Designing environmental flows in lowland river sections and estuaries is a challenge for researchers and managers, given their complexity and their importance, both for nature conservation and economy. The Ebro River and its delta belong to a Mediterranean area with marked anthropogenic pressures. This study presents an assessment of the relationships between mean flows (discharges) computed at different time scales and (i) ecological quality based on fish populations in the lower Ebro, (ii) bird populations, and (iii) two shellfish fishery species of socioeconomic importance (prawn, or Penaeus kerathurus, and mantis shrimp, or Squilla mantis). Daily discharge data from 2000 to 2015 were used for analyses. Mean annual discharge was able to explain the variation in fish-based ecological quality, and model performance increased when aquatic vegetation was incorporated. Our results indicate that a good ecological status cannot be reached only through changes on discharge, and that habitat characteristics, such as the coverage of macrophytes, must be taken into account. In addition, among the different bird groups identified in our study area, predators were related to river discharge. This was likely due to its influence on available resources. Finally, prawn and mantis shrimp productivity were influenced up to a certain degree by discharge and physicochemical variables, as inputs from rivers constitute major sources of nutrients in oligotrophic environments such as the Mediterranean Sea. Such outcomes allowed revisiting the environmental flow regimes designed for the study area, which provides information for water management in this or in other similar Mediterranean zones.

Se estudió la diferencia de abundancia, talla y biomasa de las especies de cangrejo usadas como carnada entre las localidades de Punta del Hidalgo y Finca El Apio, entre las que se asume diferente presión marisquera. Para ello se registró la talla, sexo y peso de los taxones de interés y se trató de establecer una relación entre la abundancia y el tipo de hábitat. En ambas localidades se recolectó mayor número de machos de los géneros Pachygrapsus y Xantho. Percnon gibbesi y Pachygrapsus spp. alcanzan tallas medias mayores en la Punta del Hidalgo, mientras que Xantho spp. cuenta con tallas similares en las dos zonas. El número de hembras ovígeras se mostró bajo pese a ser época de veda. Se determina que no existen diferencias significativas entre las dos localidades en cuanto al marisqueo, pero sí diferencias de abundancia a nivel de hábitat. A nivel individual, el género Pachygrapsus contó con mayor presencia en áreas donde predominan rocas de tamaño grande. Se recomienda un seguimiento de las poblaciones en periodos dentro y fuera de veda, así como desarrollar estudios centrados en la biología y ecología de dichas especies.