Length-weight relationships native fish of Southern Altiplano: Lauca National Park, Chile

Abstract Two genera of fish, Orestias (Cyprinodontidae) and Trichomycterus (Trichomicteridae), inhabit in the Lauca National Park, UNESCO Biosphere Reserve, located at the Andes highlands in northern Chile. The present study analyzed the length-weight relationship for four native species of Orestias (O. parinacotensis, O. laucaensis, O. chungarensis and O. piacotensis) and two native species of Tricomycterius (T. chungaraensis and T. laucaensis), obtained at seven different localities. Our results showed that the growth coefficient values “b” for Orestias ranged from 2.89 (O. parinacotensis) through 4.04 (O. piacotensis) and Trichomycterus between 2.53 (T. laucaensis in Parinacota wetland) through 3.14 (T. laucaensis in Caquena River). All length-weight relationships were significant (p < 0.01), with r 2 higher than 0.82. O. laucaensis, O. piacotensis and T. chungaraensis showed positive allometric growth (b > 3). However, two species O. parinacotensis and T. laucaensis showed negative allometric growth (b < 3). Only O. chungarensis showed isometric growth with b = 3. Intense anthropic activity, exotic fish introduction, and climate change present important risks for a unique endemic fish community of the high Andes.


Introduction
The Andean arid highland plain, known as Altiplano is characterized by having freshwater systems with extreme climatic conditions and located 3500 m over sea level (Ochsenius 1986;Montgomery et al. 2003;Risacher et al. 2003a;Vila et al. 2007). The present climate is dry and cold, the temperature may reach 0 as an annual average (Romero et al. 2013) and solar radiation may exceed 300 Wm À2 (Rundel and Palma 2000;Risarcher et al. 2003b;Placzek et al. 2006;S aez et al. 2007;Rondanelli et al. 2015). The rainfalls occur only during summer, phenomenon called "Invierno Altipl anico" (Altiplanic Winter), and they reach less than 100 mm/year, intensifying during the cyclic climatic event called "El Niño" (Aceituno 1996). These environmental characteristics, added to historical volcanic activity have originated endemic flora and fauna highly adapted to these conditions (Vila et al. 2013;Victoriano et al. 2015;Valladares et al. 2018).
Only Orestias has colonized the lakes where they inhabit mainly the littoral macrophytes providing food, shelter, and reproduction activities (Vila et al. 2006;2010). Orestias chungarensis and O. piacotensis are found only at Chungar a Lake and Piacota Lake, respectively. O. laucaensis inhabits Cotacotani Lake and Lauca River and O. parinacotensis inhabits Parinacota wetland (Arratia 1982;Vila et al. 2010). Meanwhile, the Trichomycterus catfish inhabit mainly the running waters of Lauca River and springs, which feed the wetlands. Thus, Trichomycterus chungaraensis has been found only at Mal Paso spring, a tributary of Chungar a Lake. Trichomycterus laucaensis inhabits the Lauca River, while T. rivulatus shows a wide distribution in the area wetlands.
Unfortunately, studies of climate change have estimated a decrease in annual rainfall around 10.5 mm and an increase of 1.9 C in temperatures, producing this way a change in the timing of the cold and warm periods for the coming years (Conaf 2018), which added to the demand of water for human consumption place at risk the integrity and stability of the systems (Dorador et al. 2003;Vila et al. 2007). On the other hand, the introduction and naturalization of an exotic species Oncorhynchus mykiss (Vila et al. 2006;Rojas et al. 2019) has also been reported, negatively affecting the survival of native species (Vila et al. 2006).
Biometric studies of Freshwater fishes of the Altiplano in relation to their size are lacking. In this context, the aim of this research proposes to estimate the length-weight relationship of the native Orestias and Trichomycterus populations inhabiting the different freshwater systems of Lauca National Park. This information will deepen our understanding and improve the conservation of these unique species living in a UNESCO Biosphere Reserve, National Park.

Materials and methods
Using electrofishing (SAMUS À725) equipment, fish were collected at Lauca National Park (18 14 0 S; 69 21 0 W, Figures 1 and 2). A total of 116 Orestias specimens and 116 Trichomycterus were obtained at seven different localities, were sampled during February and March 2016 and 2017 (Table 1). Immediately after collecting fish were anesthetized with MS 222 and measured, and they were returned after recovering. The species included in this study were Orestias parinacotensis (from Parinacota wetland), O. laucaensis (from Lauca River, Caquena River and Cotacotani lake), O. chungarensis (from Chungar a Lake), O. piacotensis (from Piacota lake), Trichomycterus laucaensis (from Lauca River, Caquena River and Parinacota wetland) and T. chungaraensis (frorm Mal Paso spring). Fish total length from mouth border to caudal fin (TL cm) were measured with a Vernier 0.1 cm precision caliper. The weight was obtained with a digital balance with 0.01 gr precision (Ohaus ScoutV R SPX1202).

Length-weight relationship
The relationship between total length and body weight (LWR) was estimated by the following formula: W ¼ a TL b (Le Cren 1951;Froese 2006). Data were changed to a  logarithmic equation as log (W) ¼ a þ b log (TL), to calculate the parameters a and b, "a" was considered constant and "b" the slope of this relationship (Gould 1966). We used the Analysis of variance ANOVA to evaluate the statistical significance of the regression model (P < 0.05) and the coefficient of determination (r 2 ) as a measure of the prediction quality of linear regression.
When the b parameter ¼ 3, fish show isometric growth. When b < 3, there is a negative allometric growth. If b > 3, there is a positive allometric growth. Through the student t-test (two tail), isometric growth is assigned when b is not statistically different of 3 (p > 0.05) and while a statistically difference of b from 3 indicates an allometric growth either positive or negative (P < 0.05). All statistical analyses were done by R 3.6.1 software (R development core team 2019).

Results
The studied specimens showed a total length between 2 and 10 cm, with weights that varied between 0.1 and 10.9 g (Table 1). Among Orestias populations, O. piacotensis showed a lower weight and length with 0.82 ± 0.55 gr and 4.18 ± 0.69 cm, respectively. Trichomycterus populations with a lower average weight were T. laucaensis at Caquena river with 1.43 ± 0.69 g. Trichomycterus chungaraensis showed the lower average length with 4.84 ± 1.49 cm (Table 1).

Discussion
The altiplanic fish have adapted to extreme freshwater systems conditions including altitude over 3500 over sea level, high salinity, solar radiation, daily, and seasonally extreme temperatures. (Vila et al. 2007;M arquez-Garc ıa et al. 2009;Rondanelli et al. 2015). These conditions added to the lack of connectivity among the systems have modeled a unique and endemic fish fauna of the diverse native killifish and catfish of the area. Overall, 50% of the length-weight relationships of the analyzed species and populations showed positive allometric growth b > 3.0 values showing a higher proportional gain of weight than length, this has been documented for species which gain weight at early age (Froese 2006). This was the case of O. laucaensis, O. piacotensis, and T. chungaraensis. Instead, the populations O. parinacotensis and T. laucaensis (Lauca river, Caquena river, and Parinacota wetland) species showed b < 3 values which show negative allometric growth. It is possible that specimens of higher size are thinner and less robust (Jobling 2002).
Only the species O. chungarensis showed a b ¼ 3 value, in which individuals' incremental weight is proportional to their incremental length (Froese 2006). It is important to consider that Chungar a Lake, where this species lives, has the higher volume and surface of water, as well as the presence of littoral macrophytes of Lauca National Park allowing this species higher resources and refuge (Vila et al. 2006;2011;Guerrero et al. 2015).
Although Lauca National Park freshwater systems have been protected by the Chilean legislation since 1965 and later during 1983 declared by UNESCO Biosphere Reserve (Rundel and Palma 2000), there is a permanent and indiscriminate water use by agriculture and mining. In addition to climatic change has caused a significant decrease in precipitation for this area producing negative hydric balances (Dorador et al. 2003;Vila et al. 2007).
On the other hand, the exotic rainbow trout O. mykiss from fish hatchery activities in the area, are affecting negatively the abundance and distribution of native species by food competence and direct depredation (Rojas et al. 2019). In view of the high degree of endemism of native taxa, these data allowed us to obtain information about the lengthweight relationships of the Lauca National Park fish for future fish studies and conservation actions this unique ancestral fish community of the Altiplano.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Notes on contributors
Pablo Rojas, Biology Teacher, Magister of Ecology and Evolutive Biology, Universidad de Chile. Limnology and fish community are his scientific working areas. He has also explored the effects of Table 2. Length-weight relationship estimations of six fish species, Lauca National Park, based on the equation log (W) ¼ log a þ b log(L); a ¼ intercept of regression line; b ¼ slope of regression line; b (95% CI) confidence interval of b parameter; r 2 : coefficient of determination; the student t-test significance conducted to verify if b is significantly different from the consensus b ¼ 3.