This study evaluated the growth and population structure of Xiphopenaeus kroyeri in Babitonga Bay, southern Brazil. Monthly trawls were conducted from July 2010 through June 2011, using a shrimp boat outfitted with double-rig nets, at depths from 5 to 17 m. Differences from the expected 0.5 sex ratio were determined by applying a Binomial test. A von Bertalanffy growth model was used to estimate the individual growth, and longevity was calculated using its inverted formula. A total of 4,007 individuals were measured, including 1,106 juveniles (sexually immature) and 2,901 adults. Females predominated in the larger size classes. Males and females showed asymptotic lengths of 27.7 mm and 31.4 mm, growth constants of 0.0086 and 0.0070 per day, and longevities of 538 and 661 days, respectively. The predominance of females in larger size classes is the general rule in species of Penaeidae. The paradigm of latitudinal-effect does not appear to apply to seabob shrimp on the southern Brazilian coast, perhaps because of the small proportion of larger individuals, the occurrence of cryptic species, or the intense fishing pressure in this region. The longevity values are within the general range for species of Penaeidae. The higher estimates for longevity in populations at lower latitudes may have occurred because of the growth constants observed at these locations, resulting in overestimation of this parameter.

Asymptotic length, von Bertalanffy, longevity, Dendrobranchiata

The impact of shrimp fisheries in tropical regions is now comparable to impacts on the world’s most intensively exploited temperate continental-shelf ecosystems. These fisheries have caused significant losses of spawning biomass and biodiversity, especially as a consequence of trawling on soft bottoms (Pauly and Christensen 1995, Pauly et al. 2002). Information about population biology can be important for understanding the life cycle of intensively fished species such as the seabob shrimp Xiphopenaeus kroyeri (Heller). This information can be developed from measurements of the size-class distribution, sex ratio, modal progression, growth, and longevity at spatial and temporal scales (Gab-Alla et al. 1990, Nakagaki and Pinheiro 1999).

Xiphopenaeus kroyeri has a wide geographical range in the western Atlantic Ocean, from Cape Hatteras (North Carolina, USA) to southern Brazil (Rio Grande do Sul) (Costa et al. 2007). Of the several species targeted by artisanal fishermen in southern Brazil, the seabob shrimp is one of the most important, and is also among the top ten penaeid species caught worldwide (Gillett 2008, Silva et al. 2013). Unfortunately, this species is overexploited in southeastern and southern Brazil (Vasconcellos et al. 2007, Almeida et al. 2012).

Many investigators have suggested that several environmental parameters and resources affect the observed patterns of population dynamics of species of decapod crustaceans. These parameters include temperature oscillations (proximate factor) and plankton productivity (ultimate factor), among others, and all are affected to various degrees by latitude (Bauer 1992, Clarke 1993, Boschi 1997, Castilho et al. 2007b, Costa et al. 2010).

Knowledge of the growth and longevity of penaeid shrimps is still limited, although these are important attributes in the study of population dynamics of heavily exploited vulnerable species (Petriella and Boschi 1997). The lack of studies on growth in decapod crustaceans can be attributed to difficulties in estimating growth, which are related to the absence of structures that can provide information about aging (Petriella and Boschi 1997, Branco 2005, Vogt 2012). Because growth in decapod crustaceans is discontinuous, frequently interrupted by successive ecdyses, the von Bertalanffy (1938) model is the most useful for the study of animals that grow rapidly, such as penaeid and sergestid shrimps (Petriella and Boschi 1997).

The present study evaluated the population biology of X. kroyeri in the Babitonga Bay region, focusing on the sex ratio at different times of the year, juvenile recruitment, growth rates, and longevity of males and females. The longevity of X. kroyeri was compared with studies of the same species at different latitudes, to determine whether the latitudinal paradigm is applicable to this population.

For Dendrobranchiata, results of several studies have revealed a predominance of females in higher size classes, e.g., for Artemesia longinaris Bate (Castilho et al. 2007a, Costa et al. 2010), Pleoticus muelleri (Bate) (Castilho et al. 2008a, Dumont and D’Incao 2008) and Sicyonia dorsalis Kingsley (Castilho et al. 2008b), as well as for X. kroyeri (Branco 2005, Castro et al. 2005, Almeida et al. 2012, Eutrópio et al. 2013, Heckler et al. 2013). These studies also collected overall more females than males, which concords with results of the present study. In penaeid shrimps, females generally reach larger sizes than males (Boschi 1969, 1989). As observed by Gab-Alla et al. (1990), Branco et al. (1999) and Yamada et al. (2007), this difference may be associated with the reproductive biology, as an increase in carapace length indicates a capacity to produce more oocytes, with a consequent increase in fecundity.

Several studies regarding X. kroyeri (Branco et al. 1994, Nakagaki and Negreiros-Fransozo 1998, Lopes et al. 2010) reported male-biased sex ratios. These fluctuations can be attributed to segregated distributions of the sexes at certain times of year (Signoret 1974, Branco 2005). Alternatively, a female-biased sex ratio may be associated with higher mortality in males (Cha et al. 2002). Kevrekidis and Thessalou-Legaki (2006), studying Melicertus kerathurus (Forskål), stated that the high metabolic demand of females during the period of gonadal maturation forces them to feed for long periods, increasing their vulnerability to sampling gear. Lopes et al. (2010) stated that a male-biased sex ratio is less common in X. kroyeri, and might be associated with migration of females to greater depths during spawning periods.

Gender is among the factors that influence growth in Penaeoidea (Petriella and Boschi 1997). Generally, males of Penaeidae show higher growth coefficients than females, resulting in lower asymptotic lengths, and males are smaller than females of equal age. In this study, females showed a lower growth coefficient and greater asymptotic length, which concords with information provided by García and Le Reste (1986) and Petriella and Boschi (1997), as well as with previous analyses of the growth of this species along the Brazilian coast (Branco 2005, Fernandes et al. 2011, Heckler et al. 2013).

On the Brazilian coast, the growth of X. kroyeri has been studied by Santos and Ivo (2000) on the coast of the state of Bahia; Heckler et al. (2013) off São Paulo; and Campos et al. (2011) off Santa Catarina, among others. Santos and Ivo (2000) obtained the lowest k values, as well as the highest asymptotic length (females: k = 0.767/year, CL_∞= 37.2 mm; males: k = 0.986/year, CL_∞= 28.0 mm), followed by Campos et al. (2011) (females: k = 2.628/year, CL_∞= 31.0 mm; males: k = 2.993/year, CL_∞= 28.0 mm) and by Heckler et al. (2013) (females: k = 2.190/year, CL_∞= 30.54 mm; males: k = 3.280/year, CL_∞= 26.38 mm). Castilho et al. (2007b) suggested that several aspects of the life cycle such as longevity, body size, and size at the onset of sexual maturity show variations related to environmental factors that can be associated with latitude. As latitude increases, individuals tend to grow larger and more slowly, probably because of the colder water temperatures. This is known as the latitudinal effect paradigm, which has been analyzed with respect to its influence on population dynamics of penaeid shrimps by several researchers (Bauer 1992, Boschi 1997, Costa and Fransozo 2004, Castilho et al. 2007b, Costa et al. 2010).

The results of growth analyses for X. kroyeri along the southern Brazilian coast do not seem to follow the latitudinal effect pattern. The lower growth coefficient (and consequently the greater asymptotic length) observed by Santos and Ivo (2000) can be explained by the methodology adopted by the authors, who used the software FAO-ICLARM Stock Assessment Tool (FISAT), which includes the ELEFAN routine (Electronic Length Frequency Analysis). Fonseca (1998) noted that this software tends to exclude extreme lengths from the analysis, which causes errors in estimating the growth coefficient. The analysis conducted by Freire (2005) on the São Paulo coast (Ubatuba) provided higher results for asymptotic length, compared to that obtained by Campos et al. (2011) on the Santa Catarina coast and to the present results. These differences can be explained by differences in sampling depth, since the study by Freire (2005) included deeper sites (up to 35 m). The results obtained in the present growth analyses are consistent with the maximum size of the shrimp observed in field sampling (females: estimate = 31.41 mm, field observation = 31.80 mm; males: estimate = 27.73 mm; field observation = 27.90 mm).

Three additional hypotheses may explain the disagreement here presented in relation to the latitudinal effect pattern. First, as observed by Gusmão et al. (2006), it is possible that two different species of Xiphopenaeus live in the southwestern Atlantic. In the brazilian coast, these authors examined an area extending from Natal in the state of Rio Grande do Norte in northeastern Brazil, to Ubatuba, and found that Xiphopenaeus sp. 1 occurred throughout the area, whereas Xiphopenaeus sp. 2 showed a discontinuous distribution, with recorded occurrences only at Natal and Ubatuba. The second hypothesis is based on the number of individuals sampled in higher size classes, which can be explained in many ways. As noted by Campos (2006), larger animals could be less vulnerable to capture, and may have some ability to escape from the sampling gear. The author also suggested that larger individuals migrate to areas that are not accessible to the artisanal fishery. In agreement with this suggestion, Freire (2005) observed an increasing trend in CL toward greater depths, explaining the higher CL values found in his study. In further support of the difference in efficiency of the fishing methods, in 2011, even with a smaller fleet (14% of 806 vessels), the industrial fishery accounted for 47% of all seabob shrimp landings in the state of São Paulo (Mendonça et al. 2013).

The third hypothesis considers the intense fishery effort off the coast of Santa Catarina, which has been studied by D’Incao et al. (2002), Branco (2005) and Vasconcellos et al. (2007), among others. Although Mendonça et al. (2013) reported that the seabob shrimp stocks off São Paulo state have stabilized in recent years, Vasconcellos et al. (2007) argued that these stocks off southeastern and southern Brazil are overexploited. This intense fishery exploitation was responsible for the decrease in yield of the seabob shrimp fishery in these areas (D’Incao et al. 2002). It is entirely possible that, due to the intense fishery effort in this area, the stocks cannot maintain their normal population structure, i.e., individuals cannot reach larger size classes.

The absence of a pattern of asymptotic length estimated for this species may be a consequence of gene flow between the populations off Santa Catarina and São Paulo, since under favorable conditions the shrimp can migrate up to 900 km, as observed by Fenucci (1988) for Melicertus plebejus (Hess) on the Australian coast. Also, similarities in reproductive biology among populations that are in some cases located thousands of kilometers apart, may indicate the existence of an open meta-population, with considerable connectivity in the southwestern Atlantic (Almeida et al. 2012).

The present results for the longevity and growth coefficient of X. kroyeri fall within the range proposed by García and Le Reste (1986) and Pauly (1984) for these parameters in penaeid shrimps. These investigators found that the shrimp complete their life cycle in approximately two years, with growth coefficients between 0.25 and 2.5. Our estimates suggest that, the seabob shrimp in the study area has a longevity of 538 days or 1.47 years for males, and 661 days or 1.81 years for females. Similarly, Campos et al. (2011) on the coast of Tijucas, Santa Catarina, estimated a longevity of 561 days for males and 641 days for females. Analyses of populations at lower latitudes have estimated greater longevities for males and females respectively: 1705 and 2192 days (Santos and Ivo 2000), 2192 and 2422 days (Santos 1997) and 1535 and 1212 days (Santos 1997). According to D’Incao and Fonseca (1999), most longevity analyses for penaeid shrimps have resulted in overestimates; based on underestimates for the growth coefficient, which increases the longevity estimate.

In general, the present results indicate that X. kroyeri completes its life cycle within the study area, because juveniles as well as adults with a range of sizes were collected. All estimates in our study concord with current knowledge of the life cycle of X. kroyeri, are within the ranges proposed by several investigators, and are similar to values observed in field sampling. This study provides a theoretical basis for informed management of this fishery along the Brazilian coast.

The authors are grateful to the Fundação para o Desenvolvimento da Unesp FUNDUNESP (#1214/2010 – DFP), the Pró Reitoria de Pesquisa – PROPE, and the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for providing financial support during field collections and travel to other laboratories (Grants 2010/50188-8), and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial support during collections and analyses. Also, we thank many colleagues from the NEBECC and LABCAM group who helped with sampling and laboratory analyses; Dr. Janet Reid for her tremendous assistance in improving the English of the manuscript; and the Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis (IBAMA) for granting permission to collect the shrimp.