Collapse of allis shad, Alosa alosa, in the Gironde system (southwest France): environmental change, fishing mortality, or Allee effect?

Abstract

Rougier, T., Lambert, P., Drouineau, H., Girardin, M., Castelnaud, G., Carry, L., Aprahamian, M., Rivot, E., and Rochard, E. 2012. Collapse of allis shad, Alosa alosa, in the Gironde system (southwest France): environmental change, fishing mortality, or Allee effect? – ICES Journal of Marine Science, 69: 1802–1811. At the end of the 20th century the allis shad population in the Gironde was the largest in Europe. During the first decade of the 21st century, catches declined dramatically by two orders of magnitude, and a fishery moratorium was implemented in 2008. This deterioration in the status of the stock was confirmed by three independent assessments (abundance of juveniles and of potential and effective spawners). Three hypotheses on the cause of the collapse were examined: (i) an environmental change in freshwater and/or in the estuary; (ii) an increase in marine and/or estuarine mortality; and (iii) the presence of an Allee effect. Changes in flow, temperature, and water quality over the period were inconclusive, but remain a possible causative factor. The instantaneous rate of marine (mean: 0.7 year−1, s.e. 0.1 year−1) and estuarine (mean: 2.6 year−1, s.e. 0.1 year−1) mortalities showed no trend between 1991 and 2003. Nevertheless, the past high estuarine (fishing) mortalities combined with a demographic Allee effect in the reproduction dynamics could explain the population collapse and hamper the stock recovery under the moratorium. This conclusion is, however, tentative as it was not possible to prove the presence of the density-dependent mechanism associated with the demographic Allee effect.

Abrupt shifts in the Gironde fish community: an indicator of ecological changes in an estuarine ecosystem

“…On the other hand, this result also corroborates the observations of other authors, who previously observed that diadromous species are sensitive to climate change and increases in temperature (see Lassalle et al 2009 for a global study, Pronier & Rochard 1998for smelt, and Cabral et al 2001and Hermant et al 2010 for flounder in the Bay of Biscay and the Tagus). Despite this global trend for diadromous species, we observed a brief recovery of allis shad abundance in the community during the beginning of this period, stressing that the driving factors are complex for this particular population (Rougier et al 2012) and are probably specific to each diadromous fish population.…”

Thermal tolerance of allis shad (Alosa alosa) embryos and larvae: Modeling and potential applications

“…This species is currently classified as vulnerable by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species and is protected across Europe (Habitat Directive, Annexes II and V; the Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR Convention), Annex V; Berne Convention, Annex III). The Gironde basin was formerly considered to be the most important habitat for allis shad in Europe (Martin Vandembulcke 1999;Chanseau et al 2005); however the population recently collapsed (Rougier et al 2012). Recruitment failure was suspected as the primary cause of the collapse (Rougier et al 2012).…”

Effect of temperature on growth, survival and respiratory rate of larval allis shadAlosa alosa

“…Although selfsustaining populations of allis shad are currently found only in France and Portugal , the species is classified as of "least concern" on the Red List of the International Union for the Conservation of Nature (Freyhof and Brooks, 2011). However, this classification is the subject of debate as the species is regarded as vulnerable by the French committee of the IUCN (Rougier et al, 2012) and is classified as endangered on the portugese red list of species (Rogado et al, 2005). Until the end of the 20th century, the allis shad population in the Gironde-Garonne-Dordogne river system was the largest in Europe, accounting for over 250 000 estimated spawners (Rougier et al, 2012).…”

“…Despite various conservation efforts, such as the construction of a fish lift designed for allis shad (Larinier and Travade, 2002) and the protection of active spawning sites in the Garonne and Dordogne rivers, the population reached its all-time low in 2012 and 2013 (Andreas Scharbert, LIFE+ project "Conservation and restoration of allis shad in the Gironde and Rhine watersheds", personal communication). The reasons for the decline are not well understood, but one possible cause is environmental changes that could have led to recruitment failure in the freshwater phase of this species (Rougier et al, 2012). According to Lassalle et al (2008), differences between the historical, current and future distribution of A. alosa can be best explained by the climate variable temperature.…”

“…However, this classification is the subject of debate as the species is regarded as vulnerable by the French committee of the IUCN (Rougier et al, 2012) and is classified as endangered on the portugese red list of species (Rogado et al, 2005). Until the end of the 20th century, the allis shad population in the Gironde-Garonne-Dordogne river system was the largest in Europe, accounting for over 250 000 estimated spawners (Rougier et al, 2012). Despite various conservation efforts, such as the construction of a fish lift designed for allis shad (Larinier and Travade, 2002) and the protection of active spawning sites in the Garonne and Dordogne rivers, the population reached its all-time low in 2012 and 2013 (Andreas Scharbert, LIFE+ project "Conservation and restoration of allis shad in the Gironde and Rhine watersheds", personal communication).…”