No abstract

SUMMARYDominant warfarin resistance and a recessive haemorrhagic trait are apparently controlled by the same allele Rw5.Twenty-eight F5 litters and 18 backcross litters of wild rats were scored for resistance when 8 weeks of age. There was a deficiency of resistant males in the F2 litters whereas phenotypic ratios were close to expectation in F5 females and hackcrosses. Any deficiency of resistant males in F5 litters could be due to the selective death of the Rw5Rw5 genotype. The size of F5 and backcross litters at birth is similar whereas by 8 weeks the former are significantly smaller than the latter.Samples of rats from populations in mid Wales were scored for resistance. There was a significant decline in the frequency of phenotypic resistance in one large population (X 500) starting when the frequency of the Rw5 allele was relatively low (c. 0l0-023). Since few alleles can be lost by selective death of Rw5Rw5 males in these circumstances it appears that, in the absence of warfarin, the heterozygote may also be at a selective disadvantage.Where rats are intensively poisoned with warfarin both Rw2 and its alternative Rw' are maintained in populations by heterozygous advantage. The ecological unreality of the concept of segregational load is discussed.Strong selection influences the frequency of the alleles of Rio. Chance must also have an important role. Populations of rats in rural areas are widely scattered and sometimes small in size. There is an unpredictable amount of movement between these populations which occur in very heterogeneous environments.

Movements of brown rats were deduced from records made during a capture-recapture study of rural populations on two farms in Mid-Wales. Populations in the hedges were sampled at monthly intervals for two years, but samples from the farm buildings were obtained only irregularly and by a variety of methods.On the main trap line, of the rats captured during at least two sampling periods 35 (78%) females and 57 (74%) males appeared to have established home ranges, the best estimates of mean home range length for each sex being 54.8 m and 66.1 m respectively. The longest recorded distances travelled during known life were 850 m for a female and 954 m for a male, although the median distances travelled were only 43 m and 52 m respectively. The median distance travelled during a sampling period (seven nights) was about 24 m for both sexes. There were no significant differences between the distances travelled by the sexes or by different age groups, and there was little seasonal variation.Infestations in the hedges were always associated with streams, and those more than about 100 m from the buildings seldom persisted through winter. During autumn and winter the rats in the more distant localities tended to move towards the buildings, but there was no suggestion of an orderly migration. Twenty-seven out of 386 rats marked in the hedges were recovered amongst 576 taken from the buildings, but only nine of them had travelled more than 100 m. No rats released in the buildings were captured in the hedges. Rats established in the buildings seemed to exclude rats immigrating from the hedges.

Because goosefish Lophius americanus arc usually landed “tails‐only” in the U.S. commercial fishery, relationships are required to provide a means to convert tail lengths or weights to total fish lengths. To accomplish this, length‐weight relationships for goosefish inhabiting the waters off the northeastern coast of the United States were calculated. The sexual maturation schedule was compared to the size frequency of individuals landed tails‐only to determine the extent to which the current fishery is landing immature goosefish. Analyses of available maturity data indicate that both sexes begin to mature at about 30 cm total length; males generally attain 100% maturity by about 50 cm and females by about 60 cm. The length at which 50% of the males from both the northern and southern components of the population matured (L50) averaged about 40 cm; L50 for females averaged about 44 cm. Mean lengths at which 75°k were mature (L75) were 45 cm for males and 52 cm for females. Overall L50 (sexes and areas combined) was 42.3 cm, corresponding to a tail length of 28.7 cm from the insertion of the first postcephalic dorsal fin spine to the end of the caudal fin (LPCDS1); overall L75 was 48.6 cm, corresponding to a tail length (from the first postcephalic spine) of 33.0 cm. These relationships may be used to evaluate the size composition and sexual maturity of the landed portion of the population and in the development of management policies to minimize the impact of' fisheries on immature fish.