Alejandra F. G. N. Santos

Quantifying seasonality along a latitudinal gradient: from stream temperature to growth of invasive mosquitofish.

Most ecosystems undergo substantial variation over the seasons, ranging from changes in abiotic features, such as temperature, light and precipitation, to changes in species abundance and composition. How seasonality varies along latitudinal gradients is not well known in freshwater ecosystems, despite being very important in predicting the effects of climate change and in helping to advance ecological understanding. Stream temperature is often well correlated with air temperature and influences many ecosystem features such as growth and metabolism of most aquatic organisms. We evaluated the degree of seasonality in ten river mouths along a latitudinal gradient for a set of variables, ranging from air and water temperatures, to physical and chemical properties of water and growth of an invasive fish species (eastern mosquitofish, Gambusia holbrooki). Our results show that although most of the variation in air temperature was explained by latitude and season, this was not the case for water features, including temperature, in lowland Mediterranean streams, which depended less on season and much more on local factors. Similarly, although there was evidence of latitude-dependent seasonality in fish growth, the relationship was nonlinear and weak and the significant latitudinal differences in growth rates observed during winter were compensated later in the year and did not result in overall differences in size and growth. Our results suggest that although latitudinal differences in air temperature cascade through properties of freshwater ecosystems, local factors and complex interactions often override the water temperature variation with latitude and might therefore hinder projections of species distribution models and effects of climate change.

When habitat complexity increases predation risk: experiments with invasive and neotropical native fishes

We tested the predator-prey relationships between a native piscivore (Salminus brasiliensis) and introduced and native fish species of the ParanaRiver, Brazil. We hypothesised that S. brasiliensis can exert biotic resistance against invasive fishes but not at the same degree for all species. Three invasive (Cichla piquiti, Oreochromis niloticus and Ictalurus punctatus) and two native (Astyanax altiparanae and Prochilodus lineatus) species were offered as prey to S. brasiliensisin 300Laquaria trials atthree levels of cover(0%, 50% and100% of artificial macrophytes).S. brasiliensis hadagreaterabilitytocapturepreyincomplexhabitats,sopredationsuccessdidnotdecreasewithhabitatcomplexityand evenincreasedonI.punctatus.Preysurvivalwasvariablethroughtimeandamongspecies,beinghighforI.punctatus.The three most consumed species (P. lineatus, C. piquiti, and O. niloticus) were less active and occupied the aquaria surfaces, changing strongly their behaviour with habitat complexity. Except for P. lineatus and C. piquiti,S. brasiliensis preferably preyed on smaller individuals of the other species. Our experiments support that S. brasiliensis is an interesting candidate to resist the invasion by C. piquiti and O. niloticus but not to control the abundance of I. punctatus. Additional keywords: biotic resistance, invasive species, ParanaRiver basin, predator-prey interactions, Salminus brasiliensis.

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

A relacao predador-presa entre juvenis de pintado Pseudoplatystoma corruscans (piscivoro nativo) e jovens do ano de especies nativas e invasoras de peixes da bacia do rio Parana, Brasil, foi testada experimentalmente. Tres especies de peixe invasoras (o tucunare Cichla piquiti, a tilapia do Nilo Oreochromis niloticus e o bagre do canal Ictalurus punctatus) e duas nativas (o lambari do rabo amarelo Astyanax altiparanae e o curimbata Prochilodus lineatus) foram oferecidas como presa para P. corruscans em microcosmos, com tres tratamentos de complexidade de habitat (0%, 50% e 100% de cobertura por estruturas submersas). A sobrevivencia de presas variou ao longo do tempo e entre especies (C. piquiti < O. niloticus < A. altiparanae < P. lineatus < I. punctatus), sendo governada, em grande parte, por diferencas especies-especificas no comportamento das presas, mas tambem pelo tamanho e defesas morfologicas das mesmas. A complexidade de habitat nao afetou diretamente a piscivoria de P. corruscans, mas, na presenca do predador, algumas especies-presa alteraram seu comportamento quanto ao uso de micro-habitat e grau de agregacao entre os niveis de complexidade de habitat. Pseudoplatystoma corruscans predou preferencialmente sobre os individuos menores das especies invasoras que apresentaram estruturas morfologicas defensivas pouco desenvolvidas e que nao formavam cardume. Em geral, os resultados obtidos contrastam com os de um experimento analogo, no qual foi usado um piscivoro diurno, sugerindo que piscivoros noturnos predam preferencialmente sobre especies de peixes diferentes (de habitos mais diurnos) e que sao menos afetados pela complexidade estrutural de habitats. Os resultados obtidos sugerem que a recuperacao de populacoes nativas de P. corruscans poderia contribuir para o controle de algumas especies de peixes introduzidas no rio Parana, especialmente C. piquiti e O. niloticus, dos quais se espera que o cuidado parental seja reduzido ou nulo a noite.

Preferência alimentar de juvenis de lambari-cachorro, Oligosarcus hepsetus, em relação a duas espécies de presas de peixe em ambiente controlado

This study was based on previously published data on natural diet of the pike characin, Oligosarcus hepsetus in a reservoir at Southeastern Brazil, and thus tested the feeding preference of this carnivore using microcosm experiments. The experiments were conducted in aquariums, in January 2011. Predators were collected in a tributary of the Paraiba do Sul River, while preys: Tilapia rendalli and Astyanax fasciatus were obtained of fish farms. We tested the feeding preference by pike characin in relation to prey size and species, and the results were compared to prey energy. Juvenile pike characin predated preferentially on tilapias with lower body length height, whereas no clear pattern of consumption was observed for lambaris. Tilapias were preferentially predated at experiments with standardized prey body length, whereas no feeding preference was recorded on any prey species at experiments with standardized prey body height. Energetic differences between prey species seem to be less important than morphological differences for the feeding preference of pike characin.

Reconciliation ecology in Neotropical reservoirs: can fishing help to mitigate the impacts of invasive fishes on native populations?

Despite the increased use as tool for conservation of native fish, the creation of non-fishing reserves is more recent in Neotropical impoundments and its effectiveness rather variable. Although not intentionally created for conservation purposes, areas located near dams could function as fish reserves, since fishing activity is often forbidden in these sites for security reasons. We investigated the effects of a fishing-restricted area in Lajes Reservoir, a Neotropical reservoir dominated by invasive species, notably the yellow peacock bass Cichla kelberi, subject to intensive local fisheries. Cichlid populations associated with rocky banks of similar size and complexity were compared between fishing and non-fishing sites, after accounting for spatial autocorrelation. Native cichlids were significantly less abundant in the non-fishing area, in contrast to invasive cichlids. Accordingly, there were smaller individuals of the two native species in the fishing-allowed sites, where the mean size of peacock bass was also smaller. Our results suggest that angling can sometimes provide demographical control of large invasive piscivores and hence predation refuges for native fish in highly modified ecosystems such as reservoirs, thus resulting in reconciliation ecology. Fishing prohibition can sometimes be counterproductive for the conservation of native populations, illustrating the importance of context-specific management strategies.