Dag Atle Lysne

A population study of cod, Gadus morhua L., in northern Norway using otolith structure and parasite tags

Abstract A total of 502 cod, Gadus morhua L., was taken during spring and autumn sampling from three different localities in northern Norway: the Barents Sea, a silled fjord (Balsfjord) and an open fjord (Malangen). Each cod was allocated, according to its otolith structure, to one of two types, coastal or Arcto-Norwegian, and was then examined for infections of four species of parasite: the myxosporeans Myxidium oviforme and Zschokkella hildae , the digenean Hemiurus levinseni , and the copepod Lernaeocera branchialis . Both types of cod were present at each locality, with coastal cod dominating in the fjords in both seasons and in the Barents Sea in spring, but with Arcto-Norwegian cod dominating in the Barents Sea in autumn. Differences in the proportions of the two types of cod and in parasite prevalences between seasons and localities were interpreted as indicating a migration of coastal cod from the Barents Sea sampling area into the fjords between March and October. We found no evidence that Arcto-Norwegian cod from the Barents Sea migrate into the fjords, but our results suggest that the fjords contain local resident populations of Arcto-Norwegian cod. It is concluded that local parasite faunas are not greatly influenced by genetic differences between the two types of cod, but that their compositions are determined largely by variations in the abundance of intermediate hosts.

The occurrence of larval ascaridoid nematodes in wild-caught and in caged and artificially fed Atlantic cod, Gadus morhua L., in Norwegian waters.

Abstract Fish farmers have long suspected that farmed cod were less heavily infected with larvae of ascaridoid worms than fish taken in the commercial fisheries. To test this hypothesis, a field experiment was designed to compare the worm burdens of caged and wild-caught fish over a 2-year period. The results showed an increase in the worm burdens of the wild fish but not in the caged fish. It is suggested that the transmission of food-transmitted ascaridoid larvae is broken through artificial feeding, which prevents new infections from establishing in the caged fish. The life expectancy of these worms in cod is probably more than 2 years.

The parasite Lernaeocera branchialis on caged cod: infection pattern is caused by differences in host susceptibility

Variation in host susceptibility causes significant differences in infection rates between hosts living in a semi-natural situation. Such knowledge has implications for population dynamics and evolutionary models of host-parasite interactions as well as for estimations of parasite abundance. Infection rates by Lernaeocera branchialis (L.) were measured through time and space on caged Atlantic cod (Gadus morhua L.). One group of hosts, identified by their infection history, developed significantly higher infection rates than the others. These were fish which had been infected previously, but had lost their infection. Differences between groups were consistent through both time and space. Two types of cod seem to have been present in the caged population; a small group of inherently susceptible fish, which were infected, and reinfected if the parasite was lost, and another group of resistant hosts with a small chance of becoming infected.

Pepsin digestion reveals both previous and present infections of metacercariae in the skin of fish

In the present study we first counted the pigmented spots that surround Cryptocotyle spp. metacercariae in the skin of fish. Thereafter we digested the skin and counted the parasites. It was then possible both to quantify the present infections and to estimate the number of metacercariae that had died recently.

Mathematical speech and practical action: a case study of the challenges of including mathematics in a school technology project

This paper presents a case study of how a teacher and nine-year-old students deal with mathematics in a practical technology project. By analysing videotaped dialogues between teacher and a pair of students working on constructing a house model, we identify challenges of meaningful inclusion of mathematics in the project. The dialogues are analysed in terms of an analytical framework where four categories of interaction patterns are combined with two main paradigms of mathematics teaching: the exercise paradigm and landscapes of interaction. The project in itself has a potential for facilitating landscapes of investigation in mathematics teaching. However, we find that the teacher as well as students adheres to the exercise paradigm when mathematics is involved in the activity. Two illustrating episodes from the project are examined and presented in detail in this paper. The findings illustrate that the conceptions teachers and students hold of what mathematics teaching means can act as an obstacle in attempts to realize mathematics teaching in creative and meaningful contexts for young students. We suggest that making the various purposes of a project more explicit may help overcome this obstacle, and that the mathematics involved might be taught in separate sessions in order to form a constructive part of a cross-curricular project.

Students’ Use of Science and Mathematics in Practical Projects in Design and Technology

Design and technology (D&T) is often seen as having a potential for students to work with science and mathematics in practical and meaningful contexts. This chapter presents results from a video study of three D&T projects run in different schools in North Norway. The study shows that knowledge in mathematics and science was represented to a very little degree in student projects in D&T, even if the projects were designed to do so. Through an inductive analysis informed by perspectives on technology and technological knowledge from research literature, we identify four issues that contribute to explaining why this is the case: (1) problem solving by other means, (2) focus on product quality, (3) task requires specialised knowledge and (4) concepts and procedures not necessary for the purpose. All these issues are related to the nature of technology as knowledge and activity rather than to pedagogy, and the results suggest that D&T as a domain of knowledge should be represented in the curriculum in its own right and not as an arena for learning science and mathematics. Still, D&T projects can be utilised as contexts for experiences forming a basis for learning in science and mathematics.