ORIGINAL

Growth and mortality of Cachana (Cynopotamus atratoensis) in the cienaga Grande de Lorica, Colombia

Crecimiento y mortalidad de la Cachana (Cynopotamus atratoensis) en la ciénaga Grande de Lorica, Colombia

Charles Olaya-Nieto¹*, M.Sc, Fredys Segura-Guevara,¹ M.Sc, Antonio Vergara-Paternina,¹ Prof. Acuicul.

¹ Universidad de Córdoba, Facultad de Medicina Veterinaria y Zootecnia, Departamento de Ciencias Acuícolas, Laboratorio de Investigación Biológico Pesquera-LIBP. Cra 23 No. 2A-20, Piso 2. Lorica, Colombia.

*Correspondence: charles_olaya@hotmail.com

Received: June 2014; Acepted: February 2015.

Objective. Growth and mortality parameters of Cachana Cynopotamus atratoensis Eigenmann, 1907 in the cienaga Grande de Lorica, Sinu river basin, Colombia, were estimated. Materials and methods. The length frequency analysis to the information collected between January 2001 to December 2002, was applied. Results. The estimated values for asymptotic length, growth rate and age at zero length were 43.4 (±0.52) cm of total length, 0.29 (±0.02) year^-1 and -0.53 years, respectively. Total mortality was estimated at 0.97 year^-1, natural mortality at 0.72 year^-1, fishing mortality at 0.25, the average catch size was 24.8 cm total length, being fully recruited to the fishery with 16.6 cm total length, with low exploitation rate (E: 0.26). Conclusions. Cachana is a fish with medium longevity and growth rate with low exploitation rate, suggesting that there is still no overfishing on the species. However, as it is an very important fish in the food security of fishermen and their families, its necessary to increase the basic biology research pointing to the management and conservation of the species in the cienaga and the Sinu basin. .

Key words: Conservation, exploitation rate, management, migration, overfishing (Source: ASFA).

Objetivos. Estimar los parámetros de crecimiento y mortalidad de la Cachana Cynopotamus atratoensis Eigenmann, 1907 en la ciénaga Grande de Lorica, cuenca del río Sinú, Colombia. Materiales y métodos. Se aplicó el análisis de frecuencia de tallas a la información colectada en el período comprendido entre enero 2001 y diciembre 2002. Resultados. Los valores estimados para la longitud asintótica, tasa de crecimiento y edad a la longitud cero fueron 43.4 (±0.52) cm de longitud total, 0.29 (±0.02) año^-1 y -0.53 años, respectivamente. La mortalidad total fue estimada en 0.97 año^-1, la mortalidad natural en 0.72 año^-1, la mortalidad por pesca en 0.25 año^-1, la talla media de captura fue 24.8 cm de longitud total, siendo reclutada totalmente a la pesquería con 16.6 cm de longitud total, con baja tasa de explotación (E=0.26). Conclusiones. La Cachana es un pez de longevidad y tasa de crecimiento medios con baja tasa de explotación, lo que sugiere que aún no hay sobrepesca sobre la especie. Sin embargo, como es un pez muy importante en la seguridad alimentaria de los pescadores y sus familias, es necesario incrementar la investigación sobre su biología básica apuntando al manejo y conservación de la especie en la ciénaga y la cuenca del Sinú.

Palabras clave: Conservación, manejo, migración, sobrepesca, tasa de explotación (Fuente: AIMS).

INTRODUCTION

Cachana Cynopotamus atratoensis (1) is a carnivorous fish found in the Ciénaga Grande de Lorica. Its main habitat is in the basin of the Sinú River, where they are mainly caught, with a maximum length of 45.3 cm TL and maximum weight of 1.1 kg.

It feeds mainly on fish such as Yalúa Cyphocharax magdalenae, Mayupa Sternopygus macrurus, Chipe Hoplosternum magdalenae, and others of its same species, indicating a tendency towards cannibalism. It maintains its preferences as it grows, year-round, and the swamp’s hydrological cycle, with a r² type reproductive strategy and two periods of spawning and high fertility levels.

The objective of this work was to estimate the growth and mortality parameters of the Cachana in the Ciénaga Grande de Lorica Sinú River basin to contribute to the knowledge of its lifecycle, fishery procedures and species management and conservation in the swamp and the Sinú basin.

MATERIALS AND METHODS

Sampling. The information was taken between January 2001 and December 2002. Some samples were collected by researchers involved in the project and the rest were taken by fishermen in their work in the study area, which were given to the laboratory. Only castnest catches were considered, a type of fishing that makes it possible to obtain different size samples and thus avoids biases due to selectivity. The following measurements were taken for each individual: total length (TL), fork length (FL) and standard length (SL) to the closest millimeter with a graduated ictiometer (Tridente, Spain) and total weight (TW) to the closest gram with an electric scale (Ohaus, USA) with a 5000 (±0.1) g capacity.

Estimating growth parameters. The information was truncated to avoid biases in the estimates. The class interval used was 1 cm to obtain the length frequency distribution, which was run through the normality test (2). Initially the growth coefficient was estimated (b) for the total length-total weight relationship for the samples collected, using the Student test to establish if it was isometric or allometric.

The linear regressions estimated were standard length-total length (SL-TL), fork length-total length (FL-TL) and standard length-fork length (SL-FL), applying the least squares methods (3):

y = a + b x

Where y is the dependent variable in centimeters, a is the regression intercept, b is the regression slope and x is the independent variable in centimeters.

ELEFAN I (4) was applied to the distribution of length frequency to estimate the asymptotic length (L_inf) and the growth coefficient (K) for the Von Bertalanffy equation (5), corrected since the growth coefficient was allometric:

L_t = L_inf {1-e ^{[-K*D (t-to)]1/D}}

Where Lt is the average length at age t, Linf is the asymptotic length or the maximum theoretical length size that the fish can reach, K is the coefficient to reach L_inf, D is equal to 3 [1- (0.6742 + 0.03574*Log₁₀ TW_Máx)], TW_Máx is the highest weight collected and t0 the “age” at zero length. Different combinations were applied to the growth parameters (L_inf and K) in an attempt to seek the best adjustment possible for the data set, selecting 10 pairs, estimating measurements and standard deviation for each parameter.

It should be noted that t₀ cannot be estimated applying the length frequency analysis, including ELEFAN, so it was obtained through this empirical equation (3):

To check if this parameter was different from zero, the Student test was applied. Finally, the empirical equation of the growth performance index (ø’) (6) was estimated, which makes it possible to compare different estimates of growth parameters within the species, order or family:

(ø’) = Log₁₀ K + 2 Log₁₀ L_inf

Estimation of mortality parameters. The instantaneous rate of total mortality Z) was estimated using the length converted in catch curve method (3) with the following equation:

Loge (N/Δt) = a + bt

Where e is the base of the neperian logarithm, N is the number of fish in the sample, Δt is the interval necessary for the fish to go from one length to the next, a is the regression intercept, b is the regression slope and t is the relative average age. The catch curve was based on the estimated average values for growth parameters, tracing the regression line above the points located in the descending part of the curve where the instantaneous mortality rate (Z) is the slope (b) with changed signs. The confidence interval at 95% and the coefficient of determination were obtained using only the values in x e y involved in the regression.

The instantaneous rate of natural mortality (M) was estimated with the equation (3):

Log₁₀ M = - 0.0066 - 0.279 Log₁₀ L_inf + 0.6543 Log₁₀ K + 0.4634 Log₁₀ T

Where T is the average annual water temperature, measured with portable multi parametric equipment (Hanna Instruments, USA). The instantaneous rate of fishing mortality (F) was calculated with F =Z-M and the exploitation rate (E) was obtained with the relation E =F/Z, routines that are part of ELEFAN II (4). To estimate the pattern of selection and recruitment, the Tresierra & Culquichicón (7) method was applied using asymptotic length data (L_inf), growth coefficient (K) and the age at zero length (t₀), previously obtained with the following equation: L_c = Linf {1-e [-K (t_50%-t₀)]}, where Lc is the length of the first catch and t_50% is the average capture age.

Confidence intervals were established at 95% for L_inf, K and Z, as well as for the morphometric relationships SL-TL, FL-TL and SL-FL. Each of these regressions is estimated for a correlation coefficient (r), and the determination (r²) for Z.

Between January 2001 and December 2002 1893 individuals were collected (Table 1), information that was combined to improve quality. The minimum length collected was 13.6 cm TL (July 2002) and the maximum length was 41.5 cm TL (January 2002). The length frequency distribution shows a bimodal curve normally distributed in both cases with modal frequencies of 22.0 and 34.0 cm TL (Figure 1).

Linear regressions obtained with 95% confidence (Figure 2) were:

TL = 0.91 (± 0.01) + 1.17 (± 0.005) SL
TL = 0.31 (± 0.06) + 1.09 (± 0.003) FL
FL = 0.58 (± 0.08) + 1.07 (± 0.004) SL

The length-weight relationship (Figure 3) was estimated at TW = 0.003 (±0.04) TL ^{3.4 (± 0.03)}, r=0.99, n=1893), with a positive allometric growth coefficient. The estimated values for L_inf, K and t₀ to the growth equation were 43.4 (±0.52) cm TL, 0.29 (±0.02) year^-1 and -0.53 years, respectively, with average annual temperatures of 28°C and confidence limits at 95%. The value of t₀ is low and is not significantly different from zero (p>0.05).

In table 2 the length-at-age key is presented, with whose data the growth curve was drawn (figure 4). The growth performance index (ø’) was 2.73. . In figure 5 it is possible to observe the length converted in catch curve and the corresponding regression. The estimated instantaneous rates were: 0.97 (±0.16) yr^-1 for total mortality (Z), 0.72 (±0.03) yr^-1 for natural mortality (M) and 0.25 yr^-1 for capture mortality (F) with 95% of confidence for all three values.

The average catch length estimated for Cachana was 24.8 cm TL, recruited to the fishery (L_c) at 16.6 cm TL (Figure 6), with an exploitation rate of 0.26.

DISCUSSION

The length frequency distribution presents a normally distributed curve, where the estimated values for asymptotic length (L_inf=43.4 cm TL) and the growth coefficient (K=0.29 yr^-1) make it possible to infer that Cachana is a fish with average longevity and growth rate, taking into account the life cycle parameters for stocks reported by Musick (8).

The estimated growth rate for the total length-total weight relationship shows that species growth under study is not isometric (b>3.0). This, and the fact that two curves in the length frequency distribution can be seen, suggests that the species makes lateral trophic or reproductive migrations in the Ciénaga Grande de Lorica to a still undisclosed location. All processes operating in floodplains are closely related to the hydrological regime, where alternation between periods of drought and flooding determines the structure and functioning of these environments (9-10). The influence of hydrological cycle attributes on migration, reproduction, juvenile survival and recruitment has been widely discussed by other authors (11-12).

However, according to the length-at-age key estimated, the species under study is recruited entirely to the fishery (L_c=16.6 cm TL) at 1.7 years, with a low exploitation rate (E=0.26), so it appears that there is still no overfishing considering that the optimal value for the exploitation rate is 0.5 and corresponds to a resource that is not being exploited in a disorganized or irrational way, which argues against the current regulations in the country. The minimum catch length established for Ciénaga Grande de Lorica and the Sinú Basin (23.0 cm SL, 27.8 cm TL) corresponds to the average length found in the length frequency distribution with a bimodal curve normally distributed as estimated in this study (22.0 and 34.0 cm TL). Therefore, it is inferred that biases could exist in the estimation of the minimum catch length established for the species if not enough larger fish were collected that correspond to older specimens of the species in question.

Therefore, the results of this study should be discussed with the fishery authority in Colombia (AUNAP) to analyze if the current legal minimum catch size established by Resolution 000520 is indeed consistent with the biology of this species. If most of fishing was focused on individuals below the legal size, the rate of exploitation would be very high, above 0.5, which is the optimum level of exploitation, and would result in overfishing; this was not observed in this study. A review, update and/or adjustment of the standard should be suggested to the AUNAP so that the fish can be caught following duly estimated growth parameters.

Unlike commercially important fish species in the Sinú River basin, Cachana is not reported in fishery monitoring studies of INPA-Urrá agreement as a fish with commercial interest, so there is little biological information regarding its growth, mortality, feeding habits and reproductive biology. However, since it is a very important fish for the food security of fishermen and their families, as well as for those who sell fish in the market places or on the streets throughout Bajo Sinú, It is necessary, as for the most commercially important species, to research on its biology and ecology for the adequate management and conservation of the species in the cienaga and the Sinu River basin.

Acknowlegements

To the fishermen and fish traders in the Ciénaga Grande de Lorica and the lower Sinú River basin, and to the researchers involved in the “Estimación de los parámetros biológicos básicos de peces comerciales del Río Sinú-Fase II” project, Code FMV-01-04, Number 1120107, which formed part of this study. To the Universidad de Córdoba for the funding received.

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