Aggression and growth of different populations of Atlantic salmon parr

In order to study aggression and growth of salmon parr of different populations and the possible effect of fin-clipping on these traits hatchery reared parr originating from three populations (2 Norwegian, 1 Swedish) were stocked in three aquaria with 90 unmarked fish in each. Three aquaria were stocked with a mixed group of 30 each of the populations. These fish were fin-clipped (adipose or pelvic fins) in order to enable identification of groups. Observation time was 15 min/day/aquarium, altogether 28 days of observation were made. Total experimental time was 8 weeks. Significant differences in agonistic behaviour as well as in growth (p<0,001) was found between the populations. An effect of fin-clipping could also be detected in this study. At least one fish per aquarium established dominancy for part of the time. Most of the dominants in the mixed groups came from the same population

Behaviour studies on fish reaction to long lines

This study is a continuation of a field investigation on the behaviour of fish towards long lines, started in 1976. The present paper gives an analysis of the effect of current and time of the day on the activity of whiting and haddock. Further, the influence of hook size and shape on hooking probability are shown

Numerical domination and herring migrations

There is accumulating evidence in favour of the hypothesis that herring migrations are influenced by social learning. The “adopted-migrant hypothesis” postulates that recruit spawning herring learn migration patterns by schooling with older individuals. However, this learning can be interrupted if the stock is unstable or if there are lack of overlap between recruits and the adult stock. There have been five reported changes in the location of the wintering area of Norwegian spring spawning (NSS) herring during the last 50 years. These changes co-occur with the recruitment of relatively strong year classes to the spawning stock. Simulations of schools containing naïve and experienced fish have shown that when abundant enough, naïve individuals repel guidance from a minority of experienced individuals. This process is referred to as numerical domination. We argue that numerical domination obstruct social learning from adults to recruits and plays a key role in establishing new wintering areas in NSS herring

The effect of shading in pen rearing of Atlantic salmon (Salmo salar)

A full scale pen rearing experiment with covers to reduce illumination was carried out. No significant effects were observed on growth, mortality, ectoparasite infection, maturation or net pen fouling

Feeding ecology of Atlantic mackerel (Scomber scombrus) in the Norwegian Sea

Previous studies on Atlantic mackerel (Scomber scombrus) feeding during the highly migratory summer period are scarce. In this study we present detailed diet analyses and investigations of prey selection for Atlantic mackerel in relation to zooplankton distribution and hydrographical conditions in the Norwegian Sea in July 2004. A wide variety of prey organisms were found in the stomachs of mackerel in all water masses. According to dry weight was Limacina retroversa the principal prey in Coastal and Atlantic water masses. L. retroversa was presumably abundant in the Norwegian Sea and taken as secondary prey in the quest for more nutritious prey. Calanus finmarchicus was less important in all water masses than previously documented, also when combined with calanoidae copepod remainders, indicating that their descend to overwinter in deeper waters had already started in late July. Mackerel showed a size selective feeding behaviour and preferred larger prey species over smaller prey. Small copepods (< 1 mm) were numerous in the zooplankton samples in all water masses, especially Coastal and Atlantic, but were not present in the stomach samples. Furthermore, the three largest copepodite stages of C. finmarchicus (CIV – CVI) were selected over the three smallest stages (CI – CIII). Pronounced prey and size selection strongly suggests that mackerel performed particulate feeding. The potential feeding competition between mackerel and herring was also investigated. There was a clear difference in the diets of the two species. C. finmarchicus and L. retroversa were the main prey of mackerel, whereas herring preferred euphausiids and amphipods. These latter species normally swim in deeper waters and only enter the upper layers for feeding when protected by the dark. Moderate feeding by herring at the end of their feeding season and more active feeding by mackerel could explain the pronounced differences in the diet and lack of feeding competition between these two important planktivorous species

Impact of herring spawning behaviour on acoustic abundance estimates

Spawning dynamics of Norwegian spring spawning herring was studied in south western Norway 29 March to 3 April 2000 using hydroacoustics. The horizontal distribution of the spawning layers shifted in a south-easterly direction during the study period indicating directional spawning. A diurnal spawning pattern was found, with layers of spawning herring recorded at night from 18 to 24 UTC (= local time - 2 hours), few herring recordings during the night from 24 to 06, and most herring recorded pelagically during the day. Recorded fish density was highest in the period 15 to 18 and lowest from 21 to 03, and schools staying pelagically had higher density than bottom layers. The observed behaviours are likely to influence acoustic abundance estimates, particularly when surveying during the night towards the end of the spawning season

Efficiency and catch dynamics of collapsible square and conical pots used in the red king crab (Paralithodes camtschaticus) fishery

We compared the capture efficiency and catch dynamics of collapsible square and conical pots used in resource assessment and harvesting of red king crabs (Paralithodes camtschaticus [Tilesius, 1815]) in the Barents Sea. After two days of soaking, square pots caught three times as many crabs as conical pots, and their catches consisted of a higher proportion of male crabs and male crabs larger than 160 mm carapace length compared to the catches in the conical pots. Catches in the square pots did not increase as soak times were increased beyond two days, which indicates equilibrium between the rate of entries into and the rate of exits from the pots. Catches in conical pots, however, increased with increasing soak times up to eight days, the longest soak time examined in this study. These findings demonstrate the higher efficiency of square pots and the importance of understanding catch dynamics when making population assessments based on catchper-unit-of-effort data. The favorable catch characteristics and handling properties of the collapsible square pot may make this pot design suitable for other crab fisheries, as well

La Piezosurgery come metodica alternativa alla strumentazione rotante tradizionale nell'estrazione di ottavi inferiori inclusi. Revisione della letteratura

La presente dissertazione si occupa di esporre la validità della piezosurgery nell'estrazione di ottavi inclusi inferiori come alternativa alla strumentazione rotante tradizionale. Ne vengono illustrati vantaggi e svantaggi con particolare attenzione al tempo impiegato per l'intervento, alla riduzione delle complicanze e al miglioramento del decorso postoperatorio, il tutto sulla base di una revisione sistematica che la letteratura propone su questo argomento corredata dall' esposizione di una serie di casi clinici

How and why acoustic detectability and catchability of herring change with individual motivation and physiological state in a variable environment: a multi-scale study on a local herring population in southwestern Norway

The understanding of distribution and aggregation in herring (Clupea harengus) can be enhanced by integrated multi-scale studies in small ecosystems. Hydro-acoustics, underwater cameras, herring and predator gillnet samples and oceanographical measurements were used to quantify herring schooling dynamics. During autumn (September) after an active feeding period, the herring was distributed in small and dense schools, mostly close to land and in relatively shallow water ( 40 m depth). All recorded herring schools were then vertically extended in the water column within the most variable temperature and oxygen profiles, presumably enabling individuals to adjust maturation rate to prevailing environmental conditions and synchronize spawning of individuals within the school. From late February prior to spawning, only one major pelagic school was observed, extraordinarily stationary at the only inlet to the inner basin. The pre-spawning herring aggregation was fairly easy to detect acoustically for more than one month. Just prior to spawning and during spawning, herring spread out and became extremely difficult to detect acoustically. Only underwater cameras and bottom set gillnets could then be used to identify herring and selected spawning areas. We argue that the dramatic seasonal changes in acoustic detectability and catchability we observe is best understood and predicted based on detailed knowledge of how herring react to a changing environment according to their physiological state and motivation. Such factors should also be taken more systematically into account when performing acoustic surveys in large marine ecosystems. We need to study in more detail the vital underlying processes behind the substantial variability observed in acoustic detectability and catchability of pelagic planktivorous fish species during their annual life cycle in order to better understand and quantify variability in acoustic surveys, and thereby improve our acoustic abundance estimation