Does global warming impact on migration patterns and recruitment of Allis shad (Alosa alosa L.) young of the year in the Loire River, France?

The hydrological and thermal changes in the Loire River were investigated to test the influence of climatic changes on a short freshwater stage anadromous fish species, Allis shad, for the 1995-2004 period. The mean water temperatures during the adult migration and juvenile growth phases showed significant increase, and mean water flow during these two phases decreased significantly. The period below the threshold of 18 degrees C shortened, and the period between 18 degrees C and the maximum lengthened, while the temperature amounts (number of degree-days) received by aquatic organisms between 18 and 24 degrees C showed an increase. The pattern of young-of-the-year downstream migration was modified. The first day when the juvenile catches reached 5% occurred 17 days earlier at the end of the 1995-2004 period than at the beginning. The first day when the juvenile catches reached 50% was related to the 18 degrees C threshold (reproductive threshold) and the temperature amounts accumulated between the 18 and 20 degrees C thresholds. The year-on-year levels of young-of-the-year abundance showed wide variations, which were not explained by environmental parameters, probably because of the long distance between the study site and the spawning grounds.