herring ( Clupea harengus L.) and sprat ( Sprattus sprattus L.) dominated the pelagic fish populations. The two methods gave similar total fish densities, but the proportion of different size classes sometimes differed by up to 30 percentage units. The acoustic data are compared with size distributions and catch per unit effort in simultaneously collected vertical gill-net samples. Size distributions in nets and acoustics were fairly similar, but the relationship between catch per unit effort in gill nets and acoustic density estimates was poor. Target strength to fish-length regressions from the two acoustic methods were similar even though individual estimates differed by up to 4 dB on some sampling occasions. When the slope of the regression is set to 20 we obtained the equation TS =20 log L -69.9 with both methods ( TS is target strength in dB and L is fish length in cm).

Improved fish detection in data from split-beam sonar

Hydroacoustic split-beam techniques have been applied to enumerate salmon migrating in the river Tana (northern Norway) during the summers 1998 and 1999. Analysing data by single echo detection and tracking was difficult. Missing echoes in tracks from fish, combined with noise in the output from the single echo detector was seen as reasons for this. An improved counting method is presented. Contours from moving targets are detected by image analysis. Then, detected single echoes within these contours are combined into tracks. This procedure reduces problems related to noise, and to tracking fish with few accepted single echoes.

Density, length distribution, and diet of age-0 arctic charrSalvelinus alpinus in the surf zone of Thingvallavatn, Iceland

SynopsisPopulation densities of age-0 arctic chaff in the surf zone averaged 1.83 and 4.70 fish m-2 in August 1984 and June 1985, respectively. Length variation of the littoral fish was low in early summer, increasing in late summer and autumn. Newly emerged charr, ∼ 20 mm long, appeared in the stony shallow water zone during both May and June. From length variation and variation in mouth position of the young charr, it is concluded that at least two of the four chaff morphs in the lake are present in the surf zone during spring and summer. In August, some of the larger age-0 charr had moved out from the surf zone, into the pelagic and the deeper epibenthic waters. The food of young littoral charr was dominated by large chironomid larvae (instar 3 and 4) and pupae

The number of pelagic fish in Lake Paasivesi, Finland, monitored by hydroacoustic methods

Abstract A Craig and Forbes algorithm-based computer program was used to make an acoustic estimate of the number of fish in the pelagic zone of Lake Paasivesi. The acoustic data were collected at night. More than 95% of the estimated fish were counted as singlefish echoes. The vertical distribution of the fish in May was uniform from 4 m down to the bottom, but in August about 90% of the fish counted were between 4 and 18 m. The total number of fish in the research area was 1.3 × 10 6 individuals in May and 2.7 × 10 6 individuals in August. The mean fish density was 260 and 530 fish ha −1 , respectively. The horizontal fish distribution in this lake is unusually uniform and the fish density unusually low in comparison with many other lakes in central Finland.

Investigating the Coupling Effects Between CubeSTAR's Communication Antenna and Deployable Payload Sensors [Measurements Corner]

CubeSTAR is a space weather nanosatellite built at the University of Oslo (UiO). Its aim is to measure the absolute electron density in the ionosphere with a spatial resolution that has not previously been possible. The satellite uses deployable sensor booms to isolate the sensors from the plasma wake effects created by the satellite body. Due to the small size of the satellite, the payload booms are situated close to the communication antenna; this may cause a coupling that can influence the directivity of the antenna. In this article, the coupling effects between CubeSTARs communication antenna and deployable payload sensors are investigated. The effects were analyzed through simulation with an emphasis on the change in the antennas directivity in reference to the separation distance between the antenna and the sensor booms.

Habitat use of arctic charrSalvelinus alpinus in Thingvallavatn, Iceland

SynopsisHabitat use by four morphs of arctic charr,Salvelinus alpinus, was investigated in Thingvallavatn, Iceland, by sampling with pelagic and benthic gill nets. Sampling was done in May/June and August/September. Greatest abundance of fish was recorded in the littoral and epipelagic zone in early autumn. Catches were low in early summer. The four morphs are partly segregated in habitat. Small (SB-) and large benthivorous (LB-) charr have a more restricted spatial distribution than piscivorous (PI-), and especially planktivorous (PL-) charr. Both benthivorous morphs are mainly found in the littoral zone, and occur in largest numbers in stony shallows at depths between 0 and 10 m. PL-charr usually dominates in numbers in all habitats. PI-charr is most abundant in epibenthic habitats, although numbers are always low. All morphs are caught in higher numbers at night than during the day, but the diurnal activity difference is highest among SB-charr. The habitat use by different morphs is as may be expected from their morphology and diets. Within the population of PL-charr, young and small fish are more abundant on the bottom than in the pelagic zone, and there is a surplus of females in the pelagic zone. Along the benthic profile, young, small and immature PL-charr are more abundant in deep than in shallow waters. The results are discussed in relation to food supply, competition and predation. Possible reasons for the occurrence of four arctic charr morphs are also discussed.

New single echo detection methods for shallow water fishery acoustics

Acoustic target detection is commonly carried out with parametric single echo detectors. These detectors test one ping at a time and look for echoes fulfilling a set of criteria such as echo duration and shape. In shallow water, noise phenomena can distort echoes from fish and false fish echoes can be generated. This causes the parametric detector to produce fractionated tracks from fish surrounded by numerous noise detections. Parametric detectors utilize only small portions of the information available in a split beam echogram. By including information from more than one ping and from the background reverberation, a more robust fish detector has been designed. This detector, called the Cross Filter Detector (CFD), has now been further improved by applying the variance in the angle measurements.