Erythrocyte parameters and condition factor of Gymnotus spp . ( Gymnotiformes : Gymnotidae ) under culture conditions

Arlene Sobrinho Ventura1, Gabriela Tomas Jerônimo2, Giovanni Henrique Ferri3, Santiago Benites de Pádua4, Maurício Laterça Martins5 & Márcia Mayumi Ishikawa6* 1Veterinary, MSc., Dr. student in Animal Sciency. Universidade Federal de Mato Grosso do Sul – UFMS, Campo Grande, MS, Brasil 2Aquaculture Engineer, Dr., Post doctorate. Universidade Nilton Lins, Manaus, AM, Brasil 3Biologist, MSc., Student in Animal Biology. Universidade Estadual de Campinas – UNICAMP, Campinas, SP, Brasil 4Veterinary, MSc., Aquaculture Product Manager. AQUA – Laboratório Biovet, Vargem Grande Paulista, SP, Brasil 5Biologist, Dr., Post doctorate. Laboratório AQUOS Sanidade de Organismos Aquáticos, Departamento de Aquicultura, Universidade Federal de Santa Catarina – UFSC, Florianópolis, SC, Brasil 6Veterinary, Dr. Embrapa Meio Ambiente – EMBRAPA, CNPMA Centro Nacional de Pesquisa de Monitoramento e Avaliação de Impacto Ambiental, Jaguariúna, SP, Brasil


Introduction
The gymnotid fish tuvira, Gymnotus spp., is currently one of the live bait species of major commercial importance in Mato Grosso do Sul, Brazil, being a source of income for many populations living close to rivers (Pádua et al., 2012).It is estimated that the catch of this fish reaches the mark of approximately 15 million baits a year, and losses during the period of storage in commercial bait houses are approximately 14% (Moraes & Espinoza, 2001).

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Erythrocyte parameters and condition factor of Gymnotus spp.(Gymnotiformes: Gymnotidae) under culture conditions This genus stands out because it belongs to the food habit of important species used in sport fishing, such as Salminus brasilienses and Pseudoplatystoma sp.(Pádua et al., 2012;Ishikawa et al., 2014).Because of the peculiar characteristics of this species, they live in environments that favor disease development, such as lentic habitats and places with high organic matter content, where they take shelter and search for food, such as fish, vegetables, sediment, protozoans, mollusks, and crustaceans (Resende et al., 2006).
Hematological studies are necessary to characterize physiologically a fish species, both in its natural environment and in captivity.They can indicate stressful conditions and anemic processes, as well as provide information on the respiratory capacity of fish species (Tavares-Dias & Moraes, 2004).They are important for the knowledge on equilibrium and pathological conditions, in determining the influence of pathophysiological conditions that can affect homeostasis, and in contributing to the diagnosis of adverse conditions, in addition to the understanding of the health status of fish (Ranzani-Paiva et al., 2013).
Sanitary studies on this fish genus are scarce, especially in the environment of storage and commercialization in the so-called "bait houses".In these places, the animals are kept after being caught in a natural environment in which sanitary conditions are not adequate to maintain the fish health (Ishikawa et al., 2014;Ventura et al., 2016), making them suitable for the development of pathogens, which can affect their health and survival.
In view of the need for information on the identification and control of stressful situations and diseases, hematological analyzes and evaluations of the condition factor assume importance as an auxiliary tool in the diagnosis, in order to ensure the health of fish.The aim of this study was to evaluate the hematological characteristics and the condition factor of tuvira, Gymnotus spp., from a "bait house" located in the Midwestern Brazil.

Material and methods
A total of 42 tuvira specimens from a commercial "bait house", located in Dourados in the state of Mato Grosso do Sul, Brazil, were evaluated.The animals were kept in masonry tanks, with high storage density and little water renewal.During the period the fish were kept in the "bait house", their feeding was based on termites, sardines, and even other crushed tuvira.
The fish were anesthetized in clove oil solution (150 mg/L -1 ) for biometry and blood collection.The 42 individuals were weighed in a semi-analytical scale and their size was measured with the aid of a ruler (Ethics Committee No. 005/2011-CEUA / UFGD).
The determination of blood glucose was performed by strips using a portable device (Accu-chek®).Total plasmatic protein concentration was determined with a portable refractometer.Length-weight relationship and condition factor were determined from the biometric data.Length-weight data were analyzed following the equation Wt=a.Lb, where Wt is the total weight in grams, L is the total length in cm, and a and b are constants.The condition factor (Kn) was calculated by equation Kn = P observed/P expected, where 'P observed' is the weight obtained from weighing each individual and 'P expected' is the weight determined by the length-weight ratio curve (Le Cren, 1951).

Results and discussion
The fish evaluated presented an average weight of 42.9 ± 31.8 g and a mean length of 23.7 ± 3.3 cm.Regarding water quality parameters, dissolved oxygen in the water was 1.05 ± 0.71 mg L -1 and temperature was 21.87 ± 1.33 °C.The temperature and oxygen observed can be considered ideal for the species, since the tuvira present bimodal breathing and highly vascularized swimming Erythrocyte parameters and condition factor of Gymnotus spp.(Gymnotiformes: Gymnotidae) under culture conditions bladder (Moraes et al., 2002), which allows them to face low levels of oxygen and survive in hypoxic environments.
The length-weight relationship of the animals (Figure 1) may be influenced by numerous factors, such as food availability, reproductive period, as well as abiotic factors characteristic of the environment in which the animals are kept.All of these can affect the estimation of the length-weight relationship, which can result in variation in the regression coefficient.The condition factor is a quantitative measure of fish welfare, which can provide a physical and physiological relation of the environment in which it lives, so it must remain constant regardless of the size the fish may have in a certain period of life (Gomiero & Braga, 2003).The factor Kn=1.00 for fish kept in the "bait house" can indicate that the fish were in good health (Table 1).
Table 1.Mean values ± standard deviation (SD), variation, and coefficient of variation (CV) of the erythrocyte parameters of tuvira, Gymnotus spp., (n= 42) from a commercial "bait house" of Mato Grosso do Sul, Brazil.

Parameters
Mean Hematological variables allow us to evaluate the health status of animals (Lopes et al., 2007).The results of this study demonstrated the erythrocyte variables of Gymnotus spp. in a commercial "bait house", which were oval with a central nucleus accompanying the cell shape, with compact chromatin and without nucleoli (Figure 2); the morphology of the erythrocytes correspond to that observed in other cultured fish species.Erythrocytes contain the respiratory pigment, which is responsible for O 2 transport and part of CO 2 transport in the blood.Any deficiency in the erythrocyte will be translated into a lack of O 2 in the tissues (Ranzani-Paiva et al., 2004).
The hematological profile of fish is directly related to the genus and gonadal development, nutritional state, seasonality, and environment (Tavares-Dias & Moraes, 2004;Babalola et al., 2009).Total plasma protein values of Gymnotus spp.were lower than those observed in tuvira after anesthesia with different concentrations of clove oil (Pádua et al., 2012).In contrast, they were similar to those found in catfish Ictalurus punctatus (Tavares-Dias & Moraes, 2007) and Sorubim lima (Bianchi et al., 2014) and they were lower than those observed in the hybrid Pseudoplatyatoma coruscans (Jerônimo et al., 2015).The total plasma protein concentration may vary according to the immune status and should be considered an interspecific difference (Tavares-Dias & Moraes, 2004).
Changes in the number of fish erythrocytes indicate responses to the oxygen levels to which the animals are exposed, so hemoglobin, MCV, and MCHC are important to identify and classify the type of anemia that can occur in fish (Labarrère et al., 2012).
Hematocrit corresponds to the volume occupied by erythrocytes contained in a certain amount of whole blood.The values of the erythrogram of Gymnotus spp.were similar to the values found for the species (Pádua et al., 2012).Because of the low concentrations of dissolved oxygen in the water, it can be stated that the fish increased the production of red blood cells to improve oxygen transport capacity, thus compensating for reduced availability in the medium (Jerônimo et al., 2011).
Glucose concentration is related to many stressors, such as temperature variation, handling, and transportation (Urbinati et al., 2004).It is an indicator of physiological disturbance as it is the main source of energy used by fish to withstand adverse situations (Morgan & Iwama, 1997).The glucose values obtained in this work were higher than those found for catfish I. punctatus (Tavares-Dias & Moraes, 2007) and smaller than those observed in silver catfish Rhamdia quelen (Higuchi et al., 2011), indicating that these differences may be species specific.
The values of red blood cells for Gymnotus sp varied, which may be due to nutritional factors and age, since the animals sampled were of different ages.However, they remained within the reference standards for the species (Rodrigues et al., 2018).However, nutritional status, age, and environmental conditions are the main factors responsible for the changes in the erythrocyte count of fish (Tavares-Dias & Moraes, 2004;Ranzani-Paiva et al., 2013).The length-weight relationship, as well as the condition factor obtained in this study demonstrated that the tuvira presented good body condition related to environmental conditions, food, and welfare.

Conclusion
The erythrocyte characteristics of tuvira, Gymnotus spp., under culture conditions in the "bait house" were similar to other cultured fish species, and they can be used to evaluate the fish health status.The fish kept in the "bait house" presented good body conditions and the total count of erythrocyte remained within the reference values for the species.