Use of trophic ecology of omnivorous fish and abiotic factors as supporting tools for assessing environmental impacts in a neotropical river

The study of diet is one of the mechanisms by which competition for resources between species that cohabit in the same ecosystem can be inferred. Therefore, the relationships of the indices that measure specialization in the diet of fish species are necessary to characterize the nutritional quality of these populations and the ecosystem's environmental health. Three species of catfish were selected: one invasive (Clarias gariepinus) and two natives (Trachelyopterus striatulus and Rhamdia quelen), with similar distribution along the Guapi-Macacu River, in the Guapimirim Protection Area (Rio de Janeiro). Fifty-nine catfish of the three species were collected in total, along 32 collection points in the Guapi-Macacu River in two periods (dry and rainy) in 2018. Non-parametric statistics showed the partition of resources between species and the influence of abiotic factors (temperature, pH, transparency, and dissolved oxygen) contributing to the selection of available resources in the environment. Diet-related indices-repletion index (RI), condition factor (K), niche width, and trophic position (TP) of the specimens collected-contributed to measuring the nutritional status of each of these catfish species, showing that R. quelen has a relationship between RI and K, tending to absorb and metabolize nutrients faster than other species. In addition, the invasive species occupies a wide range of TPs compared to native species, confirming its feeding plasticity. On the contrary, T. striatulus needs large amounts of terrestrial insects to maintain its poor condition factor. Also, the RI showed direct influences of abiotic variables, with the temperature being the most prominent. Our results suggest that the invasive species can benefit from this environment that shows signs of environmental degradation.

Experimental assessment of the effects of a Neotropical nocturnal piscivore on juvenile native and invasive fishes

We experimentally examined the predator-prey relationships between juvenile spotted sorubim Pseudoplastystoma corruscans and young-of-the-year invasive and native fish species of the Paraná River basin, Brazil. Three invasive (peacock bass Cichla piquiti, Nile tilapia Oreochromis niloticus, and channel catfish Ictalurus punctatus) and two native (yellowtail tetra Astyanax altiparanae and streaked prochilod Prochilodus lineatus) fish species were offered as prey to P. corruscans in 300 L aquaria with three habitat complexity treatments (0%, 50% and 100% structure-covered). Prey survival was variable through time and among species (C. piquiti < O. niloticus < A. altiparanae < P. lineatus < I. punctatus), depending largely on species-specific prey behavior but also on prey size and morphological defenses. Habitat complexity did not directly affect P. corruscans piscivory but some prey species changed their microhabitat use and shoaling behavior among habitat treatments in predator's presence. Pseudoplatystoma corruscans preyed preferentially on smaller individuals of those invasive species with weak morphological defensive features that persisted in a non-shoaling behavior. Overall, our results contrast with those in a companion experiment using a diurnal predator, suggesting that nocturnal piscivores preferentially prey on different (rather diurnal) fish species and are less affected by habitat complexity. Our findings suggest that recovering the native populations of P. corruscans might help controling some fish species introduced to the Paraná River basin, particularly C. piquiti and O. niloticus, whose parental care is expected to be weak or null at night.

Testicular maturation of Oligosarcus hepsetus (Cuvier) (Actinopterygii, Characidae) in a brazilian tropical reservoir

Six reproductive classes of male Oligosarcus hepsetus (Cuvier, 1829), a medium-sized carnivorous Characiform species, are described based on macroscopic and histological techniques. A total of 175 individuals were caught monthly between April 2001 and June 2002 in the Lajes Reservoir, Brazil, one of the largest impoundment areas in the state of Rio de Janeiro. The reproductive classes were based upon changes in the testicular morphology and stages of germinative cells, i.e., resting, early maturing, late maturing, mature, partially spent and totally spent. Fish in the resting class showed testes with spermatogonia and spermatocytes along the wall of seminal lobules, while spermatids were present in the lumina of the lobules. During early maturing, active spermatogenesis occurs throughout the testis; in the late maturing and mature classes, the lobules are swollen with sperm that are typical of fish in breeding condition. Spent testes presented seminal lobules with residual spermatozoa, coinciding with decreasing GSI and greatly reduced sperm production. Overall, the testicular morphology and class of maturity development of O. hepsetus in the Lajes reservoir did not differ significantly from those of closely related species in other lentic environments. Lower GSI values in the oligotrophic Lajes reservoir than in other eutrophic natural lakes suggest that this species may be modifying this aspect of its reproductive strategy in response to the artificial environment.

Spatial and seasonal changes in the diet of Oligosarcus hepsetus (Characiformes, Characidae) in a Brazilian Reservoir

We assessed spatial and seasonal changes in the diet of Oligosarcus hepsetus in order to describe the strategy developed by this species that allows their very high abundance in Lajes reservoir, Rio de Janeiro, Brazil. Fish samplings were carried out using gill nets, deployed during ca. 12 and 24 hours, between April 2001 and May 2002. A total of 289 individuals were examined, of which 97 showed gut contents. We used the index of relative importance (IRI) to compare probable dietary shifts, and the frequency of occurrence (% OC) to analyze possible ontogenetic influences on feeding. O. hepsetus showed carnivorous habits, feeding preferably on fish and insects, the latter of which occurred in 71.0% of the guts presenting contents. O. hepsetus consumed different items along the three reservoir zones: insects (61.0% IRI) and Cichla monoculus (38.9% IRI) in the lower zone; Lepidoptera (57.0% IRI) in the middle zone; and C. monoculus (77.0% IRI) in the upper zone. Food items changed seasonally with C. monoculus predominating in autumn 2001, and Hymenoptera and Lepidoptera in the winter. In spring almost all food was Lepidoptera (99.8% IRI), while in the summer Hemiptera dominated in the diet. In autumn 2002 Hemiptera (97.0% IRI) was dominant, in significant contrast with the previous autumn. Individuals smaller than 190 mm SL fed heavily on insects, while fishes predominated in the diet of individuals larger than 190 mm SL. Shifts in prey-capture ability among length classes suggest decreasing intraspecific competition. A higher food plasticity seems to be the strategy employed by this opportunist species, which used food resources available in the reservoir.

Hystological analysis of ovarian development of the characiform Oligosarcus hepsetus (Cuvier, 1829) in a Brazilizn Reservoir

A histological analysis was performed of the ovarian development of Oligosarcus hepsetus (Cuvier, 1829), a medium-sized carnivorous Characiform species in the Lajes reservoir, Brazil. A total of 125 fishes, collected monthly between April 2001 and June 2002 were examined by routine macroscopic and histological techniques. Eight phases of the oocyte development were described, and 4 stages and 4 sub-stages of gonadal development were proposed. Spawning in reservoirs, similarly to that in rivers, is in batches, which favors juvenile survival and suggests that the impoundment of this area did not influence the reproductive strategy of this very successful species in a dammed environment.