F. Scasso and H. Campos

Introduction

Fish productivity depends on the trophic status of the aquatic system to which the fish belong.  Theoretically, a body of oligotrophic water give rise to a fish biomass which is less than that corresponding to a superior trophic level (Stockner, 1987; Ryding & Rast, 1990; Downing et al., 1990; Plante & Downing, 1993). In the presence of a eutrophication phenomenon, salmonid communities respond initially with an increase in the body growth rate, but later, with a reduction in the natural reproduction. Finally, the taxa are replaced by others that are able to survive in the modified environment (Colby et al., 1972). Thus, the nutrients indirectly control not only the production of fish in lakes, but also the specific composition of the community.  Species of economic importance (i.e. trout) are reduced or disappear completely in enriched environments and are generally replaced by other fish like Cyprinus carpio, Carassius carassius and Carassius auratus (FAO, 1982; Andersson, 1985),  of less economic and recreational importance (OECD, 1982; Treviño et al., 1984).
Oncorhynchus mykiss (rainbow trout) is one of the species that are negatively affected in terms of its productivity by elevated increments in the trophic levels in the environments which they inhabit. This species was introduced in Chile at the beginning of the century (Golusda, 1927), and today abundant populations can be found in lakes and rivers in the southern and central part of the country (Campos, 1970; Campos et al., 1993).

Materials and Methods

In order to study the populations of O. mykiss, three lakes representing distinct trophic levels were selected. These lakes are: Lake Icalma (38º 48’ S, 71º 18’ W) classified as oligotrophic (Parra et al., 1993); Laguna Chica of San Pedro (36º 51’ S, 73º 05’ W) classified as mesotrophic (Parra et al., 1989a); and Laguna Grande of  San Pedro (36º 51’ S, 73º 06’ W) classified as eutrophic (Parra et al., 1989b). All three lakes belong to the Biobío river basin in central Chile.
Three series of samples were taken from Lake Icalma during the months of November 1993, and January and May, 1994.  Five samples were taken from Laguna Grande and Laguna Chica of San Pedro during the months of November 1993, and January, March, June, and September of 1994.
Fish were detected with hydroacoustic methods utilizing a Lowrance X16 echo sounder.  The transect volumes were calculated according to Oldani (1986).  A set of four gill nets 40 m long, 2.5 m high, with inter-knot distances of 32, 35, 45, and 55 mm, were used.  The fishing effort was the same in the three lakes.
The population parameters was compared by using ANOVA tests.
 

Results

The physical and chemical characteristics of the lakes have been described by Parra et al. (1989a, 1989b, 1993). The captures obtained by gill nets reveal the presence of 7 distinct species, 6 of which were found in Laguna Grande of San Pedro, 4 in Laguna Chica of San Pedro, and 3 in Lake Icalma.  Of the 6 species found in Laguna Grande of  San Pedro, 19% of the biomass corresponded to O. mykiss, 40.9% to Odontesthes bonariensis, 25.1% to Cyprinus carpio, 12.6% to Percichthys trucha, 1.4% to Cichlasoma facetum and 1.1% to Carassius carassius. In Laguna Chica of San Pedro, O mykiss comprised 26.9% of the total biomass, Odontesthes bonariensis 72.2%, Cichlasoma facetum 0.5% and Nematogenis inermis 0.4%.  In Lake Icalma, the percentage of biomass of O. mykiss was 24.2%, Salmo trutta 71.1%, and Percichthys trucha 4.1%.
Significant differences were found between mean densities of O. mykiss in Lake Icalma and Laguna Grande of San Pedro, and between Lake Icalma and Laguna Chica of San Pedro (both p<0.01).  The highest mean density of rainbow trout was registered in Laguna Chica of San Pedro with a wide range. Laguna Grande of San Pedro presented a very narrow range, which was located in the lower extreme of the previous range.  In Lake Icalma, O. mykiss was found to present lower densities than those found in the other two lakes (Fig. 1).
The calculated standing biomass of rainbow trout presented significant differences only between Laguna Chica of San Pedro and Lake Icalma (p<0.01).  The maximum annual average was obtained in Laguna Chica of  San Pedro (Fig. 2).
The average total length of O. mykiss in Lake Icalma, 40.7 cm, was longer than the average total length taken  from fish in the other environments.  Furthermore, this lake presented the largest range of lengths.  In Laguna Grande of San Pedro and Laguna Chica of San Pedro the average values (32.2 and 35.6 cm respectively) and the ranges were very similar (Fig. 3).  However, significant differences were found (p<0.001) between the total lengths in the three lakes.
Production of O. mykiss was calculated according to the multiple regression equation of Downing & Plante (1993).

log10P =  - 0.25 + 0.901log10B - 0.151log10W + 0.291log10TP

where: P = fish production (kg ha-1 yr-1), B = standing biomass (kg ha-1), W = body-mass of the largest size class of fish in the population (g), and TP = phosphorus concentration (?g l-1)

The respective values for TP and W were: 40 ?g l-1 and 940 g in Laguna Grande of San Pedro; 20 ?g l-1 and 909 g in Laguna Chica of San Pedro; and 4 ?g l-1 and 2930 g in Lake Icalma. Considering the calculated average values and ranges of B, production of  rainbow trout varied between 34.3 and 54.8 with an average of 46.6 kg ha-1 yr-1 in Laguna Chica of San Pedro, between 31.3 and 64.3 with an average of 41.1 kg ha-1 yr-1 in Laguna Grande of San Pedro, and a range of 2.2 and 11.7 with an average of 8.1 kg ha-1 yr-1 in Lake Icalma.

Discussion

O. mykiss was the only species in common captured in all of the lakes.  This species represented a similar percentage of the biomass in each of the three environments.  In Lake Icalma, almost the entire remaining percentage corresponded to another salmonid (Salmo trutta), confirming the tendency for salmonids to dominate in temperate oligotrophic lakes.  However, in Laguna Grande of San Pedro and Laguna Chica of San Pedro, lakes with higher trophic levels, the percentage of remaining biomass was comprised of species other than salmonids.
The density, standing biomass, and production obtained from the three bodies of water appeared to be in agreement with their inherent trophic levels, noting a clear difference between Lake Icalma and the other two lakes, which yielded clearly higher values. In these two lakes which exhibited proceses of eutrophication, a difference appeared that confirms the hypothesis proposed by Colby et al. (1972). That is to say, in an intermediate phase of eutrophication there exist a larger increase in the populations of salmonids as compared to an environment with high levels of enrichment. This is clearly observed in the mesotrophic Laguna Chica of San Pedro which exhibited higher abundance, standing biomass, and production than that demonstrated in the eutrophic Laguna Grande of San Pedro.
In these lakes it does not exist a routine extraction of fish (either commercial or sporting) that alters the populations. The fish mortality is being regulated by the environmental factors of these bodies of water.
In environments that are relatively more productive, with more food availability, fish are found that grow rapidly with a higher initial growth rate. On the other hand, they would reach smaller asymptotic sizes, as shown by Lafarga & Guerrero (1991), Campbell (1971), and Legget & Power (1969), for rainbow trout and other salmonids. This appears to be confirmed with the populations in this study since O. mykiss in Lake Icalma were found to attain larger maximum sizes than those from the other two lakes. In addition, individuals from Laguna Chica of San Pedro attained grater sizes than those from Laguna Grande of San Pedro.

Acknowledgements

The authors would like to thank Dr. Homero Urrutia for the critical revision of the manuscript and Dr. Jared Rubin for the translation of the text into English. This project was financed by Centro EULA-Chile of the University of Concepción, the Investigative Project 94.310.08-6 of the Dirección de Investigación of the University of Concepción, and by the FONDECYT project 92-200 of Dr. Hugo Campos.

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