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The visually indistinguishable eggs of various fish species may be identified by species-specific zymograms. Polymorphic loci in identified eggs permit estimation of allele-frequencies for use in population structure analyses. General principles and practical procedures in such work are described, with references to results from recent applications. Potential applications in cod aquaculture are considered.

A total of 1205 immature and mature cod (Gadus morhua L.) of both sexes, caught during 1976-1981 in the Trondheimsfjord, Norway, were investigated in a population-genetic study by means of the polymorphic haemoglobin locus HbI (SICK 1961). Genetic heterogeneity at this locus was observed both between and within samples, even when taken on the spawning ground. In 1977, 602 specimens from the spawning group were genotyped for HbI, tagged with Lea hydrostatic tag, and released. The spatio-temporal distribution of genotypes in recaptures during four years reinforced the impression of genetic imbalance at HbI for cod in this area. Much of the HbI heterogeneity in adults was shown to be correlated with fish size, and an analysis of the length distribution among immatures revealed significant differences in mean lengths between the three common HbI genotypes. The goodness of fit, to our data, of two different population models is evaluated; in model 1, the observed HbI heterogeneity and genotypic size differences are explained as effects of physical population mixing while in model 2, these observations are interpreted as effects of genotypic growth differences and size-selecting fishing gear. The observations in the present study seemed most consistent under the latter model, although the former could not be formally excluded. The present findings are related to previous studies concerning the selection aspect of the cod HbI polymorphism, and the necessity of experimental testing of some field observations is pointed out. However, it is concluded that there at present is reason to doubt the reliability of HbI characteristics when used in cod population structure analyses.

The two common alleles at the polymorphic haemoglobin locus HbI (Sick, 1961) in the Atlantic cod (Gadus morhua L.) code for proteins whose functional properties are differentially influenced by environmental temperatures (Karpov and Novikov, 1980). In the present study HbI genotypic mean lengths in a total of 275 immature specimens (0.5 - 1.5 year of age) from 8 samples taken in the Trondheimsfjord and Oslofjord during 1977-1981 were investigated by rank correlation analysis. In males the overall genotypic mean length rank was HbI^2-2 >HbI^1-2 >HbI^1-1, while i n females HbI^l-2 >HbI^2-2 >HbI^1-1; both ranks were highly significant. The age at first spawning (A_m) was determined by otolith reading in a sample of 118 male cod caught during spawning in the Trondheimsfjord in 1983. In this sample the individual A_m ranged from 4-7 years with a significantly lower mean for the HbI^2-2 genotype. The A_m of heterozygotes was intermediate but not significantly lower than for the HbI^1-1 genotype. Some population genetic and aquacultural implications of these results are briefly discussed.

Summary of thesis: The aim of this thesis was to investigate the fate of escaped farmed cod and detect any introgression in the local stock. Phenotypic discrimination of wild and farmed cod, and genetic baseline values for wild and farmed characteristics were established. The thesis also included a simulated escape (tag and recapture experiment), to study dispersal and presence of escapees in the Trondheimsfjord estuarine system, and an investigation of pen-spawned pelagic cod eggs and larvae in the neighbourhood of the cod farm. The potential impact escape by spawning on local cod populations was assessed. Two large escape events and extensive pen spawning has been reported for the farm in the Trondheimsfjord, still the genetic evidence did not show a robust sign of introgression of farmed cod into the wild stock. The results also indicate a limited presence of escaped farmed cod, suggesting a low ability to adapt to a life in the wild. Sammendrag av avhandling: Målet med doktorgraden var å undersøke effekten rømt oppdrettstorsk kan ha på en lokal vill torskebestand. Forskjellen mellom oppdrett og villtorsk ble beskrevet ved bruk at fenotypiske karakterer og genetiske markører. Oppgaven inneholder også et eksperiment der en har simulert rømming ved å slippe ut merket oppdrettstorsk og registrere gjenfangster. Dette ble brukt til å studere hvor oppdrettstorsken vandrer og å gi et estimat for overlevelse. Gyting i merd ble undersøkt ved å ta vannprøver i nærheten av oppdrettsanlegget. Potensialet gyting i merd kan ha på en lokal torskebestand ble også estimert. To store rømminger og ustrakt gyting i merd har blitt rapportert for anlegget. De genetiske resultatene viser ikke robuste tegn på at det har skjedd en hybridisering av oppdrettstorsk og villtorsk. Resultatene viser også en begrenset tilstedeværelse av oppdrettstorsk som indikerer at oppdrettstorsken kan ha vanskelig for å tilpasse seg et naturlig miljø.

Trondheimsfjorden is one of approximately one dozen Norwegian fjords where the crown jellyfish, Periphylla periphylla (Scyphozoa, Coronatae) has had massive blooms. It has established large local populations during the last two decades. The population in Trondheimsfjorden is mainly established in three innermost basins. The growth and proliferation have been under close monitoring by regular research vessel cruises done by Trondheim Biological Station (TBS), Department of Biology, Norwegian University of Science and Technology (NTNU). The Periphylla population seems to have reached a local carrying capacity year 2007. Simultaneously with the increase in the jelly population, local artisanal fisheries for codfishes have suffered from reduced catches. The reduction in catches is caused by clogged nets, fish quality reduction due to burn marks, and longer working hours caused by longer travelling distances and cleaning of nets. Together these factors have resulted in a negative economic development, and reduced the number of fishermen. This study estimates the economic consequences of the Periphylla bloom in the Trondheimsfjord. A simple ecologic model, public available fisheries statistics from the Norwegian Directorate of Fisheries, and data provided by Yajie Liu, have been applied. The model is based on calculations and estimations of the energy budget (measured in units of carbon), for the growth and maintenance of the current Periphylla population. The calculations include estimates of biomass and density, respiration rates, carbon demand and size distribution of Periphylla in the entire fjord. The economic and ecologic impacts have focused on the inner fjord basins: beyond the shallow sills at Tautra. The results show a total biomass of Periphylla of 101 466 tons, an average density of 1.83gm-3, average production of 2.01 gCm-2year-1, and an average carbon turnover rate of 0.0175 per day. The size distribution of the jellies is quite different between the four basins. The distribution are also changing on year to year basis within the basins. The yearly average possible codfish production, which could have been produced with this carbon budget, is estimated at approximately 70 tons. After the year 2007 when the jelly population assumingly reached the local carrying capacity, this would sum up to an average amount of 600 000 900 000 NOK per year, for the fisheries. For the entire period 2007-2015 the total loss is estimated to be 900 000-950 0000 NOK.

Periphylla periphylla (Péron and Lesueur 1809) is a deep sea coronate scyphomedusa distributed in all oceans of the world, where it is normally found in low abundances. Over the last decades, however, it has established itself in some Norwegian fjords where it has reached very high biomasses and taken over the role as top-predator in the ecosystems. It is naturally adapted to the deep and dark environment of world oceans, but has nevertheless proliferated in comparably shallow waters of many Norwegian fjords. This MSc project aimed at shedding light on the physical factors, environmental characteristics and biological traits which has enabled this jellyfish to establish itself so successfully in the Trondheimsfjord, where it has multiplied rapidly to a large population and to some degree out-competed its ecosystem rivals. The recruitment appears to be a crucial success factor. Thus to achieve the listed goals, studies were focused on the recruitment cycle of the Trondheimsfjord Periphylla periphylla stock, in fresh as well as preserved historical plankton samples from the fjord. The distribution in time and space of the eggs and young specimens of the jellyfish among the plankton in the water column on five fixed plankton sampling locations in the fjord was investigated.The results indicate that Periphylla periphylla spawns continuously in the Trondheimsfjord, with a possible peak in autumn and winter months. The main spawning areas seem to be the middle and inner basins of the fjord (the Ytterøya and Beitstadfjord basins). The pelagic eggs are probably transported outwards from the inner parts of the fjord, as eggs were most abundant on the most coastward sampling station. From its first appearance to a recent peak in population size in the Trondheimsfjord it seems to have taken approximately 10-12 years.

Periphylla periphylla (Scyphozoa, Coronatae) is a deep sea jellyfish first described by Péron & Lesueur in 1809. It is distributed in all of the world s oceans except for the Arctic. P. periphylla avoids light and is well adapted to a life in the dark. It performs diel vertical migrations (DVM) in the water column.Relatively recently, since first observation in Lurefjorden near Bergen in the 1970s, it has established dense populations in several Norwegian fjords including the Trondheimsfjord, the focus of this study. The main goal of this thesis was to estimate the current biomass of the P. periphylla population in the inner Trondheimsfjord (Beitstadfjorden), which has three smaller basins with different depths. Most of the data was collected by a Lightweight Video Profiling Platform (LVPP) which provided information on the vertical distribution of jellyfish, their numbers, and the size (CD: coronal diameter) of each individual. The population in Verrabotn, the shallowest and innermost basin of Beitstadfjorden, had a higher percentage of large individuals (CD >121 mm) than the other basins. Most of the jellyfish individuals in the other two basins tended to be small, with CD ≤ 40 mm. However, the total biomass estimated at each location was mainly made up by large P. periphylla. Comparing the present biomass estimate with a previous one in 2007, it appeared that the population had decreased.However, the presence of large numbers of small individuals of different sizes suggests that a successful local recruitment is still taking place.

The stellate sturgeons as well as the other sturgeon populations inhabiting the Lower Danube River system (LDR) and the NW part of the Black Sea have experienced a severe decline due to heavy fishery, river regulation and water pollution. Traditionally, sturgeons have represented a great economic value for the countries along the Danube, being fished mainly for their eggs, the black caviar . The human population along the Danube River took advantage of the anadromous behaviour of these fishes and over decades of overfishing they depleted the stocks. In 2006 the Romanian government banned the commercial fishing of sturgeons for a period of 10 years and started a Supportive Stocking Programme of Danube with farmed juveniles originating from wild parents. The present study focuses on whether there are genetically isolated subpopulations among the adult cohorts of stellate sturgeons migrating into the LDR in the year 2010, and whether the genetic characteristics of YOY samples in the same year supports the existence of such subpopulations. Samples from fall and spring migrants and from YOY stellate sturgeons naturally born in the river in 2010 were genetically analyzed for mtDNA cyt b and D-loop regions as well as a panel of nuclear microsatellites, and revealed considerable individual variability. Hardy-Weinberg conformity tests revealed heterozygote deficit at 3 out of 4 microsatellite loci. Based on the genic differentiation at the microsatellite loci and mtDNA and cluster analyses, a genetically distinct group were demonstrated among the 2010 YOY samples, pointing to the existence of a previously unknown population substructure within the stellate sturgeons of The Lower Danube River. This was supported by biological characteristics of the YOY samples.