Egg weight and fecundity of Clyde spring-spawning and autumn-spawning herrings, Clupea harengus L., in the nothern North Sea and North Minch in different years were analysed. For Clyde spring-spawning herring high variation in egg weight was found and it was attributed to more than one spawning group spawning in the area. Annual variation in fecundity between 1963-1984 was found. Fecundity in 1984 was found to be about 10% higher than previous years. For autumn-spawning herring, fish spawn in the North Minch produce about 1.18 times heavier eggs than those spawning in the northern North Sea. Regional annual variation in fecundity between 1971-1984 was found. In 1984 fecundity of samples collected from Shetland was 1.12 higher than those of North Minch. Fecundity indices for Shetland samples were found to be 1.23 to 1.77 higher in 1984 than 1971-1973. Annual variation between 1971-1973 was also found. It was concluded that fecundity is a potential contributor to population regulation not necessarily related to population density. Monthly analysis of energy content of soma and gonad was carried out for Clyde spring-spawning herring. Differences in the magnitude and timing of energy allocated to the gonad was found between sexes. Energy budget was prepared for each sex at ages 2 and 3. No deficient of energy during winter was apparent due to gonad maturation and metabolic activity. Routine metabolic rate was measured in a group of herring in a respirometer. Oxygen consumption was estimated to be 31.3, 93.3 and 278.6 mgO2 kg-1 h-1 at 5, 10 and 15oC respectively. Standard oxygen consumption was derived from oxygen consumption and theoretical swimming speed to be 20.4, 59.5 and 164.8 mgO2 kg-1h-1 at 5, 10 and 15oC.

The northern Norwegian killer whale (Orcinus orca) is an important predator but little is known about its population dynamics, particular in response to changes in its main prey, the highly dynamic Norwegian spring spawning (NSS) herring (Clupea harengus). The main aims of this thesis were to estimate killer whale population parameters, to explore the future viability of the population, and to explore the response of this predator to changes in distribution and abundance of its main prey over the last 25 years. Population size was estimated as ~ 700 individuals, taking heterogeneity of capture probabilities into account and correcting for unmarked animals. Apparent survival rates of 0.974 (SE = 0.006) for adult males and 0.984 (SE = 0.006) for adult females were estimated accounting for temporary emigration, transience and trap-dependency. Temporary emigration was greater for males than females. Calving intervals ranged from 3 to 14 years (mean = 5.06); equivalent to 0.197 calves per mature female per year. Future viability of the killer whale population was evaluated under various plausible scenarios. The baseline scenario using the best available information predicted a viable population and indicated that the population may be increasing size. Analysis of data on naval sonar activity, killer whale sightings and herring abundance showed that naval sonar activity appeared to have a negative effect on killer whale presence during a period of low prey availability. A time lag of four years was found between the first sign of NSS herring changing its distribution and reduced killer whale presence inside the fjord system. Analysis of energy budgets showed that killer whales spent more time travelling/foraging in 2005/06 than the 1990s. The fjord system was inferred to be a preferred habitat for killer whales when there was a higher density of NSS herring in this area compared to offshore area.