Interannual Variability in Mantle Length Structure ...

Interannual Variability in Mantle Length Structure, Recruitment, and Sex Ratio of Jumbo Squid, Dosidicus gigas, in the Central Gulf of California, Mexico Author(s) :José Iván Velázquez-Abunader, Agustín Hernández-Herrera, Susana Martínez-Aguilar, Juan Gabriel Díaz-Uribe and Enrique Morales-Bojórquez Source: Journal of Shellfish Research, 31(1):125-134. 2012. Published By: National Shellfisheries Association URL: http://www.bioone.org/doi/full/10.2983/035.031.0116

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Journal of Shellfish Research, Vol. 31, No. 1, 125–134, 2012.

INTERANNUAL VARIABILITY IN MANTLE LENGTH STRUCTURE, RECRUITMENT, AND SEX RATIO OF JUMBO SQUID, DOSIDICUS GIGAS, IN THE CENTRAL GULF OF CALIFORNIA, MEXICO

JOSE´ IVAŃ VELA´ZQUEZ-ABUNADER,1* AGUSTIŃ HERNAŃDEZ-HERRERA,2 SUSANA MARTIŃEZ-AGUILAR,3 JUAN GABRIEL DIÁZ-URIBE3 AND ENRIQUE MORALES-BOJO´RQUEZ4 1 Centro de Investigaciones y de Estudios Avanzados del Instituto Politećnico Nacional, Unidad Me´rida, Antigua carretera a Progreso km 6, CP 97310, Me´rida, Yucatań, Me´xico; 2Centro Interdisciplinario de Ciencias Marinas, Instituto Politećnico Nacional, Av. IPN. s/n. Colonia Playa Palo de Santa Rita, CP 23000, La Paz, Baja California Sur, Me´xico; 3Instituto Nacional de Pesca, Centro Regional de Investigacioń Pesquera La Paz, Carretera a Pichilingue s/n km 1, CP 23020, La Paz, Baja California Sur, Me´xico; 4Centro de Investigaciones Biolo´gicas del Noroeste SC (CIBNOR), Mar Bermejo 195, Col. Playa Palo de Santa Rita, CP 23090, La Paz, Baja California Sur, Me´xico ABSTRACT Changes in the number and abundance of the cohorts of jumbo squid are a demographic response associated with high variability in recruitment, and have implications for availability and accessibility to the fishing fleets. In this study, we analyzed the interannual changes in the size structure, recruitment, and sex ratio of jumbo squid Dosidicus gigas in the central Gulf of California, Mexico. Data were analyzed for the 2000 to 2009 fishing seasons (from March to November). The biological data were collected biweekly at the port of Santa Rosalı´ a, Baja California Sur, during each fishing season. We recorded mantle length and mantle weight, and sex (male or female) was identified from morphochromatic properties of fresh gonads. We concluded that the mantle length structure of jumbo squid changed between 1 cohort and 3 cohorts from 2000 to 2009. In the study zone, the presence of 2 cohorts is common. The species shows positive allometric growth, and the females are more abundant than the males in the region. The comparison between the most important fishing grounds in the central Gulf of California (Santa Rosalı´ a and Guaymas) showed similar patterns, such as the number of cohorts, sex ratios, growth pattern, and migration pattern identified between both coasts. We believe that this could be evidence of one population that is widely distributed in the central Gulf of California. KEY WORDS: jumbo squid, Dosidicus gigas, mantle length structure, cohorts, recruitment, sex ratio, growth

INTRODUCTION

The jumbo squid (Dosidicus gigas d’Orbigny 1935) is widely distributed in the eastern Pacific, from Alaska to Chile (Cosgrove 2005, Wing 2006, Zeidberg & Robison 2007). This new distribution pattern was observed between Monterey, CA, and Chile (Ehrhardt 1991). The presence of this species in Monterey has always been assumed to be a temporal invasion; the first reports of this phenomenon were documented during the 1930s (Croker 1937). Consequently, jumbo squid is identified as an endemic species to the eastern Pacific Ocean (Nesis 1983, Nigmatullin et al. 2001). In Mexico, most commercial catches of jumbo squid are harvested from the central Gulf of California (between 22°N and 28°N, and 109°W and 113°W). However, fishing areas can vary depending on squid distribution and oceanographic conditions. For example, during 1998, when a strong ENSO occurred, high catches were reported from the southern Gulf of California, beyond traditional fishing areas (Morales-Bojo´rquez et al. 2001a). Unusual catches from Loreto, Bahı´ a de La Paz, Bahı´ a La Ventana, and even in Bahı´ a Magdalena on the Pacific coast account for a probable range expansion of the Gulf of California jumbo squid population. Recent occurrences of jumbo squid in Ensenada, near the northern border of Mexico from 2007 to 2009, and in Bahı´ a Magdalena during 2009 and *Corresponding author. E-mail: [email protected] DOI: 10.2983/035.031.0116

2010, have resulted in an increased demand for squid-fishing permits along the occidental coast of the Peninsula of Baja California. In this fishery, there are different fishing fleets. The fishing effort was increased as follows: 2 fleets from Baja California Sur (Santa Rosalı´ a and Bahı´ a Magdalena), 3 fleets from Baja California (Ensenada), and 2 fleets from Sonora, Mexico. In the Gulf of California, the landings vary seasonally as a function of the availability of squid. Off Baja California Sur, fishing occurs during spring and summer, and off Sonora, during fall and winter (Fig. 1). Baja California Sur and Sonora have artisanal fleets. The traditional artisanal fleets consist of small open boats (known locally as pangas), each operated by 2 fishermen. The fishing gear is a hand jig with 6 rings of barbless hooks; only 1 jig (14 cm) per line is attached to the end of a nylon line (300–400 m). In addition, Sonora has a fishing fleet of shrimp trawlers of various characteristics equipped with a manual jigging system or hand jigs. The Ensenada region has different fleets: an artisanal fleet similar to those observed in Sonora, commercial trolling, and trawler (midwater) vessels; both fleets are adapted and use a manual jigging system with a light system. Population dynamics studies on jumbo squid in the Gulf of California have shown that the number of cohorts is variable. Although Ehrhardt et al. (1983) reported 5 cohorts in the Gulf of California from January to September 1980, HernańdezHerrera et al. (1998) found only 1 cohort between November 1995 and November 1996. More recent studies over a wider time frame have shown a variable interannual number of cohorts

125

VELA´ZQUEZ-ABUNADER ET AL.

126

TABLE 1.

Number of squid sampled per fishing season at the port of Santa Rosalı´ a, Baja California Sur, Mexico. Year Month March April May June July August September October November

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 100

140 141 257 298 121

292 279 269 145 31 177

215 92 178 265 136 153

73 107 94

336 273 256 247 138 123 118

91 101 146 206 224 227 184

96 213 213 100 231 83

246 254 236 200 112

153 204 210 235 101 188 206 215

223 247 235 250 243 252 250

morphochromatic properties of fresh gonads (Dı´ az-Uribe et al. 2006). The number of squid sampled each month per fishing season is shown in Table 1. Statistical Analysis Figure 1. Study area of Dosidicus gigas in the central Gulf of California, Mexico. The shaded area represents the fishing ground of jumbo squid at Santa Rosalı´ a, Baja California Sur.

from 1–3 (Morales-Bojo´rquez et al. 2001b, Morales-Bojo´rquez & Neva´rez-Martı´ nez 2010, Neva´rez-Martı´ nez et al. 2010, Vela´zquez-Abunader et al. 2010). The origin of the cohorts has been proposed to be either a phenotypic (Nesis 1983, Keyl et al. 2008, Keyl et al. 2011) or genetic response (Nesis 1983). Nigmatullin et al. (2001) explained the complicated intraspecies size structure of jumbo squid along the latitudinal gradient in the eastern Pacific. They suggested the presence of 3 groups on the basis of mantle length (ML): small, ML of 13–26 cm in males and 14–34 cm in females; medium, ML of 24–42 cm in males and 28–60 cm in females; and large, individuals with an ML of 50–120 cm. In this last group, the females are larger than the males. Some mechanisms that can, in theory, cause variability in the size structure included environmental factors (Hill 1985), the population area, the spatial distribution of individuals (Winters & Wheeler 1985), the abundance (MacCall 1976), density dependence of the squid population and fishing by different fleets, and schooling behavior (Ye & Mohammed 1999). Changes in the number and abundance of the cohorts of jumbo squid are a demographic response that have implications for availability and accessibility to the fishing fleets. In this study, we analyzed the interannual changes in the size structure, recruitment, and size at first capture of jumbo squid in the central Gulf of California, Mexico.

To estimate the relationship between ML and MW, the power equation MW ¼ aMLb was used for each fishing season and the total data, where a is the average condition factor and b is the coefficient of allometry, indicating isometric growth when equal to 3 and allometric growth when significantly different from 3 (Esmaeili & Ebrahimi 2006, Aguirre-Villasenõr et al. 2008). The estimated value of b was analyzed with Student’s t-test (Sokal & Rohlf 1995, Zar 1999) to determine whether growth was isometric or allometric. Sex ratio data were calculated monthly as the number of females divided by the sum of males and females, and the data were categorized by a 4-cm ML class. To determine whether the sex ratio can be regarded as 1:1, a chi-square hypothesis test was carried out (chi-square, a ¼ 0.05) (Sokal & Rohlf 1995). Mantle Length–Frequency Distributions

The mantle length–frequency distribution of jumbo squid in each fishing season was represented graphically as frequency histograms. In this way, the observed modes were fitted to a multimodal model defined by Pfxi jn; p1 ; p2 ; ::. . . ; pkg ¼ n!

i

i¼1

xi !

(1)

where xi is the number of times a type i event occurs in n samples, n is the sample size, and pi are the separate probabilities of each of the type k events possible. To estimate the model parameters, it is necessary to transform Eq (1) into a likelihood expression. Therefore, the new equation is

MATERIALS AND METHODS

Data from the 2000 to 2009 fishing seasons in the Gulf of California were analyzed. Two biological sampling sessions per month were carried out during each fishing season (usually from April to November) at the port of Santa Rosalı´ a, Baja California Sur (Fig. 1). Random samples of squid in commercial catches were selected to record the individual ML (±0.1 cm) and mantle weight (MW; ±0.1 kg), whereas sex was identified from

k Y pxi

In L fxi jn; p1 ; p2 ; :::::; pkg ¼

n X i¼1

½xi Inðpi Þ

(2)

The main assumption for the parameter estimation is that the size distribution for each mean ML or mode can be analyzed with a normal distribution, determining that each mode corresponds to a different size group or cohort in the squid population

INTERANNUAL VARIABILITY OF DOSIDICUS GIGAS (Keyl et al. 2011). Using this condition, the estimations of the relative expected proportions of each ML category are described using the density function pML

1 pffiffiffiffiffiffi 3 e ¼ sn 2p

ðMLmn Þ2 2s2

(3)

where mn and sn are the mean and SD of the ML of each size group. Expected frequencies were estimated with the logarithmic function of the multinomial distribution (Eq 2), and final values of the model parameters were assigned by comparing the observed and expected frequencies (Haddon 2001, AguirreVillasenõr et al. 2006). The objective function for parameter estimation was defined as n k X X î L In LfMLjmi ; si g ¼ (4) Li Inðpî Þ ¼ Li In P î L i¼1

i¼1

where mi and si are the mean and SD of each modal group present during a fishing season. The initial values for parameters in Eq 4 were assigned using 2 criteria: (1) a visual inspection of the mantle length–frequency distribution (Montgomery et al. 2010) and (2) previous knowledge of the recruitment events (Neva´rez-Martı´ nez et al. 2006, Neva´rez-Martı´ nez et al. 2010).

127

The model parameters were estimated when the negative loglikelihood function (Eq 4) was minimized with a nonlinear fit using Newton’s algorithm (Neter et al. 1996). When the sample had more than 1 modal group, the separation index (SI) was used (Sparre & Venema 1992): SI ¼

MLj MLi ! $2 S2j + S2i 2

(5)

where MLj and MLi are the mean MLs of j and i modal groups, and S2j and S2i are SDs for the j and i modal groups. If SI < 2, then it was not feasible to separate the normal components of the frequencies observed (Sparre & Venema 1992). The previous analysis was used to determine the number of size groups and to estimate the recruitment of jumbo squid. We defined recruitment as the number of individuals at a certain age, size, or stage added to the exploitable stock each year as a result of growth and/or migration into the fishing area. The choice of the age and stage varies (Myers 2002). In marine fisheries, recruitment usually refers to the first age at which individuals are targeted for harvest (Hilborn & Walters 1992, Quinn & Deriso 1999, Haddon 2001). Boyle & Rodhouse (2005) defined recruitment as the number of individuals that reach

Figure 2. Monthly mantle length–frequency distribution from the 2000 to 2009 fishing seasons. The frequency histograms per month are represented as relative frequency (measured as a percentage) and mantle length (in centimeters).


128

a specified stage of the life cycle (e.g., metamorphosis, settlement, joining the fishery). Different measures of recruitment are valid, and the choice often depends on the ease of measurement. We used the accepted concept of recruitment given by Hilborn & Walters (1992), Quinn and Deriso (1999), Haddon (2001), and Myers (2002). Mantle Length at First Capture

ML at first capture was calculated with the cumulative ML frequency of squid using 4-cm ML intervals. The ML at which 50% of the jumbo squid are captured (MLc) was determined by fitting the logistic model (Neva´rez-Martı´ nez et al. 2010): Pi ¼

1 1+ exprðLi L50 Þ

(6)

where Pi is the cumulative probability at i interval, Li is the midpoint of the ML at i interval, r is the intercept, and L50 is MLc (in centimeters). Fitting was performed by nonlinear regression and minimizing the squared sum of residuals with a nonlinear fit using Newton’s algorithm (Neter et al. 1996). RESULTS

For the fishing seasons between 2000 and 2009, 11,429 individuals were analyzed. Most biological samples were obtained from May to October, when the squid are mainly distributed in the fishing grounds off Santa Rosalı´ a, Baja California Sur. When the fishing season extended to November (2000, 2001, 2004, and 2008) or started earlier in March or April (2004 to 2009), sampling was also achieved for these months. For some fishing seasons such as 2001, 2007, and

2009, a hurricane and tropical storms prevented us from monthly sampling during September or October. In 2000 and 2003, some logistical problems prevented us from regular sampling throughout the entire fishing season (Table 1). The smallest individuals were observed during 2004 (ML ¼ 27 cm), 2008 (ML ¼ 28 cm), and 2009 (ML ¼ 29 cm), whereas the largest ones were sampled during 2001 (ML ¼ 93 cm) and 2008 (ML ¼ 102 cm; Fig. 2). The ML–MW annual relationships showed positive allometric growth for jumbo squid for all fishing seasons (b 6¼ 3; Student’s t-test, P < 0.05). The b values varied between 3.2 (2003) and 3.6 (2005); the parameters of the power equation during the study period are shown in Table 2. The same pattern of positive allometric growth was estimated during the entire study period; the comparison between the power equation from 2000 to 2009 (a ¼7 3 10–5, b ¼ 3.2, R2 ¼ 0.92, F(1,11287) ¼ 0.13 3 10–4, P < 0.0001) and each fishing season is shown in Figure 3. Estimates of the sex ratio showed that females were more abundant than males during the spring and summer months of the 2001 to 2003 and 2007 to 2009 fishing seasons (chi-square, P < 0.05); the same pattern was observed during autumn of the 2000, 2002, and 2004 to 2006 fishing seasons (chi-square, P < 0.05). Males were more abundant than females during the spring months of 2006, 2008, and 2009 (Fig. 4). This pattern in the sex ratio showed that in the Santa Rosalı´ a region, females are predominant in the population. ML–frequency distributions of jumbo squid in each fishing season showed that the size structure changes between 1 size group and 3 size groups in the population (Fig. 5, Table 3). Only 1 size group composed of adult individuals was identified during

TABLE 2.

Mantle length–mantle weight relationship (a, b) parameters. Mantle Length (cm)

Parameters

Maximum

a

b

R2

35

89

31

93

2002

1,039

37

90

2003

274

26.2

100.9

2004

1,591

27.4

84.9

2005

1,048

36

82.9

2006

1,027

29.8

87

2007

1,088

30

93.6

2008

1,512

27.6

97.8

2009

1,700

24.9

82.1

3.35 (3.27–3.42) 3.32 (3.27–3.37) 3.53 (3.49–3.57) 3.30 (3.21–3.40) 3.35 (3.30–3.39) 3.59 (3.53–3.64) 3.36 (3.29–3.42) 3.30 (3.26–3.34) 3.45 (3.41–3.49) 3.31 (3.26–3.35)

0.88

1,193

4 3 10–6 (2 3 10–6–5 3 10–6) 4 3 10–6 (3 3 10–6–5 3 10–6) 1.8 3 10–6 (1.9 3 10–6–2 3 10–5) 4 3 10–6 (2 3 10–6–6 3 10–6) 3 3 10–6 (2 3 10–6–4 3 10–6) 1.4 3 10–6 (1.1 3 10–6–1.8 3 10–6) 3 3 10–6 (2 3 10–6–5 3 10–6) 4.8 3 10–6 (4.1 3 10–6–5.6 3 10–6) 2.6 3 10–6 (2.2 3 10–6–3 3 10–6) 6 3 10–6 (5 3 10–6–7 3 10–6)

Year

n

2000

957

2001

Minimum

0.94 0.96 0.94 0.93 0.93 0.90 0.96 0.95 0.91

Sample size (n), mantle length interval (minimum and maximum), and the coefficient of determination (R2) are shown. Confidence intervals are presented in parentheses.

INTERANNUAL VARIABILITY OF DOSIDICUS GIGAS

129

Figure 3. Mantle length (in centimeters) and mantle weight (in kilograms) relationship for Dosidicus gigas. The dashed line represents the mantle length and mantle weight relationship for all fishing seasons; the solid line represents the mantle length and mantle weight relationship for each fishing season analyzed.

2000 and 2006, whereas 3 size groups were found in 2003. In the remaining fishing seasons, 2 size groups were identified (Table 3), compared with only 1 size group during 2000 (ML ¼ 73.2 cm) and 2006 (ML ¼ 63.8 cm). These size groups were identified as individual adults in the population. Figure 6 shows the mean ML of the smallest size group of each fishing season. ANOVA showed significant differences in the ML of these size groups (F(9,7035) ¼ 1,258, P < 0.0001), and the post hoc test (P < 0.05) showed that the largest mean ML in the historical series occurred in the 2000 and 2006 fishing seasons. The estimated MLc showed the influence of the number of size groups identified in the population. The highest MLc values were estimated during the 2000 (69 cm) and 2003 (71.3 cm) fishing seasons. During the first period, only 1 size group was observed (large). However, during the 2003 fishing season, 3 size groups were estimated—large, medium, and small—although the large group was the most abundant (Fig. 5). The variation of MLc, excluding the 2000 and 2003 fishing seasons, ranged from 42.8 (2009)–60.9 cm (2002; Table 4). According to the ML– frequency distribution and the dominant size group, the size at first capture depends on the number and abundance of size

groups in the population, which could explain the changes in the MLc and ML of recruits in the fishery. DISCUSSION

As is the case for many other squid species, because of their short life span the stock reduction method is commonly used for abundance estimation, which assumes one single cohort in the population (Morales-Bojo´rquez et al. 2001a). Thus, length– frequency analyses are valuable tools to gain insight into the population dynamics of exploited resources, and to identify problems such as inconsistent year-class strength, slow growth, and excessive mortality. Therefore, it is important to identify the link between size groups and the number of cohorts in the population. For jumbo squid from the Peruvian waters, between 1 cohort and 6 cohorts were determined by mode discrimination using ML–frequency distribution (Keyl et al. 2011). The variability observed in the ML structure of the jumbo squid population in the central Gulf of California suggests that the number of cohorts varied interannually between 1 and 3, with the common presence of 2 cohorts. This variability could be

130


Figure 4. Monthly sex ratio for Dosidicus gigas at Santa Rosalı´ a, Baja California Sur. The asterisk denotes significant differences (chi-square, P < 0.05) in sex ratio. Females (n) and males ( ).

attributable to 3 key factors: (1) variability in recruitment (Morales-Bojo´rquez et al. 2001a), (2) variability in individual growth (Markaida 2006), and (3) environmental influence such as an El Ninõ or La Ninã events (Neva´rez-Martı´ nez et al. 2006). Hernańdez-Herrera et al. (1998) analyzed the annual ML– frequency for the fishing grounds off Guaymas and found evidence of only 1 cohort with annual recruitment. During fall and winter 1995, MLs between 60 cm and 80 cm were found, whereas a drastic change in the size composition was detected in spring 1996, with an ML between 20 cm and 40 cm. However, analysis of the catch-per-unit effort (CPUE) for the same fishing periods allowed 3 cohorts to be identified in the jumbo squid population, assuming that CPUE is an index of relative abundance (Morales-Bojo´rquez et al. 2001b). If only 1 cohort supported the population of jumbo squid, then the expected trend of CPUE would show a constant decrease; however, the index of relative abundance showed recoveries, with 3 consistent peaks of catchability. The peaks occurred at 3 ML intervals: the first between 29 cm and 33 cm, the second between 53 cm and 57 cm, and the third between 65 cm and 71 cm. (Morales-Bojo´rquez et al. 2001b). Three different cohorts were also identified by Vela´zquez-Abunader et al. (2010) using ML– frequency distribution from research oceanographic cruises

between 1997 and 2008. These discrepancies in the number of cohorts of the jumbo squid in the Gulf of California estimated from the ML composition of the commercial catch were reanalyzed by Morales-Bojo´rquez and Neva´rez-Martı´ nez (2002) and Morales-Bojo´rquez et al. (2008), who concluded that the jumbo squid population has several cohorts in its ML structure, and that the most abundant cohort will be identified as recruitment. Considering recruitment as a key process in a dynamic population, and that recruits can be defined as the first size interval subject to fishing pressure (Myers 2002), then the variability in size composition of the jumbo squid can be used as a good indicator of recruitment. In the current study, during the 2000 and 2006 fishing seasons, there was only 1 medium-tolarge size group. The absence of the small size group commonly caught by the fishery in these years probably indicates a failure in the recruitment of this group. In the Gulf of California, the 3 groups described by Nigmatullin et al. (2001) on the basis of ML, the small size group, a medium size group, and a third group with larger individuals, have been observed previously at Guaymas (Neva´ rez-Martı´ nez et al. 2006, Neva´rez-Martı´ nez et al. 2010). Despite its mathematical simplicity, the weight–length relationship represents a complex interaction of different factors,


131

Figure 5. The mantle length–frequency distribution of Dosidicus gigas for each fishing season. The observed data are shown as bars, and the line represents the model fitted to the observed data.

such as food availability, feeding rate, gonad development, and spawning period (Bagenal & Tesch 1978, Anderson & Gutreuter 1983). Isometric growth is defined when b ¼ 3, which means that any length increment is related to a proportional increment in weight. If b 6¼ 3, an allometric growth is observed. So the positive allometric growth (b > 3) estimated for giant squid, means that the growth in weight is proportionately greater than the growth in length. This growth pattern was also found by Neva´rez-Martı´ nez et al. (2006, 2010) at Guaymas along the Sonora coast between 1996 and 2008. The relationship between ML and MW was allometric and varied between fishing seasons, with negative allometry for the 1996 to 1997 fishing season (b < 3) and positive allometry (b > 3) for the others. Ehrhardt et al. (1983) reported b ¼ 2.9, close to isometric or a slightly negative allometric growth, and Hernańdez-Herrera et al. (1998) estimated b ¼ 3.4 by 1995 to 1996. Consequently, positive allometry seems to be the growth pattern of jumbo squid in the central Gulf of California. Given the fast growth and massive early reproduction strategy, positive allometry is congruent with the life history of jumbo squid (Nigmatullin et al. 2001). The importance of environmental variability for the recruitment strength among cephalopod populations is well known; however, there is no general agreement on criteria for the recognition of recruits. Typically, recruits are defined simply

by their body size. Despite the apparently simple life cycle of most fished cephalopod species, analysis of size– or age– frequency of the catch usually results in a series of multiple size or age groups, presumably representing successive subannual waves of recruitment. The reasons for this phenomenon are not clear, but perhaps it is mainly caused by the presence of more than 1 seasonal cohort (Boyle & Rodhouse 2005). Neva´rezMartı´ nez et al. (2010) identified a pattern in the variability of recruitment according to environmental variability. Warmer waters caused by El Ninõ conditions in the California Current were coincident with low recruitment. Cooler waters identified with negative anomalies in the sea surface temperature caused by La Ninã conditions were related to high recruitment. Markaida (2006) explained that in the Gulf of California, the response of the jumbo squid to changes in environmental conditions presented a certain degree of delay. The abundance of squid decreased even well after El Ninõ had ended and during the subsequent transition to La Ninã in 1998 to 2001. This could be caused by a failure in squid recruitment during the previous year. The great abundance of medium-size jumbo squid in the Gulf of California in early 1999 followed the development of the mature La Ninã conditions more closely. Markaida (2006) analyzed MLs of 157 jumbo squid between May 10, 1998, and October 29, 2000, to assess environmental


132 TABLE 3.

TABLE 4.

Number of cohorts estimated based on mantle–length frequency distributions for Dosidicus gigas at Santa Rosalı´ a, Baja California Sur, Mexico.

Estimates of the mantle at which 50% of the jumbo squid are captured (MLc), the r parameter, and residuals (SSQ).

No. of Cohorts Year

1

2000 2001 2002 2003 2004 2005

56.8 (56.3–57.3) 49.5 (49.2–49.8) 50.9 (50.1–51.8) 47.5 (47.1–47.8) 45.6 (45.4–45.8)

2006 2007 2008 2009

44.1 (43.8–44.4) 52.4 (52.0–52.8) 41.8 (41.5–42.1)

2 73.2 (72.8–73.5) 74.8 (74.3–75.2) 69.9 (69.5–70.4) 70.2 (69.4–71.0) 62.8 (62.4–63.2) 59.3 (58.8–59.8) 63.8 (63.3–64.3) 67.3 (66.8–67.7) 70.9 (70.5–71.2) 64.0 (63.7–64.4)

Year

r

MLc

SSQ

2000

0.22 (0.20–0.24) 0.13 (0.12–0.14) 0.13 (0.11–0.14) 0.10 (0.09–0.11) 0.15 (0.14–0.16) 0.19 (0.18–0.20) 0.17 (0.16–0.18) 0.12 (0.10–0.14) 0.14 (0.12–0.15) 0.16 (0.13–0.18)

69.09 (68.52–69.41) 60.31 (59.83–60.73) 60.90 (60.01–61.81) 71.35 (69.98–72.72) 52.98 (52.50–53.45) 56.61 (56.37–56.86) 58.21 (57.94–58.48) 59.08 (57.85–60.32) 56.40 (55.68–57.13) 42.89 (41.91–43.88)

0.008

3 2001 2002 2003 86.4 (85.8–87.0)

The mean value estimated by cohort is shown in bold type. Confidence intervals for each cohort are shown in parentheses.

factors and their impact on the size composition of jumbo squid in the Gulf of California. The locations in the Gulf of California were Isla Tortuga, Loreto, Topolobampo, and Santa Rosalı´ a. The most important fishing ground in the Gulf of California off Guaymas, Sonora, was not sampled (Morales-Bojo´rquez et al. 2001b). Markaida (2006) showed that the size composi-

2004 2005 2006 2007 2008 2009

0.005 0.017 0.033 0.005 0.001 0.002 0.032 0.012 0.026

The confidence intervals for MLc and r are shown in parentheses.

tion changed among locations, and the abundance of males and females varied. Our results also showed that females dominated the sex ratio at Santa Rosalı´ a, principally during spring and summer months, and occasionally during autumn months. The abundance and sizes of females could explain the changes in the ML structure. In our study, MLc varied from 42.8–71.3 cm; however, the interval estimated at Guaymas varied from 35.7–61 cm during 1995 to 2002, and from 56.6–67 cm during 2003 to 2008. The variation of MLc between locations could suggest segregation by size group in the central Gulf of California. We conclude that the ML structure of jumbo squid at Santa Rosalı´ a fluctuated between 1 and 3 cohorts from 2000 to 2009. In the study zone, the presence of 2 cohorts is common, defined as the medium size group (ML, 28–60 cm) and the large size group (ML, 50–120 cm). The species shows positive allometric growth, and the females are more abundant than the males in the region. MLc is larger than those caught at Guaymas. Last, the comparison between the most important fishing grounds in the central Gulf of California (Santa Rosalı´ a and Guaymas) showed similar patterns in the number of cohorts, sex ratios, growth pattern, and migration pattern identified between both coasts (Markaida et al. 2005, Gilly et al. 2006). We believe that this could be evidence of a single population that is widely distributed in the central Gulf of California. ACKNOWLEDGMENTS

Figure 6. Changes in the average mantle length (in centimeters) of the recruits of Dosidicus gigas at Santa Rosalı´ a, Baja California Sur. The confidence interval is shown as the upper and lower limits.

We thank the Centro Regional de Investigacioń Pesquera de La Paz, and CICIMAR for support in obtaining biological and statistical data of jumbo squid. AHH received support from COFAA-IPN and is an EDI-IPN fellow.


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