Abstract : The ecology of the young stages of allis shad Alosa alosa is poorly documented,
although they can be exposed to many pressures during their freshwater phase and
their downstream migration. When passing through systems such as the Gironde-
Garonne-Dordogne watershed (GGD, SW France), they can be subjected to high
temperatures and low levels of oxygen (hypoxia). The aim of this work is to assess
the tolerance of young Alosa alosa at four ages (c. 10, 30, 60 and 85 days old) by challenging
them to different temperatures (18, 22, 26 and 28C) together with decreasing
oxygen saturation levels (from 100% to 30%). Survival of the 10-day-old
individuals was not influenced by oxy-thermic conditions, but high stress levels were
detected and perhaps this age class was too fragile regarding the constraint of the
experimental design. Survival at 30 and at 60 days old was negatively influenced by
the highest temperatures tested alone (from 26C and from 28C, respectively) but
no effect was detected at 85 days old up to 28C. A combined effect of temperature
and oxygen level was highlighted, with heat accelerating survival decrease when
associated with oxygen level depletion: essentially, survival was critical (<50%) at
30 days old at temperature ≥22C together with 30% O2; at 60 days old, at temperature
= 28C with 30% O2; at 85 days old, at temperature ≥26C with ≤40% O2. Tolerance
to oxy-thermic pressures appeared to be greater among the migratory ages
(60 and 85 days old) than among the 30-day-old group. Based on environmental data
recorded in the GGD system and on our experimental results, an exploratory analysis
allowed a discussion of the possible impact of past oxy-thermic conditions on the
local population dynamics between 2005 and 2018. The oxy-thermic conditions that
may affect Alosa alosa at ages when they migrate downstream (60 and 85 days old)
were not frequently recorded in this period, except in cases of extreme episodes of
heat together with hypoxia that occurred in some years, in summertime in the turbidity
maximum zone of the Gironde estuary (particularly in the year 2006). Interestingly,
oxy-thermic conditions that are likely to threaten the 30-day-old individuals
occurred more frequently in the lower freshwater parts of the GGD system between
the years 2005 and 2018. In the context of climate change, a general increase in
temperature is predicted, as well as more frequent and severe hypoxic events, therefore
we suggest that local Alosa alosa population recruitment could encounter critical
oxy-thermic conditions more frequently in the future if no adaptive management of
water resources occurs