Factors determining length distribution and abundance of the European eel, Anguilla anguilla, in the River Mondego (Portugal)

• Published in: Freshwater Biology
• Main Authors: DOMINGOS, ISABEL, COSTA, JOSÉ LINO, COSTA, MARIA JOSÉ
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2006
• Subjects: Anguilla anguilla; European eel
• Online Access: http://dx.doi.org/10.1111/j.1365-2427.2006.01656.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1365-2427.2006.01656.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2427.2006.01656.x

Summary 1. The distribution and abundance of the European eel, Anguilla anguilla , were investigated in the Mondego River, a strongly modified river in central Portugal. Nine freshwater sites, located below the first impassable obstacle to eel migration, were surveyed for 2 years. The study was conducted monthly during the first year and seasonally during the second year. Fish were caught by electric fishing and, physical and biotic variables potentially influencing eel length distribution and abundance were determined at each sampling session. 2. Canonical correspondence analysis (CCA) was used to explain the relationships between abiotic and biotic habitat variables, and eel length distribution. Spatial variation was strongly associated with distance from the sea, number of obstacles, river width and percentage of instream cover. Depth, river flow, water temperature and cannibalism had weaker effects on the longitudinal distribution, but contributed as explanatory variables for the model. The influence of spatial variables outweighed the importance of temporal variables as predictors in this model. 3. Length distribution changed in space from a dominance of small eels (<100 mm) closer to the sea to larger eels (≥250 mm) in the upper reaches of the river. The smallest length class (eels <100 mm) was strongly associated with wider stretches where instream cover, mainly composed of aquatic submerged macrophytes, was abundant (>75% area coverage). On the contrary, larger eels (≥250 mm) preferred deeper stretches with stronger river flow and less instream cover. 4. The effect of biotic interactions with conspecifics as well as with the non‐native red swamp crayfish Procambarus clarkii , in sites with poor instream cover, modified this general pattern of distribution leading to fewer eels <100 mm at some sites closer to the sea. Therefore, despite the ubiquity and plasticity shown by this species, there were clear ontogenetic variations in habitat use such that the first two canonical axes of a CCA ...