LENGTH-WEIGHT RELATIONSHIPS OF 29 SPECIES OF TELEOST FISH FROM SHALLOW ESTUARINE ENVIRONMENT OF SAMBOROMBON BAY ( RIO DE LA PLATA , ARGENTINA )

The length–weight relationships (LWR) of 29 teleost fish species from the shallow estuarine environments of the Samborombón Bay wetland (Río de la Plata, Argentina) were estimated. A total of 6,376 individuals belonging to 19 families were measured and weighed. The best represented family was Sciaenidae with 6 species, followed by Clupeidae (n = 3), and Heptapteridae and Characidae with 2 species each. Significant length–weight relationships with high correlation coefficients were found for all species, which mostly presented positive allometric growth. This study reports the first findings regarding the LWR of 29 species in the Samborombón Bay waters. New maximum sizes and geographic distribution records of certain species are also commented.


INTRODUCTİON
Samborombón Bay is a shallow estuarine ecosystem located in the southern boundary of Río de la Plata. Its ichthyofauna encompasses a combination of freshwater, estuarine and marine species and the 80 % of the fish captures is dominated by juvenile (LASTA, 1995). This environment provides a unique habitat for feeding, mating, spawning and nursery ground for many species with commercial and recreational interest among others (BARLETTA et al., 2010;LASTA, 1995). The shallow environments of the Río de la Plata estuary are used by juveniles species for a short period of time and, subsequently, adults disperse other habitats spatially segregated (JAUREGUIZAR et al., 2003;2006;2009). So, analysis of the fish fauna of estuarine environments provides information of the biology at early stages of some species, some of them important for the future of fisheries activities (LASTA, 1995).The parameters estimation of the length-weight relationship (LWRs) is considered a key element to fishery scientist; the LWR can be used for the estimation of other life history parameters through empirical relations, and for the conversion of length structures into weight structures (FROESE, 2006).
The objective of this study was to provide for the first time the baseline data on LWRs for several juvenile estuarine teleosts fish species from the south-western Atlantic, among these there are many species that are targeted by the artisanal and commercial fisheries of the region.

MATERİAL AND METHODS
The length-weight relationship of 29 fish species was calculated, the fish samples were collected in shallow environments (< 2 m deep) of Ajó River and San Clemente Stream. Both environments are located in the southernmost portion of the Samborombón Bay wetland, on the west margin of the Río de la Plata, Buenos Aires Province, Argentina (Figure 1).
Sampling was conducted bimonthly between September 2007 and August 2009. With the objective of covering the environmental gradient from the freshwater to estuarine, five points on the Ajó River and three on San Clemente Stream were sampled using the modified Garlito/Bituron fixed net (Figure 1) (COLAUTTI, 1998).
The specimens were measured to the nearest cm (0.1 cm) and weighed (0.1 g). For the analysis, standard length (SL) measures were used, except for Symphurus plagusia where total length (TL) was used.
The parameters for the equation W = aSLb (RICKER, 1973) were estimated by least-square the LWR as follows: log 10(W) = log10a+b*log10(SL), where log10a is the intersection of the regression line (coefficient related to body shape), and b is the slope of the regression line. Values of b indicates whether the growth is isometric (= 3) or positive allometric (> 3) or negative allometric (< 3). The 95 % confidence intervals (95 % CI) of the parameter estimates were computed by assuming normally distributed errors. The 95 % CI of b was used to test the hypothesis of isometry.

RESULTS
A number of 6,376 specimens belonging to 29 species included in 19 families were analyzed (Table 1) The data presented below comes from samples taken seasonally during two annual cycles, thus, the values of a and b must be taken as mean annual values for each of the species analyzed.
The linear model described above was highly significant for all species (P < 0.05) and the coefficient of determination (r2) ranged from 0.95 to 0.99. Excluding Trachinotus carolinus, Dactylopterus volitans and Syngnathus folletti, the interspecies variability of the parameter b was low (s.d. = 0.21; CV = 6.91 %), with a mean value of 3.08 ranging between 2.5 and 3.5, results that its agreement with the suggestion of CARLANDER (1969). The minimum value (2.69) was recorded for Parona signata, and the maximum (3.47) for Brevoortia aurea. The parameter b obtained for S. folletti was higher than 3.5 (3.61) event related probably to body shape (eel-like shape) exhibited by this species. In the case of T. carolinus and D. volitans, the parameter b of the LWRs was low (Table 1), and it may be associated with the low sample size. However, this specimens were Bol. Inst. Pesca, São Paulo, 42(1): 236-240, 2016 included in the analysis because they are the first record of these species in the Samborombón Bay. The analyses of the regression coefficient b and their 95 % CI shows that only 4 species (13.8 %), have isometric growth; 17 (58.6 %), positive allometric growth; and8 (27.6 %), with negative allometric growth (Table 1).

Figure 1.
Study area in Samborombón Bay, Río de la Plata, Argentina. I to VIII: samples stations.
As expected from estuarine environments, 11 of the species analyzed (38 %) were represented only be juveniles (Table 1) and over 50 % of these fishes had positive allometric growth. This situation indicates a higher somatic condition for larger size individuals, highlighting the importance of shallow estuarine areas as nursery habitats for fish marine species.
Other species such as D. volitans, Prionotus punctatus, Macrodon ancylodon, Menticirrhus americanus and Micropogonias furnieri showed a lower b value than the one obtained by SEGURA et al. (2012), VIANNA et al. (2004), and HAIMOVICI and VELASCO (2000 at the nearby areas.These discrepancies could be due to several factors such as differences in the number of sampled specimens, narrower or wider size ranges and/or differences in the environment habitat condition (temperature, salinity, food) that affect the growth.The results obtained in this study highlight the importance of shallow estuarine areas as nursery habitats for estuarine and marine fishes, contributing to the knowledge of fish populations in this area plus assist fisheries scientists and managers for future studies.  Observations: n: sample size, W: weight ranges in g (minimum and maximum), TL: total length ranges in cm (minimum and maximum), SL: standard length ranges in cm (minimum and maximum), log10a: intercept of the relationship, b: slope of the relationship, b 95 % CI: 95 % confidence interval of b, r 2 : coefficient of determination, Growth: I: isometric, A+: allometric positive, A-: allometric, * species in which only juveniles were analyzed, in bold: species with new records of maximum sizes.