Bjorn Bjornsson

Marine and Freshwater Research Institure · Aquaculture

Temperature fluctuations impact the distribution of various marine fish species and typically result in distributional changes, seeing either a geographical expansion or, possibly, a retraction. Survival and fitness in the new habitat depend, however, on the species’ adaptability capacity. Here, we describe changes in geographical distribution of I...

Natural mortality (M) is difficult to measure directly in exploited fish stocks. Therefore, in stock assessments, M has usually been assumed to be constant for age classes of fishable size. However, M is likely to vary annually due to temporal changes in food availability, predation, fish size at age, and other biological and environmental factors....

This study supports the hypothesis that well-fed cod (Gadus morhua) seek higher temperatures to increase growth rate, and poorly fed cod select lower temperatures to save metabolic energy. Depth and temperature of free-ranging adult cod (44–79 cm) were studied with data storage tags as part of a ranching project in an Icelandic fjord. Forage fish w...

Food searching behaviour in a group of individually tagged 1–5 kg Atlantic cod Gadus morhua was studied in a set of three experiments in a sea cage with two underwater platforms, where restricted amounts of food was delivered several times per day during an acoustic training period. It took c. 1 week to train 20 naïve cod to associate low frequency...

In marine fisheries, considerable development has occurred in capture technology. Yet, some of the current fishing methods impact the environment by large greenhouse gas emission, harmful effects to benthic communities, and/or high bycatch of juvenile and unwanted species. It is proposed that for some fish species these deficiencies could be mitiga...

Knowing movement and structure of fish populations is a prerequisite for effective spatial fisheries management. The study evaluates migration patterns and connectivity of two groups of cod (Gadus morhua) associated with offshore feeding and nursery grounds. This was achieved by investigating (i) migration pathways of cod tagged at the feeding area...

Changes in stock size of a top predator have been found to cascade through the trophic levels but the mechanism has not been fully clarified. Using data from annual trawl surveys within a fjord during 1988-2015, we present evidence that changes in abundance of cod Gadus morhua and haddock Melanogrammus aeglefinus have an immediate effect on the dis...

The study analyses the home ranges and spatial segregation of spawning components of Icelandic cod Gadus morhua based on: (1) conventional tag (CT) recaptures and (2) depth, temperature and tidal locations from electronic data storage tags (DSTs). A total of 5987 CT recapture positions from tagging in geographically distinct spawning grounds from 1...

In recent years, about one-third of the cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) catches in Icelandic waters have been obtained with longline. Although longlining has been regarded as a conservation-oriented fishing method, a serious drawback of this fishing gear is the high catches of undersized fish. Our purpose was to locate are...

Predatory fish often forage in schools, but observations of actual schooling behaviour in deep-water ecosystems are scarce. In a ranching study of wild Atlantic cod Gadus morhua L. in a fjord in east Iceland, observations of peculiar synchronous and vertically undulating schooling behaviour of the fish were recorded. These fish had been conditioned...

Induction of triploidy has been suggested as an effective tool to prevent spawning of farmed fish. This experiment examined the growth potential of triploid cod when reared communally with diploid ones after the juvenile stage. Pressure treatment was used to induce triploidy in a batch of cod eggs in April 2009. The resulting offspring were reared...

The effects of stocking density on the productivity in a juvenile cod farm were studied in a flow-through system for fish weighing initially 44 g. The fish densities increased throughout the experiment from 4 to 21, 11 to 54 and 22 to 95 kg/m3. Growth rates were negatively affected at densities above 50 kg/m3 and at the end of the experiment the me...

Útdráttur The effects of different harvesting strategies and increased fish biomass on the feasibility of ranching wild cod in ‘herds’ formed with anthropogenic feeding were studied. The five following scenarios using purse seine as the method of capture were considered: I. Basic harvesting, II. Harvesting with size-grading and selling the smaller...

The feasibility of ranching wild cod in 'herds' was compared with three other scenarios: on-growing of wild cod in sea cages, full-cycle cod farming with hatchery produced juveniles, and commercial cod fishery. In the calculations it was assumed that an existing fishing company could either fish, ranch or on-grow 200 tons of cod quota to increase t...

Two year‐classes of Atlantic cod were reared in indoor tanks from November 2002 to November 2004. The average annual mortality was 21% and 36% for males and females respectively. The highest monthly mortality rates in females were observed around and during the spawning periods from March to July, maximally 7% and 20% for year‐class 2001 and 18% an...

The interaction between two species of gadoid and a shrimp stock was studied in a 40 km long two-armed fjord in north-west Iceland. On the basis of acoustic and trawl surveys in 2005 and 2006, immature cod Gadus morhua and haddock Melanogrammus aeglefinus were found to migrate to the inner part of the fjord in late summer, concurrent with rising te...

Björnsson, B., Karlsson, H., Thorsteinsson, V., and Solmundsson, J. 2011. Should all fish in mark–recapture experiments be double-tagged? Lessons learned from tagging coastal cod (Gadus morhua). – ICES Journal of Marine Science, 68: . Cod (Gadus morhua) were tagged outside and inside “herds” formed by anthropogenic feeding in an Icelandic fjord. Re...

Anthropogenic feeding was carried out for 21months in a fjord in Northwest Iceland to assess the potential of aggregating and growing free-ranging cod (Gadus morhua). Practical feeding and harvesting methods were developed and four sizable herds of wild cod formed at four feeding stations, where net bags with frozen trash fish were deployed 2–5 tim...

To study the effects of anthropogenic feeding on the behaviour of cod (Gadus morhua), a 30km2 area was reserved in Arnarfjördur, a fjord in Northwest Iceland. At four feed stations, frozen trash-fish was dispensed 2–3 times per week from April 2005 to December 2006. Four sizable ‘herds’ of coastal cod were rapidly formed. Cod were tagged outside th...

This book contains 27 chapters on finfish aquaculture. The chapters were designated into four main topics: I. Aquaculture diversification an introduction (contains the first seven chapters namely: fish culture: achievements and challenges, fish culture: the rearing environment, fish culture: feeds and feeding, farmed species and their characteristi...

Six size groups of hatchery produced turbot (2–499g) were reared at six constant temperatures (10–25°C). The results were used to develop a mathematical model for growth rate and feed conversion in turbot. At each temperature there were linear relationships between logarithms of (a) specific growth rate (G%/day) and body weight (Wg), and (b) feed c...

The effect of thermal environment on condition factor was examined for six different size-classes of Atlantic cod Gadus morhua fed to satiation. A weight–length relationship for 8 to 1303 g fish reared at 4–20°C indicated an allometric growth (W = aLb, a = 0.0045, b = 3.257) of cod. Changes in relative condition factor (Krel) with temperature were...

Results from several laboratory experiments showed that at each temperature there was a linear relationship between the logarithms of specific growth rate (G%/day) and body weight (W g) of Atlantic cod fed to satiation: lnG = α + βlnW. Both α and β were found to be a function of temperature (T °C): α = a + bT + cT2; β = d + eT; a = − 0.7620, b = 0....

In a 5-month experiment where groups of juvenile cod were reared in a flow-through system at low density (Group 1) and in a recirculating system at low (Group 2) and high densities (Group 3), the recirculated water had negative effects on growth rate, nutritional condition, and mortality. After the first month, mean weight was significantly larger...

The optimal temperatures for growth of four groups of hatchery-reared cod larvae (geometric mean weight: 73, 191, 249 and 251 μg), reared on rotifers at four or five constant temperatures between 4 and 16° C for 14, 12, 9 and 16 days were 9.7, 12.3, 12.7 and 13.4° C, respectively. The maximum growth rate also increased with size and was 6.5, 9.6, 1...

The results indicate that seawater recently disinfected with ultraviolet radiation (254 nm) can increase the frequency of cataract in juvenile cod. In a 5-month experiment where groups of cod were reared in a recirculating system at two stocking densities (54 and 300 fish/m3), the frequency of cataract increased after the fish were exposed to UV-tr...

Nephrops was found to be of low quality as food for cod. In a laboratory experiment the mean specific growth rate of 1 kg cod was 0.184 and 0.415% d−1 when fed to satiation on Nephrops and capelin, respectively. This large difference in growth rate resulted not only from less intake of Nephrops (1.19 kg cod−1) than capelin (1.55 kg cod−1) but also...

The growth rate of wild, free-ranging cod increased substantially in a 17-month feeding experiment in Stödvarfjördur, a small Icelandic fjord. The feed, mainly capelin and herring, was dispensed in the fjord from a boat three times a week, 2–10 tonnes per month. During feeding an audio signal was transmitted and the response of the fish observed wi...

Results from laboratory experiments showed that food-unlimited growth rate (G) of Atlantic cod (Gadus morhua) declined linearly with fish weight (W) on a log-log scale at six different temperatures: 2, 4, 7, 10, 13, and 16degreesC. The intercept (alpha(i)) and slope (beta(i)) of these regressions increased linearly with temperature (T), implying th...

In the lake Ellidavatn, southwest Iceland, the diel activity of Arctic char and brown trout was studied in autumn (September) and spring (April) by removing the fish from gillnets every 3 hours for three days. Additional fish samples, evening and morning, were taken at other times of the year. The catch per unit effort was about 10 times higher dur...

The concept of large-scale feeding of a predatory fish stock by natural prey species is introduced and evaluated for the Icelandic cod (Gadus morhua L.) stock. The paper addresses the question of whether fisheries yield can be enhanced by relocating food supply in an ecosystem from areas of surplus prey abundance to areas where predator abundance i...

In the lake Ellidavatn, southwest Iceland, fish samples were collected with gillnets of one mesh-size once or twice a month for two years. Stomach samples of 961 Arctic char and 429 brown trout were analysed. The annual diet of char consisted of chironomid larvae (31%), cladocerans Eurycercus lamellatus (19%), bivalves Pisidium spp. (16%), water sn...

The immune response of cod (Gadus morhua L.) is unusual in that specific antibody response is limited or absent. In the present study cod was immunised with haptenated and non-haptenated protein antigen at two different temperatures and the antibody response monitored over a period of 18 months. Other humoral parameters of immunological importance...

Five size groups of hatchery reared cod (2-109 g) and three size groups of wild cod (447-2213 g) were reared at a range of constant temperatures. All groups were fed to satiation, the hatchery fish on dry feed and the wild fish on capelin and shrimp. The optimal temperature for growth decreased linearly with fish weight on a log scale from 17°C for...

The effects of environmental temperature on certain humoral immune parameters in Atlantic cod (Gadus morhua L.) were studied. Serum samples were collected from captive cod, of wild origin, kept at different temperatures for 12 months. It was found that immunoglobulin and natural antibody levels increased with increasing temperature whereas the tota...

The effects of size and gender on several humoral immune parameters in cod were examined under different environmental conditions. Serum samples were collected from wild cod of different sizes. Two samplings were undertaken: In the spring in relatively cold waters off the north west coast of Iceland and in the fall in relatively warm waters off the...

The aim of this study was to determine the optimal temperature for growth and growth efficiency for different size-classes of immature halibut. In experiment A, lasting for 99 days, six groups of fish of initial mean weight 8 g were reared at 7.3, 10.0, and 12.8oC, two replicates at each temperature. In experiment B, lasting for 216 days, six group...

The weight gain of 180 tagged halibut (88 females and 92 males) was studied in a 3-year rearing experiment conducted at 7 °C. Two size-classes of young halibut (mean weight 1.8 and 3.2 kg) were stocked at two densities (11 and 22 kg/m2). The fish were fed frozen capelin and herring to satiation 6 days a week. The males became sexually mature at an...

The aim of this study was to estimate optimal stocking density for halibut. Two size-classes of halibut of initial mean weight 1.8 and 3.2 kg respectively were stocked at three different densities: 11, 22 and 33 kg/m2 and reared in six large circular tanks (8 m) for 3 years at 7°C. The fish were fed to satiation 6 days a week with frozen fish (cape...

A laboratory study was performed on how young Atlantic cod (Gadus morhua) regulate their swimming speed according to available food. A circular tank 15 m in diameter was divided into six sectors, each containing two cod ranging in length from 29 to 42 cm. Live fish 5 cm long on average were used as prey. For the first 48 d food intake was changed e...

1.0 Inngangur Á Íslandi hefur lengi verið áhugi á þorskeldi og hafbeit. Í lok nítjándu aldar var bent á þá miklu möguleika sem frjóvgun hrogna á rúmsjó gæti haft á afrakstur fiskistofna (Sveinbjörn Egilsson 1897). Á þessum árum voru m.a. Bandaríkjamenn og Norðmenn með stórtækar sleppingar á kviðpokaseiðum. Þessum sleppingum var hætt um 1970 þar sem...