The biology of the early upstream migrant River lamprey has been studied using sample taken from the cooling water intake screens of the Oldbury Power Station in the Severn Estuary, Examination of the numbers of lampreys caught at different times suggests that increased fresh water discharge is the predominant environmental factor responsible for initiating the movement from the sea into the estuary. The migrants could be separated into typical and praecox forms whose mean lengths were approximately 300 and 230 ram respectively. The typical forms were occasionally found in the estuary as early as July and as late as April with peak abundance generally being reached in November, whereas the praecox forms were present mainly between January and April. The ratio of typical to praecox forms over the four years of sampling was approximately 4:1, In the typical forms, evidence was found for a correlation between high numbers and an increased proportion of males. Measurement of a number of different characters, including gonadosomic and gut ratios, condition factors and body intervals, suggest that although the time when the typical forms enter the estuary is variable, the onset of the changes leading LO sexual maturity are far more synchronous. A small number of the later typical migrants, however, exhibited different characteristics to those of the majority of the animals comprising this size category. Measurements of the plasma constituents of animals from Oldbury indicate that they can regulate their ion in salinities at least as high as 70% of full strength sea water. The mean haematocrits and haemoglobin concentrations of approximately 40% and 14 g% respectively were greater than those of spawning animals. Differences were found between the ions of both the liver and the gonad of males and females. Muscle/plasma ratios for sodium and potassiura were 0.056 and 33.23 respectively. In the female River lamprey (Lampetra fluviatilis), progesterone increased steadily from 31 ng 100 ml-1 in October/ November to 1705 ng ml-1 in May, Testosterone levels in these females was very low in October/November (5.5 ng ml-1) and undetectable thereafter. By contrast, in male River lampreys, testosterone concentration increased from 3331 ng ml-1 in October/ November to 5344 ng ml-1 by May. Progesterone values in males showed a marked seasonal variation, ranging from 12.6 - 22.2 ng ml-1 In female Petromyzon marinus, captured in June at the onset of spawning, the testosterone concentrations ranged from 694 - 1669 ng ml-1 with corresponding progesterone values between 1303 - 2367 ng 100 ml-1. In males however, testosterone concentrations ranged from 2980 - 8846 ng 100 ml-1, with progesterone values from 568 - 1241 ng 100 ml-1.
|Date of Award||1977|