Spatial diversity of ichthyoplankton in the lower stretch of the Amazon River, Pará, Brazil

Background. This study analyzed the variation of abundance and spatial distribution of fish eggs and larvae sampled from December 2017 to April 2018 along approximately 23 km in the downstream/upstream direction (departing from the city of Santarém) in the lower stretch of the Amazon River, with emphasis on gradients: vertical (subsurface and depth) and horizontal (in the mid channel and the shore proximity). Materials and methods. Fish eggs and larvae were collected at six sampling stations, each representing the mid channel, the shore proximity areas, and both the subsurface (by means of horizontal trawling) and the depth of 10 meters (by means of a conical plankton net with a mesh size of 300 µm). Results. During the sampling period, a total of 62 eggs and 8188 fish larvae were collected. The larvae represented seven orders, 21 families, 29 genera, and 46 species-level taxa: Strongylura timucu, Leporinus sp., Rhytiodus microlepis, Schizodon fasciatus, Schizodon vittatus, Brycon amazonicus, Characidae gen. sp., Potamorhina altamazonica, Potamorhina latior, Psectrogaster amazonica, Psectrogaster rutiloides, Rhaphiodon vulpinus, Anodus elongatus, Hemiodus immaculatus, Hemiodus microlepis, Hemiodus unimaculatus, Prochilodus nigricans, Semaprochilodus insignis, Semaprochilodus taeniurus, Mylossoma albiscopum, Mylossoma aureum, Piaractus brachypomus, Triportheus auritus, Triportheus spp., Rhinosardinia amazonica, Engraulidae gen. sp., Pellona castelnaeana, Pellona flavipinnis, Microphilypnus sp., Plagioscion squamosissimus, Trachelyopterus galeatus, Tatia sp., Cetopsidae gen. sp., Doradidae gen. sp., Heptapteridae gen. sp., Brachyplatystoma filamentosum, Brachyplatystoma juruense, Brachyplatystoma sp., Calophysus macropterus, Hypophthalmus marginatus, Pimelodus blochii, Pseudoplatystoma punctifer, Pseudoplatystoma tigrinum, Sorubim elongatus, Sorubim lima, Colomesus asellus. Among them, 65% perform reproductive migrations and about 80% are of commercial interest in the region of the lower Amazon River. Regarding the spatial variation, the egg concentration was uniform and no significant differences were observed. For the larvae, there was both a horizontal (t-test, df = 4, t = 23.827, P < 0.001), and vertical (t-test, df = 4, t = 4.086, P = 0.015) spatial variation, with the highest abundance values recorded in the shore proximity areas and in the subsurface of the water column. Conclusion. The studied environment functions as an important mechanism of exchange between spawning zones and nursery sites. Any alteration in these areas can certainly modify the composition, abundance, and distribution of the ichthyoplanktonic community and affect the processes of biological recruitment of fishery resources in the region