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Extensive sampling of the intake screens of power stations in the Severn Estuary (Berkeley, Oldbury-upon-Severn and Uskmouth) and Bristol Channel (Hinkley Point) yielded a total of 97 species of lampreys, elasmobranchs and teleosts. Data were most comprehensive for Oldbury in the inner estuary where samples of all the fish collected over 24 h were obtained on four occasions in each month between July 1972 and June 1977. The Gadidae was the most abundant family at Oldbury, both in terms of numbers of individuals (51934) and species (13). The fifteen most abundant species at Oldbury included two anadromous (river lamprey (Lampetrafluviatilis), twaite shad (Alosafallax)), one catadromous (European eel (Anguilla anguilla)), one estuarine (common goby {Pomatoschistus microps)) and one freshwater species (3-spined stickleback (Gasterosteus aculeatus)). The remaining ten species, which fall within the broad category of estuarine-dependent marine species, contained a large proportion of o + individuals. This group comprised a species complex consisting of two morphologically very similar sand gobies {Pomatoschistus minutus and Pomatoschistus lozanoi), which were only separated during one year of the study, and the whiting (Merlangius merlangus), flounder (Platichthys flesus), bass (Dicentrarchus labrax), sea snail (Liparis liparis), poor cod (Trisopterus minutus), thin-lipped grey mullet {Liza ramada), herring (Clupea harengus), sprat (Sprattus sprattus) and bib (Trisopterus luscus). Juveniles of the last nine species took on average between 11-15 and 38-42 weeks to enter the shallows in the middle of the inner estuary from their spawning grounds, often having previously passed further up the estuary as postlarvae. These species showed a markedly seasonal pattern of occurrence at Oldbury, with the majority of each usually being collected within distinct two month periods. The number of species, and to an even greater extent the total number of fish, underwent consistent seasonal trends, with maximum and minimum values for the latter occurring between September and January and between March and May respectively. The seasonal trends for species richness (D), Shannon-Wiener (//') and Evenness indices (J) were similar, with maximum and minimum values generally occurring in the winter and summer respectively. A comparison between our data and those of earlier workers indicates that no major change has occurred in the composition of the fish fauna of the Severn Estuary during this century, except for the establishment of two 'northern' species, northern rockling (Ciliata septentrionalis) and Norway pout {Trisopterus esmarkii), during recent times.

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Gonadal developnient has been studied throughout the larval period of Lumpetru fluviatitis and comparisons have been made with conditions in the larvae of the closely relatcd nonparasitic species, L. planeri. The two forms differ in the time of onset or mitotic division in germ cells and in the period when oogenesis is initiated. These phases begin in L. pluneri when the ammocoetes are one year old and in L. fluviatilis at the end of the second year. The course of sex differentiation appears to be sinlilar in both forms. While in the early stages, the oocytes of L. planeri are larger than those of L. j7uviatili.Y at comparable ages, by the end of the third year they are sinlifar in size in both spccics. It is suggested that in 15. planeri, during the extension of the larval period, gonad growth is retarded and during this phase the ammocoete accumulates body reserves which are used to meet the metabolic requirements of the rapidly developing gonads after metamorphosis. The differences observed in early gonadogenesis are believed to account for the reduced fecundity of the L. plunen ammococtc and their adaptive significance is discussed. I. INTRODUCTIONAs a result of the studies that have been made on gonadogenesis in ammocoetes of both parasitic and non-parasitic lampreys (Hardisty, 1960(Hardisty, , 1965a(Hardisty, , b, 1969a it has been possible to formulate certain views on the relationships between these larval processes and the fecundity of the adult forms. With regard to the phasing of gonadogenesis, it has been demonstrated that the three forms that have been studied: the anadromous sea lamprey Petromyzon marinus L., the landlocked form of the same species from North America and the non-parasitic species Lampetra pluneri (Bloch), form a series showing progressively increasing acceleration of gonadal processes relative to the age and lengths of the larval forms. These differences are believed to have influenced the reduction in oocyte numbers that has occurred in the female ammocoete of L. planeri and also to a lesser extent, in the landlocked sea lamprey (Hardisty, 1961a(Hardisty, , 1963(Hardisty, , 1964 and may have played a significant role in the evolution of these lampreys from their ancestral parasitic and anadromous forms.With regard to the position of the brook lamprey L. pluneri a more pertinent comparison would be between this species and the closely related paired form, the parasitic river lamprey L. Juviatilis. This has hitherto been impracticable because of the difficulties in distinguishing between the ammocoetes of the two species. Fortunately, it has been possible to find a few sites on the River Teme where larval populations of L. Juviatilis occur, apparently without any serious contamination with L. planeri. At these same sites, larvae of the sea lamprey P. rnarihus are also found in smaller numbers, but these are easily identified by the characteristic shape of the caudal fin and the distribution of pigment on the tip of the body axis (Hardisty, 1969b).

Eight species of the Gobiidae were recorded in weekly samples taken between July 1972 and June 1977 from the intake screens of Oldbury Power Station in the inner Severn Estuary. Buenia jeffreysii, Aphia minuta, Gobius paganellus and Gobius niger occurred infrequently and Crystallogobius linearis was only common in the late spring and summer of 1975 and 1976. Pomatoschistus microps, which peaked in numbers in the winter, increased in abundance each year between 1972 and 1977. The most numerous gobies were those belonging to the Pomatoschistus minutus complex which were separated into P. minutus and P. lozanoi only between June 1974 and July 1975. Seasonal trends in the abundance of these two species were similar, with elevated numbers occurring between July and September 1974 and in January 1975. Gonadosomatic indices and other data indicate that the decline in numbers after the latter month represented an emigration to spawning areas further down the estuary or in the Bristol Channel. Although P. minutus attained a larger body size than P. lozanoi, both these species and P. microps typically had a one year life cycle with only a small number of individuals surviving for a further few months. The fecundity of P. lozanoi, which ranged from 410–2453 (mean 1270), was lower than that reported for P. minutus in this and other studies. Analysis of stomach contents showed that the diet of both species was similar, consisting predominantly of gammarids and mysids. While both species were infected with Ligula intestinalis, the incidence of infection and the parasite index was greater in P. lozanoi than in P. minutus.