The effect of the migration of Alosa fallax fallax (Laécpède) into fresh water, on branchial and gut parasites

Abstract

The prevalence and relative density of branchial and gut parasites taken from Almafullax in the estuary of the River Severn, at the start of the freshwater phase of its spawning migration, were compared with those taken from their spawning grounds in fresh water. Mazocrues alosue showed no significant difference in prevalence between the two habitats, though their relative density did fall significantly. Because of the direct relationship between length and relative density, this difference may be explained by the estuary samples of A. fullux being of fish significantly larger than those from fresh water. Other possible causes are discussed. The prevalence and relative density of M. alosae showed a trend to increase with age of the host. Repeat spawning A. fallax had a significantly greater relative density and prevalence of M. alosue when compared to virgin fish. Analysis of the sampled population showed that Clavellisu emarginata declined significantly in both prevalence and relative density in fresh water, and this was confined to the younger and first time spawning A. fullux. Thynnuscuris aduncum showed significant reductions in prevalence and relative density of infection in fresh water when compared to the estuarine samples. The species Hemiurus uppendiculatus and Pomphorhynchus laevis showed no significant difference in prevalence and relative density of infection, and Pronoprymna ventrzcosa no significant difference in prevalence, between the two habitats.

Variation in the Age at First Spawning of Female Twaite Shad (Alosa Fallax Fallax) From the River Severn, England.

“…However, the study of TAVERNY (1991) did show that Alosa fallax from the GirondeGaronne-Dordogne (France) system were confined to a relatively small area close to the mouth of the estuary, with individual cohorts remaining together. This suggests that the Severn population has a center of distribution somewhere in the Irish Sea, a suggestion corroborated by their parasite fauna (KENNEDY, 1981 ;APRAHAMIAN, 1985).…”

The Influence of Water Temperature and Flow on Year Class Strength of Twaite Shad (Alosa Fallax Fallax) From the River Severn, England.

“…Part of the unexplained variance (32 %) may relate to the fact that not all fish experience the same level of mortality as a result of (1) spatial segregation and (2) variation in the age at maturity. It has been hypothesised that the population congregates at sea (APRAHAMIAN, 1985 ;APRAHAMIAN and LESTER, 2001) and are thus considered to be subject to the same or similar environmental pressures. However, it is realised that an unknown portion of the population re-enter the estuary the following April (APRAHAMIAN, 1988) and are thus unlikely to be subject to the same abiotic and biotic conditions as those fish which remain at sea.…”

Helminth parasites of twaite shad, <I>Alosa fallax</I> (Actinopterygii: Clupeiformes: Clupeidae), from the southern Baltic Sea

“…It is worth noting that twaite shad spawn in rivers and estuaries (Heese 2000), and may accumulate freshwater parasites and simultaneously lose marine ones during this phase in their life history. This may explain the fall in infections by some parasites of twaite shade during their spawning migrations reported by Aprahamian (1985).…”

“…In the past, Nybelin (1922) did find the presence of E. fragile in the Baltic and recognised this as evidence of the host immigration from the Danish straits. Aprahamian (1985) was of the opinion that E. fragile may be used as an indicator of fish migrations.…”