The influence of water temperature and flow on year class strength of twaite shad (Alosa fallax fallax) from the River Severn, England.

Year class strength (YCS) was estimated for the,25 year period between 1972 and 1996. The index (YCS) was based on the mean number of six year old female fish caught per tide (1972-1991) in a net fishery at the start of the freshwater phase of their spawning migration, or predicted from juvenile data (1992-1996). The variation in recruitment as measured by the coefficient of variation for the 20 year period (1972 to 1991) was 124.5 %. There was good spawning success in 1976 and 1989 and particularly poor recruitment in the periods 1977 to 1981 and 1985 to 1988. Since 1990 recruitment has remained relatively stable. Water temperature was positively correlated to YCS with mean July temperature explaining the greatest proportion of the variance in YCS (67 %), followed by August (48 %), June (31 %) and October (20 %); May and September temperatures did not significantly explain the variation in YCS. Flow in the months June to August were, significantly inversely correlated with YCS, with the greatest proportion of the variability explained by August flows (41 %), followed by flows in July (36 %) and June (25 %). Flows in May, September and October were not significantly correlated with YCS. Combining, environmental variables in a multiple regression indicated that mean daily temperature between June and August accounted for 77.1 % of the variability in year class strength. The inclusion of flow did not increase significantly the amount of variation explained.