Gross and histological observations of ovarian development in twaite shad, &lt;i&gt;Alosa fallax fallax&lt;/i&gt;, from Rivers Mira and Guadiana (Portugal)

José Pedro Andrade,Eduardo Esteves,Teresa Pina

doi:10.3989/scimar.2003.67n3313

Abstract

SUMMARY: In order to describe the stages of oogenesis of twaite shad, Alosa fallax fallax, 265 females were collected between March and June 1997, February and June 1998 and January and April 1999 in the Rivers Mira and Guadiana. From the histological study of ovaries a total of eight developmental stages were delineated. Gross examination of paired ovary revealed that they could be placed into one of seven maturity stages according to their stage of development. Two stages of atresia, alfa and beta, were identified. Upon cessation of spawning, the ovaries still contained some oocytes at various stages of development but with a greater number of atretic oocytes. The simultaneous occurrence of oocytes at different stages of development in the ovary indicates asynchronous oocyte development. Oocyte size frequency distributions do not show a gap in size between cortical alveoli and vitellogenic oocytes during the spawning season. This may represent the ability of twaite shad to push oocytes through vitellogenesis from a previtellogenic condition during the spawning period. This has important implications for twaite shad fecundity, because fish with this type of oocyte development depend on estimates of batch fecundity and spawning frequency to determine potential annual fecundity.

Highlights

The twaite shad, Alosa fallax fallax, is an anadromous species that inhabits the North-East Atlantic ‡Corresponding authorOcean and adjacent waters, from the ChristianaFjord to the Iberian Peninsula and the coast of Morocco (Quignard and Douchement, 1991).Anadromous species are highly vulnerable and threatened by anthropogenic activities.twaite shad is rare or even extinct in Europe, despite the existing legal protection for OOGENESIS OF TWAITE SHAD 313FIG. 1. – Map of Portugal, showing the areas in which the samples were collected: 1, River Mira; 2, River Guadiana.migratory species (Lelek, 1980)
June 1997, February and June 1998 and January and April 1999 over the spawning areas of twaite shad in the upper estuaries of the Rivers Mira, South-west Portugal (Odemira) and Guadiana, South-east Portugal (Foz de Odeleite and Mértola)
The process of oocyte development in A. fallax fallax follows the same basic progression as that described for Alosa sapidissima (Mylonas et al, 1995; Olney et al, 2001) and other teleost fish (Tyler and Sumpter, 1996)

Summary

INTRODUCTION

The twaite shad, Alosa fallax fallax, is an anadromous species that inhabits the North-East Atlantic. This species still migrates into the River Mira and the River Guadiana, Portugal, to spawn In these rivers, spawning migration is triggered by favourable environmental conditions, such as the increase in water temperature, and starts between March and April, when adult twaite shad congregate in the sea near the mouth of the river. This study was undertaken as part of a project studying the recruitment, reproduction, age and growth, and genetics of shad populations in Portuguese rivers, and constitutes the first description of the oogenesis of twaite shad. The outcomes of this project will be used to develop conservation and management programmes that endeavour to halt further declines and to improve twaite shad conservation status. As a basis for further studies, the present investigation provides a detailed description of the different stages of oocyte development

MATERIAL AND METHODS

Maturing

Partially spent

DISCUSSION