VARIATION IN THE AGE AT FIRST SPAWNING OF FEMALE TWAITE SHAD (ALOSA FALLAX FALLAX) FROM THE RIVER SEVERN, ENGLAND.

Abstract

The mean age of maturity of female Alosa fallax fallax based on annual samples ranged from 4.28 to 5.26 years, with an overall mean ± 95 % CI of 4.80 ± 0.021 years. The mean age at first spawning was determined for the 1973 – 1992 year classes and ranged from 4.41 years to 5.40 years. A significant power relationship existed between the mean age at first spawning and the combined biomass of the 0-2 and 0-3 age groups (P < 0.05), explaining 40.7 % and 31.6 % of the variability in the age at which a year class matures. The relationship between maturity and biomass indicates that the onset of maturity was related to density-dependent processes and fish, that matured early had significantly faster growth rates compared to those that matured later (P < 0.05). The findings suggest that the immature fish at sea congregate in a fairly restricted area where it is likely that the older year classes impact younger ones. The fact that the relationship between maturity and the biomass of various components of the stock breaks down after the age of four may relate to the onset of maturity, as the mature fish enter the rivers to spawn and are effectively isolated from the main population.

Keywords

• age at maturity
• growth
• Alosa fallax fallax
• River Severn