Ecology of the early life-history stages of shad Alosa fallax fallax (Lacépède, 1803) in the River Mira, with a note on Alosa spp. larvae in the River Guadiana

Tese dout., Ecologia, Universidade do Algarve, 2006

Allis and Twaite shad Alosa spp. are anadromous clupeid species, which still migrate into several rivers along Portugal to spawn but are regarded as Endangered or Vulnerable (using IUCN criteria). Within the framework of a countrywide research project, this dissertation reports the work carried out (from February 1998 until July 2000) to study the biology and ecology of the early life-history stages of Twaite shad, Alosa fallax fallax, in the River Mira (with a final note on Alosa spp. in the River Guadiana). Twaite shad larvae were more abundant in the surface waters during the day whereas no diel abundance pattern was observed for embryos (i.e. yolk-sac larvae). Furthermore, eggs, embryos and larvae were only found in the upstream stations of River Mira from late-March until mid- June. As expected, no habitat selection occurs at either the egg or embryonic life stages. Conversely, shad larvae abundances were related to several environmental co-variates through a complex non-linear generalised additive model (GAM). Paired translucent and opaque rings in a regularly-recurring sequence deposited around the sagittae of A. fallax fallax were readily visible. The high proportion of sagittae used suggests the usefulness of otolith microstructural analysis to estimate age and to study growth and morality of larvae. Growth and mortality rates varied seasonally (ranging from 0.36 to 0.60 mm d -1 and from 16 to 30 % d -1 , respectively). In a typical year, larvae are in better nutritional condition (RNA/DNA ratio) during two relatively short time-periods, around mid-April and mid to late?May when water temperatures average 22 ?C and levels of prey are <80 nauplii 100 m -3 . Conversely, RNA/DNA are significantly reduced when water turbidity is higher than 2 mg DW m -3 and potential prey are readily available (>20 nauplii 100 m -3 ). The few occurrences of Alosa spp. embryos and larvae in the River Guadiana and the tributary Ribeira de Odeleite precluded sensible analysis. Future topics of research to clarify some issues or directed at unanswered questions are proposed.