Why and when do freshwater fish migrate? Observations of migration patterns of the native fishes from the Iberian Peninsula (SW Europe)

Abstract

To find out the possible reasons and periods associated with fish migration, we collected most of the available information (publications and databases) of fish passage assessments and on freshwater fish biology and ecology from all over the Iberian Peninsula.

Almost all Iberian freshwater fish (95.7%; 70 species from 17 families) clearly migrate. The great majority are potamodromous species (70.4%; 50 species from 6 families) but diadromous species (25.4%; 18 species from 9 families) also exist, subdivided as anadromous (10 sp.) and catadromous (8 sp.).

Spawning period is a primary driver of freshwater Iberian fish migration. In addition, there is information about feeding and associated to refuge. A combination of inner and environmental cues and triggers are associated to this behaviour. Sexual maturity is the most important cue that stumilate fish migration. Water temperature, river flow, currents, hydrology and meteorology, salinity and diurnal/nocturnal rhythm or photoperiod are also very important. Moreover, data regarding tidal cycle, large oceanic features, moon cycle and turbidity are available.

The spawning and migrating periods are extended (females have multiple spawning), long, and change between years, adapted to the great year variability, which is characteristic of Mediterranean climate present in the Iberian Peninsula.

Most of spawning and other migration movements are quite extensive and vary between different years. Combining the various present species in each site, it covers all or practically all the year. So, Iberian freshwaters should be always connected –without transversal obstacles- or their fish passes should be permanently, or practically always, in operation.