Population Characteristics of Spotted Rose

Snapper Lutjanus guttatus Caught as Shrimp Bycatch in the Gulf of California

Oscar A. González Ochoa. Doctoral candidate in Marine Biology, Centro de Investigaciones Biológicas del Noroeste (CIBNOR), La Paz, Mexico.

Juana López Martínez. D.Sc. in Marine Sciences, Centro Interdisciplinario de Investigaciones Marinas, Instituto Politécnico Nacional (CICIMAR-IPN), Mexico. Researcher, CIBNOR, Mexico. Address: CIBNOR, Unidad Sonora. Km. 2.35 Carr. Las Tinajas, Col. Tinajas, Guaymas, Sonora, 85460 México. e-mail: jlopez04@cibnor.mx

Norma Y. Hernández Saavedra. D.Sc. in Biotechnology, CIBNOR. Researcher, CIBNOR, La Paz, Mexico.

There are limited population biology studies of the spotted rose snapper Lutjanus guttatus. Adults of this highly valued commercial species are fished with gillnets and hook-and-line, while juveniles are caught as shrimp bycatch and usually discarded. The effects of this practice have not been studied. As a first step, this study assessed some population parameters of juvenile snapper caught by the Gulf of California shrimp fishery. We looked for early growth stages and determined by the least squares method the weight to standard length relationship as W= 0.000092, SL3.0509. Length frequency distributions were constructed; using the ELEFAN I method, von Bertalanffy growth parameters were found to be L¥= 515mm (standard length) and K= 0.13. Natural mortality (M= 0.35) was estimated from Pauly’s empirical and Ralston equations; and total mortality was calculated by the catch curve equation. The recruitment pattern extended throughout the year, with spring and summer peaks. The sex ratio was 1:1 and the length at capture was 80mm (standard length). With an apparently high fishing mortality levels, it is recommended that abundance and distribution studies be performed to determine the impact of shrimp fishing on this population.

Son escasos los estudios sobre la biología poblacional del pargo lunarejo Lutjanus guttatus. Los adultos de esta especie, de alto valor comercial, son capturados con redes agalleras y anzuelos mientras que los juveniles son capturados como fauna de acompañamiento de la pesca de camarón y usualmente descartados. Los efectos de esta práctica no han sido evaluados. Como un primer paso, en este estudio se estiman algunos parámetros poblacionales de juveniles del pargo lunarejo capturados en la pesquería de camarón del Golfo de California. Se indagaron estadios de desarrollo y se determinó mediante mínimos cuadrados la relación longitud estándar-peso como W= 0,000092; SL3,0509. Se construyeron distribuciones de frecuencia de talla, y usando el método ELEFAN I se encontraron los parámetros de crecimiento de von Bertalanffy L¥= 515mm (longitud estándar) y K= 0,3. La mortalidad natural (M= 0,35) fue estimada por la ecuación empírica de Pauly y la ecuación de Ralston, y la mortalidad total se calculó mediante la ecuación de la curva de captura. El patrón de reclutamiento se extendió a lo largo del año, con máximos en primavera y verano. La proporción sexual fue 1:1 y la talla media de captura fue 80mm (longitud estándar). Con una tasa de mortalidad pesquera aparentemente alta, se recomienda evaluaciones de la abundancia y distribución de la especie para determinar el impacto de la pesquería del camarón sobre esta población.

São escassos os estudos sobre a biologia populacional do "pargo lunarejo" Lutjanus guttatus. Os adultos desta espécie, de alto valor comercial, são capturados com redes de emalhar e anzóis enquanto que os juvenis são capturados como fauna de acompanhamento da pesca de camarão e usualmente descartados. Os efeitos desta prática não tem sido avaliados. Como um primeiro passo, neste estudo se estimam alguns parâmetros populacionais de juvenis do "pargo lunarejo" capturados na pescaria de camarão do Golfo da Califôrnia. Indagaram-se estágios de desenvolvimento e se determinou mediante mínimos quadrados a relação longitude estandar-peso como W= 0,000092; SL3,0509. Construiram-se distribuições de frequência de tamanho, e usando o método ELEFAN I se encontraram os parâmetros de crescimento de von Bertalanffy L¥= 515mm (longitude estandar) e K= 0,3. A mortalidade natural (M= 0,35) foi estimada pela equação empírica de Pauly e a equação de Ralston, e a mortalidade total se calculou mediante a equação da curva de captura. O padrão de recrutamento se extendeu ao longo do ano, com máximos em primavera e verão. A proporção sexual foi 1:1 e o tamanho médio de captura foi 80mm (longitude estandar). Com uma taxa de mortalidade pesqueira aparentemente alta, se recomenda avaliações da abundância e distribuição da espécie para determinar o impacto da pescaria do camarão sobre esta população.

The spotted rose snapper Lutjanus guttatus (Steindachner 1869) is a highly valued species captured by hook and line and gillnets along the tropical eastern Pacific coast, from Mexico (SAGARPA, 2006) to Peru (Fischer et al., 1995; Robertson and Allen, 2002). This species is part of a multi-species fishery. Under local common names (huachinango, pargo), various lutjanid species are landed without recording the catch of each species (CONAPESCA, 2002, 2003; SARGARPA, 2006). There is no minimum legal size or closed season (SARGARPA, 2006) and the species is considered overexploited (Andrade, 2003; Díaz et al., 2004; Amezcua et al., 2006) in recommendations for regulated conditions for the fishery. Juveniles are caught as shrimp bycatch and discarded or sold at lower prices than if they were obtained as a targeted fishing (Andrade et al., 2003; Amezcua et al., 2006). Claims of overfishing are countered by arguments that economic hardships make specialized fishing unattractive. This argument is not valid because snappers are captured in multi-species fisheries or as bycatch and not routinely evaluated, have a high market value that would increase as the species becomes scarcer (Dulvy et al., 2003); the apparent abundance of the genus may mask overfishing of one or several species in the complex (Marko et al., 2004). Exploitation thus affects species in a way that partly depends on intrinsic life history traits (Rochet et al., 2000). Excessive fishing pressure on immature individuals reduces potential yield by growth (Kristiansen et al., 2006). In addition to direct fishing, bycatch has detrimental effects on slow growing long-lived species under intense fishing pressure (Coggins et al., 2007). When trying to predict population size, population dynamics is controlled by recruitment and death from fishing (Caley et al., 1996). If there is rapid growth, this can be used as an indicator to predict species vulnerability or create a plan for better management (Jennings et al., 1998). Unfortunately, the relationship between size, age, and mortality is unknown for early stages of most marine fish species (Broadhurst et al., 2006). Given the lack of knowledge about the spotted rose snapper, specifically about the bycatch fraction, the goal of the present study was to use the bycatch of spotted rose snapper caught in shrimp trawling to estimate population parameters in the Gulf of California.

Each sample consisted of 20kg of bycatch. Spotted rose snappers were separated and keys were used to identify the species (Allen, 1995; Roberston and Allen, 2002). Measurements of total length (TL; ±1mm), standard length (SL; ±1mm), and total weight (W; ±0.1g) were taken. Sex and adult sexual maturity were determined using a six-stage morphochromatic scale: I (virgin), II (quiescent), III (ripening), IV (mature), V (reproducing), VI (spawned). Females in Stages III–VI were considered as mature (Nikolsky, 1963). The log weight-log standard length relationship was used to detect outliers and to look for growth phases in small fish (Froese, 2006). The standard length-total to length relationship (SL-TL) was determined using the least-squares method to fit the linear model TL= a + bSL. The weight-standard length relationship (SL-W) was determined by using a non-linear estimation to fit the potential model W= a + SL^b. We looked for a relationship between individual size and depth, estimated a condition factor: CF= 100W/SL³ (Fulton, 1904), and looked for significant differences between months, using analysis of variance. Monthly length frequency (SL) distributions were constructed to estimate growth, using the von Bertalanffy equation L_t=L_¥1-e^-K(t-to)), where L_t: SL at age t, L_¥: asymptotic length, K: instantaneous growth rate, and t_o: theoretical age at zero length. A tentative initial L_¥ value was found by the Powell-Wetherall method (Powell, 1979; Wetherall, 1986). With this L_¥ value, the Shepherd method (Shepherd, 1987) was used to estimate K. The Powell-Wetherall method for the estimation of L_¥ gave unusual K values for lutjanids; therefore, an average from values reported by Rojas (2001) and Amezcua et al. (2006) was used as L_¥ value. A new estimate of K was made using the ELEFAN I method, keeping L_¥ fixed (Pauly and David, 1981). The parameter t₀ was obtained by Pauly’s empirical equation (Pauly, 1980)

Longevity was estimated as t_max= t_o+3/K (Taylor, 1958), where t_max is longevity (years) and other parameters are as described earlier. Total mortality rate (Z) was estimated by the linearized length-converted catch curve, Ln(N_i/Dt_i)=a+bt_i (Pauly, 1984b), where N_i number of fish in length class i; Dt_i: time needed for the fish to grow through length class i; t: relative age where mid-length is reached in class i; and b: an estimate of Z (sign changed). Natural mortality (M) was calculated using Pauly’s empirical equation

(Pauly, 1980), where T is the mean annual sea surface temperature (°C) in the species’ habitat. Fishing mortality (F) was calculated from Z= N+F and the exploitation rate (E) was assumed to be E= F/Z (Jones, 1984). Additionally, M was estimated according to the method of Ralston (1987) as M= 0.0189+2.06K. The recruitment pattern was estimated using the ELEFAN II method (Moreau and Cuende, 1991), providing an indirect idea of the reproductive season. From cumulated relative frequency, length at first capture was calculated by nonlinear least-squares fitting (Prager et al., 1987) using the logistic function

,

where P is the proportion of organisms caught and SL₅₀ is the standard length at which 50% of the population is caught.

Larger individual size was correlated with a deeper depth of capture (D= -0.0359SL+21.74, R²= 0.0152, P= 0.001; Figure 3). The condition factor did not vary markedly (F_{a, 0.05, 9, 677}= 1.444), showing only slight, non-significant (P= 0.16) higher values during autumn and lower values during summer. Length frequency distributions (Figures 4 and 5) reveal that the smallest individuals are better represented from Sep. through Dec. and larger sizes appeared from Aug. through Nov.

In the initial L_¥ estimate, unusual K values were obtained from what is commonly observed for lutjanids (Martinez, 2003). The estimations from Amezcua et al. (2006) and Rojas (2001) were averaged for a new K estimate. The final K estimate was arrived at by the ELEFAN I method, with L_¥ fixed; the growth equation was determined as SL= 515(1-e^{(-0.13(t-0.39))}) and longevity (t_max) estimated as 23 years (Figure 6).

The pattern of recruitment extended throughout the year with higher values during spring and summer (Figure 7). We found 25 sexually mature males and 40 females (M:F= 1:1.63) with non-significant differences from a 1:1 ratio (X²_{a, 0.05,} = 3.46, P= 0.06). Total mortality (Z) was 1.00 ±0.56. Considering Pauly’s equation at 23°C and the Ralston method, natural mortality (M) was 0.39 and fishing mortality (F) was 0.61; the exploitation rate was 0.61.

Data adjusted to the sigmoid function, considering cumulative frequency of all individuals, was

,

where length at first capture was 80mm SL (Figure 8).

Snapper availability was limited to the shrimp fishing season and research survey. Most of the catch came from the most heavily fished areas offshore. Small snapper were most common in the shrimp bycatch (Andrade, 2003; Amezcua et al., 2006). When large snapper appeared from Jul to Nov, they were present in small numbers. The trawl net, trawling velocity, bottom habitat conditions, and dietary preferences of juveniles may influence the findings. For example, when juvenile snapper are found over soft bottoms, they feed mainly on shrimp and, in smaller proportions, on fish (Rojas, 1996; Saucedo and Chiapa, 2000).

In the W-SL relationship, the b value suggests negative allometric growth (Rojas, 2001; Andrade, 2003). The relationship depends on local and seasonal conditions such as food abundance and availability. Additionally, sampling in the gulf mainly yielded small juvenile snapper. Therefore, even when the allometric trend is clear, values of the relationship should not be considered because results were not the typical representation of the species, but part of the natural variation between certain limits (Froese, 2006). We used all pooled specimens and found that the condition factor varied only slightly. This result may be a consequence of the scarcity of mature snapper and that seasonal gonadic development strongly influences body proportions (Froese, 2006).

In the analysis of length frequency distribution, two cohorts were observed and yielded the L_¥= 51.5cm SL estimation, that is, L_¥= 65.3cm TL (from the TL-SL relationship). This is less than the 66.2cm TL obtained by Amezcua et al. (2006) using ring counting on vertebrae, and is also less than the 66.4cm TL obtained by Andrade (2003) using otoliths to establish age. However, the present results are much below the 70.6cm furcal length obtained by Rojas (2001) using vertebrae readings. The first two authors included small snapper from shrimp bycatch and medium to large snappers from artisanal fishery, while the third author sampled only the artisanal fishery. Aside from different age determination and adjustment methods, as a higher proportion of larger snapper are included, higher L_¥ values are obtained for estimating growth. Methods of analysis of length frequency, such as ELEFAN, have been criticized for underestimating L_¥ (Martínez, 2003; Amezcua et al., 2006); nevertheless, we suggest that the results are determined by the length frequency structure itself, as well as by the initial L_¥ value used in the estimation (García and Duarte, 2006). Even while direct age determination methods usually lead to higher and more accurate growth and longevity values, the present estimate of longevity (23 years) is similar to that in studies of lutjanids of medium to large size. This value is above the lower limit (<20 years) set by Martínez (2003).

The obtained estimate of Z (1.0) is between 0.33, the value obtained by Amezcua et al. (2006) and the estimated 1.24 by Rojas (1996), whereas the estimate of F (0.61) lies between the lower values (0.8-0.15) obtained by Amezcua et al. (2006) and the higher values (0.946-1.014) obtained by Rojas (1996). Estimates of F are partly dependent upon estimates of Z, which, at a given time, depend on the age or length frequency structure used in the converted catch curve method. Rojas (1996) included only large snappers caught by the local finfish fishery, whereas Amezcua et al. (2006) considered the mortality at the local fishery and fishing fleets. The present estimate, which appears high in comparison, would represent the additional mortality from juveniles caught by trawling. Ralston (1987) suggested that Pauly’s empirical equation (Pauly, 1980) results in underestimated M of slow-growing fishes. However, direct and reliable estimates of M are difficult to obtain, and the equation was derived from data on 175 fish stocks composed of 84 species from freshwater and diverse marine environments. Therefore, we averaged the results from both methods, would likely give a more accurate estimate of M. This value (0.39) is slightly higher than the value (0.20-0.29) reported by Amezcua et al. (2006) and that of 0.28 reported by Andrade (2003), being very similar to the mean of 0.38 reported for data of the Lutjaninae subfamily (Martínez, 2003). The exploitation rate is slightly higher than optimum (0.5) and may indicate overfishing (Gulland, 1983).

In the present study mean size at capture was ~80mm SL, which appears very low compared to length at maturity (235mm FL) reported by Rojas (2001). Even when age appears to be more critical than size in reaching sexual maturity (Martínez, 2003), the difference between the two lengths is considerable. Since the shrimp fleet captures young snapper before they reach reproductive age, there is a potential recruitment problem caused by overfishing (Díaz et al., 2004). Andrade (2003) has suggested creating normative conditions to regulate direct and indirect fishing mortality of L. peru and L. guttatus. Notably, the shrimp bycatch contains many species (van der Heiden, 1985) and fishing effects are likely to be different for each species, depending on life history traits such as age at maturity, growth rate, and potential rate of population increase, which can influence abundances (Jennings et al., 1998). In most of the limited reports on juvenile spotted rose snapper, they were caught by shrimp trawlers, and estimates may be biased by the localities where caught, including their preferred habitat. Even when juvenile spotted rose snapper are not dependent on estuarine coastal lagoons, they may inhabit these environments (Martínez, 2003) which are closed to industrial shrimp fishing fleets. A shrimp and fish diet (Rojas, 1996; Saucedo and Chiapa, 2000) could also extend their habitat over hard bottoms. The scarce landing records indicate that the snappers are fished mainly over large areas of the central and southern Mexican Pacific coast, where almost no shrimp fishing takes place (CONAPESCA, 2003). Therefore, it is important to determine the extent to which shrimp trawling affects populations of species subject to indirect fishing mortality.

The authors thank E. Herrera-Valdivia, J.G. Padilla-Serrato, A.M. Siaruki-Quijano, and R. Morales-Azpeitia of CINBOR, who assisted with identifying specimens, biometrics, and preparing the database. Funding was provided by Secretaría de Agricultura, Ganadería, Desarrollo Rural, Pesca y Alimentación (SAGARPA-CONACYT grant 2003-C01-089) and CIBNOR (Project EP1.1.). The Instituto Nacional de la Pesca of Mexico also provided funding and fish samples. O.A.G.O. is a recipient of a doctoral fellowship from Consejo Nacional de Ciencia y Tecnología of Mexico.

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