Morphometric relationships, age, growth, and mortality of the geoduck clam, Panopea generosa, along the Pacific Coast of Baja California, Mexico.
Article Type: Report
Subject: Clams (Observations)
Clams (Distribution)
Clams (Growth)
Aging (Observations)
Morphometrics (Biology) (Observations)
Authors: Calderon-Aguilera, Luis Eduardo
Aragon-Noriega, Eugenio Alberto
Hand, Claudia M.
Moreno-Rivera, Victor Manuel
Pub Date: 08/01/2010
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2010 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: August, 2010 Source Volume: 29 Source Issue: 2
Topic: Event Code: 690 Goods & services distribution Advertising Code: 59 Channels of Distribution Computer Subject: Company distribution practices; Company growth
Product: Product Code: 0913030 Clams NAICS Code: 114112 Shellfish Fishing SIC Code: 0913 Shellfish
Geographic: Geographic Scope: Mexico Geographic Code: 1MEX Mexico
Accession Number: 234712472
Full Text: ABSTRACT A commercial fishery for geoduck Panopea generosa in Baja California began in 2002 and has since achieved landings comparable with those in Washington state and British Columbia, Canada, where geoduck fisheries developed in 1970 and 1976, respectively. This investigation was motivated to acquire the knowledge of population biology of P. generosa at the southernmost extent of its distribution that is required to establish a biological basis for fishery management decisions. This article was further motivated to assess the appropriateness of the current minimum legal size of 130 mm established by the Mexican fishing authorities. Morphometric relationships, age, and growth, and natural mortality of P. generosa were investigated with animals collected from Bahia de San Quintin and Islas Coronado, Baja California, Mexico, collected monthly from April 2008 through January 2009. Ages were obtained by counting growth bands in cross-sections of the shell hinge plate and utilizing cross-dating methods to increase accuracy. Individual P. generosa ages ranged from 3-96 y over both sites. Growth is rapid during the first 10 y and then slows for individuals older than 20-25 y. The asymptotic length was found to be 134 mm, the mean total weight was 764 [+ or -] 255 g, the Brody growth coefficient (k) was 0.191, and the natural mortality rate was estimated to range from 0.0274-0.046, depending on the method used. These parameter values are similar to those of more northern populations of the same species. Because the proposed minimum legal size is close to the asymptotic length, and considering that geoduck size is difficult to determine before harvest, we suggest that the use of a minimum legal size is not an effective management strategy for geoduck clams.

KEY WORDS: Panopea generosa, geoduck, clam, fishery, growth, age, Baja California

INTRODUCTION

Species of the genus Panopea, commonly called the "geoduck clam" (family Hiatellidae), are found throughout the world. The genus comprises many species, including but not limited to P. abbreviata (Valenciennes, 1839) endemic to the southwestern Atlantic, P. zelandica (Quoy y Gaimard, 1835) found in the southwestern Pacific, P. japonica (Adams, 1849) distributed off the Aleutians and Japan to South Korea, P. globosa (Dall, 1898) endemic to the gulf of California, and P. generosa (Gould, 1850), which ranges from Alaska to Baja California, Mexico. According to Coan et al. (2000), P. generosa is synonymous with P. abrupta (Conrad, 1849). It was recently concluded that P. generosa is the proper name for the geoduck species harvested from Alaska to Baja California, Mexico (Vadopalas et al. 2010).

Several species of geoduck clams support commercial fisheries, and their population characteristics have been studied, including P. zelandica in New Zealand (Gribben & Creese 2005); P. abbreviata in Argentina, South America (Morsan & Ciocco 2004); and P. generosa in the Pacific Northwest of North America (Goodwin 1976, Sloan & Robinson 1984, Campbell et al. 2004). The commercial fishery for the Pacific geoduck, P. generosa, started in 1970 in Washington state, and in 1976 in British Columbia, Canada, where current average landings (2006 to 2008) are 1,600 t and 1,335 t, respectively (data available at www.st.nmfs.noaa.gov and www.env.gov.bc.ca/omfd). More recently, exploitation of geoduck species has extended to northwest Mexico: P. generosa along the Pacific coast of the Baja California peninsula and P. globosa in the Gulf of California. Production estimates for both species have increased from 49 t in 2002 to more than 1,200 t annually (2006 to 2008), rivaling the established fisheries to the north. The current value of the fisheries for both geoduck species in Baja California is comparable with the Canadian fishery at US$30-35 million (Khan 2006, SAGARPA 2007). The geoduck fishery of Baja California is therefore becoming a significant contributor to world geoduck production, and yet its development has been in the absence of site-specific biological and stock information.

The management strategy of the Pacific geoduck clam in Canada is a combination of limited entry and individual quotas (Zhang & Hand 2006). Surveys are conducted to estimate biomass, and target fishing mortality is derived from age-structured modeling. In Washington, the fishing mortality rate is based on the output of an age-structured equilibrium yield model (Bradbury & Tagart 2000), which relies on the 3-parameter von Bertalanffy growth model. Hoffmann et al. (2000) conducted a study in 4 regions of Washington state and found that growth parameters were different among regions and among sites within regions. Similar studies were conducted by Bureau et al. (2002) and Campbell and Ming (2003) in locations throughout British Columbia, with similar results: Parameters of the von Bertalanffy growth model were different among locations. In addition, Campbell et al. (2004) and Zhang and Hand (2006) have found that key parameters required to estimate biomass and productivity differ among regions.

The estimation of growth parameters requires accurate estimates of age. The method of counting the internal growth increments on shell cross-sections has been shown to be effective for establishing the age of many bivalves, and has been validated for geoduck by known-age cohort tracking (Shaul & Goodwin 1982, Gribben & Creese 2005) and, more recently, cross-dating methods (Black et al. 2008). The technique has been implemented using thin sections to determine the age of P. zelandica (Gribben & Creese 2005) and P. abbreviata (Morsan & Ciocco 2004), and using acetate peels for and P. generosa (Goodwin & Shaul 1984, Bureau et al. 2002, Bureau et al. 2003).

The Mexican fisheries authority, Comision Nacional de Acuacultura y Pesca, attempts to pursue a conservative approach in regulating the fishery by limiting license entry, imposing a minimum legal size of 130 mm, restricting fishing to within identified beds, and imposing a maximum allowable catch of 0.5% of total estimated biomass for prospective fisheries (for which there are no catch data) or 1% for commercial fisheries (postprospective beds that have catch data). The 130-mm shell length for minimum legal size is an arbitrary reference point, following the precautionary approach for responsible fisheries established by the Food and Agriculture Organization.

Because nothing is known about the population biology of P. generosa in Baja California, and given that such knowledge is required to establish sustainable harvest strategies and to anticipate harvest impacts for this emerging commercial fishery, a biological sampling program was conceived to determine estimates of population age distribution, natural mortality rate, morphometric relationships, and the parameters of the von Bertalanffy growth model. In this article we present the results of analysis of these samples and, furthermore, aim to evaluate the rationale and suitability of a minimum legal size management strategy for this species.

MATERIALS AND METHODS

Stud)' Site and Samph, Collection

Morphometric relationships, age, and growth of P. generosa were investigated with animals collected from Bahia de San Quintin and Islas Coronado on the west shore of the Baja Peninsula, Baja California, Mexico (30[degrees]23'N, 115[degrees]55'W and 32[degrees]25' N, 117[degrees] 15'W, respectively). Monthly samples were collected from the subtidal between 15 m and 25 m depth from April 2008 through January 2009, with sample sizes as follows: April (30 m), May (30 m), June (17 In), July (30 m), August (30 m), September (30 m), October (30 m), November (0 m), December (22 m), January (45 m). No samples were collected in November 2008, because high levels of suspended material in the water column made it impossible to locate geoduck siphon holes. Divers used a low-pressure compressor and water jet to loosen the clams from the substrate and harvest one at a time. After extraction from the sediment, the clams were transported to the laboratory in coolers and were processed immediately upon arrival. Individual shell length (SL; straight-line distance between the anterior and posterior margins of the shell), shell height (SH; distance between the dorsal and ventral margins), and shell width (SW; distance between the closed left and right valves with ventral margins touching) were measured to the nearest millimeter using vernier calipers. Individual total wet weight was obtained after draining water from the body cavity, and tissue wet weight and shell weight were measured by separating the shell from the tissue and blotting both dry to remove excess water. Gonads were kept for a separate investigation of age at maturity and spawn timing. All weight measurements were determined to the nearest 0.1 g using an electronic balance (UVD 500, A & D, Co., Ltd., Seoul, Korea). Sex was identified by histological analysis.

Morphometric Relationships

The length weight relationship was estimated using a model of the form: W = [alpha] x [L.sup.[beta]] + [[epsilon].sub.j], where W is total wet weight, L is SL, [alpha] and [beta] are parameters, and [[epsilon].sub.1] [approximately equal to] N(0, [[sigma].sup.2.sub.1]) is a normal variate.

Aging

Age was estimated using the acetate peel technique of Shaul and Goodwin (1982). The right valve from each individual was cut with a low-speed diamond saw through the center of the hinge plate. The cut surface was polished, etched with 5% hydrochloric acid solution for 1.5 min, rinsed with distilled water, and dried. An acetate peel impression was made by pressing acetate film, softened with 1 drop of acetone, onto the etched cross-section of the hinge plate. Growth rings imprinted on the acetate peel (Fig. 1) were counted using a Carl Zeiss microscope (Primostar, Carl Zeiss Imaging, Gottingem, Germany). Each peel was counted by 2 readers to confirm the estimated age. Cross-dating methodology was used to demonstrate the validity of the ring-counting method for aging Mexican geoducks and to increase accuracy (Black et al. 2008, Black 2009). Cross-dating relies on a growth synchrony within a population, in response to climate signals, and the identification of "signature years" that can be used accurately to identify the calendar year of each growth increment. Signature years in our samples were identifiable as narrow bands (1932, 1939, 1942, 1948, 1958, 1959, 1963, 1974, 1984, 1985, 1994, 2002) or as wide bands (1943, 1949, 1962, 1964, 1970, 1983, 1986, 1992, 2003) of growth rings imprinted on the acetate peel. Age data were double-checked for accuracy by staff of the schlerochronology laboratory at Fisheries and Oceans Canada Pacific Biological Station. The possible relationship between age--class strength and the Pacific decadal oscillation index (PDO; http://www.cdc.noaa.gov/) was explored through a cross-correlation analysis. The cross-correlation coefficient [r.sub.xy](k) represents the correlation between 2 series x and .v, where x is lagged by k observations (Box & Jenkins 1976).

Growth

Growth parameters were determined from size-at-age data. Von Bertalanffy growth curves were fitted to all data points as described by the equation

[L.sub.t], = [L.sub.[infinity]](1 exp(-k(t - [t.sub.0]]))) + [[epsilon].sub.2],

where t is age in years: [L.sub.t] is SL in millimeters at age t; [L.sub.[infinity]] is the mean asymptotic length; k is the Brody growth coefficient, which determines the rate of increase or decrease in length increments: to is a phase variable, suggesting the hypothetical age at which the organism would have been at 0 mm in length; and [[epsilon].sub.2] [approximately equal to] N(0, [[sigma].sup.2.sub.1]) is a normal variate. The parameters [L.sub.[infinity]], k, and [t.sub.0] were estimated using the Levenberg-Marquardt algorithm (nonlinear least squares (More 1977)). The loss function was the sum-of-squares difference:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

[FIGURE 1 OMITTED]

Mortality

Instantaneous natural mortality (M) was estimated with four approaches: Pauly's equation (Pauly 1980), catch curve analysis, longevity (Hoenig 1983), and the Chapman-Robson estimator (Chapman & Robson 1960). Pauly's equation estimates natural mortality by means of an empirical relationship between M, two parameters of the von Bertalanffy growth function and mean environmental temperature, as follows:

M = [k.sup.0.65] x [L.sup.-0279.sub.[infinity]] x [T.sup.0.463.sub.c],

where M is the instantaneous annual natural mortality rate (proportion per year) and [T.sub.c]. is the mean habitat (water) temperature measured in degrees Celsius. The mean annual sea surface temperature was 19.6[degrees]C for San Quintin and 17.8[degrees]C for Islas Coronado (December 1981 through March 2009, data from the National Oceanic and Atmospheric Administration Climate Prediction Center, available at http://www.emc.ncep.noaa.gov/research/cmb/sst_analysis/#_cch2_1007146782).

Catch curve analysis estimates M as the slope of the regression line of ln abundance versus estimated age. Natural mortality was estimated on age data grouped into 3-y age classes. Longevity data are used to estimate M as

M = ln(p)/A,

where A is the maximum age and p is the proportion of the population that is assumed to reach maximum age. A value of P = 0.01 has been used for other geoduck species (Breen et al. 1991, Gribben & Creese 2005) and was used in this analysis.

The Chapman-Robson (CR) estimator of M is calculated from catch and age data as

CR = ln (1 + [bar.a] - 1/n/[bar.a]),

where [bar.a] is the mean age and n is the sample size.

RESULTS

Morphometric Relationships

We processed 264 geoducks, 198 from San Quintin and 66 from Islas Coronado. From the sites combined, there were 82 females, 61 males, and 121 that could not be sexed because gametes were reabsorbed.

Table 1 presents the descriptive statistics of SL, SH, SW, total weight, tissue mass, and shell weight, as well as the correlation matrix of all variables. All correlations were significant (P < 0.05), except those between tissue mass and both SH and SW. Because SL is correlated to all 3 weight metrics, only the relationship between SL and total wet weight is presented. The parameter b ([+ or -] SE) of the SL--weight relationship was statistically significant (2.428 [+ or -] 0.136), but the parameter a was not (0.005 [+ or -] 0.003).

Despite the large number of unsexed animals, we attempted separate analysis by sex. Overall geoduck SL ranged from 96-162 mm (average, 132 [+ or -] 13 mm (SD)), with a mode of 135 mm at Islas Coronado (Table 2, Fig. 2). The SL of geoducks from Islas Coronado were slightly larger than those from San Quintin, but not significantly so [analysis of variance (ANOVA), P = 0.41]. There was no difference between male and female SL (Fig. 3; ANOVA, P - 0.60).

Age and Growth

Of the 264 geoducks, only 233 could be aged as a result of shell breakage, mislabeled shells, or indecipherable growth rings. Signature years were found (Fig. 1), which improved the accuracy of age estimates. Age ranged from 3 96 y, with the oldest being an unsexed animal 141 mm long, the youngest clams were 3 y old, 2 females that were 100 mm and 110 mm, and one unsexed clam of 105 mm SL. Age frequency histograms (Figs. 4 and 5) show that the 20-25-y age group is by far the strongest mode in the distribution of ages. Mean age in Islas Coronado was higher than in San Quintin, but San Quintin had older individuals.

Length-at-age data show that growth is rapid during the first 10 y and then decreases dramatically, slowing for individuals older than 20-25 y (Fig. 6). There is considerable variability of SL within each estimated age. The Brody growth coefficient was 0.189 and the asymptotic length was 134 mm, for all organisms combined. The von Bertalanffy parameters were not significantly different between sites or sexes (P > 0.05).

Mortality

Estimated mortality rates varied with the method used. Pauly's equation produced an unrealistically high estimate of M, at 0.617. Catch curve analysis estimated M to be 0.027, whereas the longevity and Chapman-Robson estimators yielded estimates of 0.046 and 0.034, respectively.

DISCUSSION

This is the first report on the biology of P. generosa from the southernmost extent of its geographical distribution. Although there is a wealth of information about this species from Washington and British Columbia (see Feldman et al. (2004) for a review), there is only one short note from California (Moore 2001), where it is exclusively a sport fishery, and nothing is published from locations further south. P. generosa populations in Baja California are the target of a fast-growing fishery, and an understanding of population parameters is critical at this stage to develop sustainable harvest strategies.

[FIGURE 2 OMITTED]

The existence of signature years in the growth patterns of shell cross-sections lends validity to the ring-counting methodology for the Baja California P. generosa. There is no reason to suspect that internal growth ring deposition does not occur during winter months in Pacific Baja California as it does in the species farther to the north.

Aged individuals spanned year-classes from 1912 to 2005, but the strongest cohorts were from the 1980s, accounting for 36% of all sampled animals. Significant (P < 0.05) cross-correlation function r between the year-class and the PDO Index were abundance obtained without and with time lagged PDO values (lag: -1, -9, 10, 9, and 10 y). Nielsen et al. (2008) found that the PDO exerts the strongest influence on the shell growth of this species in British Columbia during the very start of the growing season, which agrees with our findings in Baja California. This is a promising area of research that requires additional sampling and further investigation.

[FIGURE 3 OMITTED]

In the Baja California peninsula, the estimated asymptotic length is 134 mm SL, which was achieved at 10 y. This asymptotic length is within the range found for P. generosa in British Columbia of 129-147 mm (Bureau et al. 2002, Bureau et al. 2003, Campbell & Ming 2003) and in Washington state of 132-173 mm (Hoffmann et al. 2000), and is larger than species of geoducks from the southern hemisphere: 106 mm SL for P. abbreviata (Morsan & Ciocco 2004) and 116 mm and 104 mm for P. zelandica (Breen et al. 1991, Gribben & Creese 2005).

Because the mean asymptotic length of 134 mm is only slightly larger than the minimum legal size of 130 mm that is currently imposed in the fishery, the size limit would appear to be too high. A more important issue is whether any minimum size is appropriate for this species. The digging appendage is vestigial in the adult geoduck (Goodwin & Pease 1989) and hence the animal cannot rebury itself once removed from the substrate. Based solely on the limited biological sampling from this study, the proportion of animals smaller than the legal size that would likely be left on the sea floor could be as much as 38%. Although Gribben and Creese (2005) found that the distance between siphon holes could be used to predict shell length for P. zelandica ([r.sup.2] = 0.57), Andersen (1971) did not find any such relationship for P. generosa. Regardless, it is unlikely that commercial harvesters would be able to make, or even attempt, such judgments while fishing. In consequence, imposing any legal size limit would result in unreported mortality and wasted resource. Hoffmann et al. (2000) claimed that the growth constant k in the von Bertalanffy model is the key parameter in estimating fishing mortality rate. In this study, we found k = 0.19, which is intermediate to those estimated by Hoffmann et al. (2000) in Washington state (0.113 0.235), and among those found by Campbell and Ming (2003) in Canada (0.146 and 0.189). In this case, the k parameter is also similar to that found for the southern hemisphere geoduck" 0.183 in P. abbreviata (Morsan & Ciocco 2004) and 0.16 in P. zelandica (Breen et al. 1991). The fit of the growth model to data, and the value of k itself, depends on the demographic structure of the studied population. Higher estimation of k is expected when target populations are composed of younger individuals (Hoffmann et al. 2000).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

There are many examples of P. generosa older than 100 y (Goodwin 1976, Goodwin & Shaul 1984, Sloan & Robinson 1984, Bureau et al. 2002, Bureau et al. 2003, Campbell & Ming 2003). In this study, we found a male of 96 y old, and the nearest was another male of 74 y old; both were sexually reproductive. This agrees with findings of Sloan and Robinson (1984), who found ripe individuals ranged from 7-107 y old in males and 8 89 y old in females.

[FIGURE 6 OMITTED]

Estimated natural mortality rates of Baja California geoduck (0.027-0.046) were within the range found for geoducks in Washington state and British Columbia of 0.016, 0.0226 (Bradbury & Tagart 2000), 0.035 (Sloan & Robinson 1984), and 0.036 (Zhang & Campbell 2004). The mortality rate estimated by Pauly's method, 0.617, is considered unrealistic because it would imply a life span of 7.5 y (estimated as the time required for a geoduck to reach 95% of its asymptotic length, from the inverse of the von Bertalanffy equation, [t.sub.max] = (-1/k) ln[l- (0.95 [L.sub.[infinity]])/[L.sub.[infinity]]]), where [t.sub.max] is mean life span). Because natural mortality, recruitment, and growth are primary drivers of the productivity of a geoduck population, improving estimates of these parameters through increased biological sampling on P. generosa in Baja California is a research priority.

[FIGURE 7 OMITTED]

ACKNOWLEDGMENTS

This study was funded by the Baja California State Secretary of Fisheries and Aquaculture. Consejo Nacional de Ciencia y Tecnologia supported the sabbatical leave of E. A. A. N. at Centro de Investigacion y Educacion Superior de Ensenada and L. E. C. A at Centro de Investigaciones Biologicas del Noroeste. V. M. M. R. received aging training at the Fisheries and Oceans Canada Pacific Biological Station (PBS) at Nanaimo. Judy McArthur from PBS assisted as a growth ring reader and A. E. Romo did the second reading of growth rings. The authors are grateful to two anonymous referees for their constructive comments on the original text.

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LUIS EDUARDO CALDERON-AGUILERA, (1) * EUGENIO ALBERTO ARAGON-NORIEGA, (2) CLAUDIA M. HAND (3) AND VICTOR MANUEL MORENO-RIVERA (1)

(1) Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Km 107 Carretera Tijuana--Ensenada, Ensenada 22860 Baja California, MEXICO; (2) Centro de Investigaciones Biologicas del Noroeste, Unidad Sonora, Km 2.35 Camino al Tular, Estero Bacochibampo, Guaymas, Sonora 85454, Mexico; (3) Fisheries and Oceans Canada, Nanaimo, British Columbia V9T 6N7

* Corresponding author. E-mail: leca@cicese.mx
TABLE 1.

Descriptive statistics of Panopea generosa morphometrics and
correlation matrix from Baja California, Mexico.

               Shell Length    Shell Height    Shell Width
                   (mm)            (mm)            (mm)

Mean                132             82              55
SD                  13               8              8
n                   258             255            249

               Total Weight     Tissue Mass    Shell Weight
                    (g)             (g)            (g)

Mean               763.6           492.2          130.1
SD                 254.6           142.9           49.8
n                   264             264            263

                               Shell Height    Shell Width
               Shell Length        (mm)            (mm)

Shell height       0.75#
Shell width        0.61#           0.61#
Total weight       0.74#           0.64#          0.59#
Tissue mass        0.67#           0.51            0.49
Shell weight       0.69#           0.69#          0.68#

               Total Weight        Total       Tissue Mass

Shell height
Shell width
Total weight
Tissue mass        0.79#
Shell weight       0.57#                          0.46#

All r values in bold type are significant (P < 0.05).

Note: All r values in bold type are significant (P < 0.05)
are indicated with #.

TABLE 2.

Sample size, mean shell length ([+ or -] SD), mean age ([+ or -]
SD), and von Bertalanffy growth parameter estimates ([+ or -]
SE) derived from shell length at age for P. generosa at 2
locations in Baja California, Mexico.

Location              n       Mean SL (mm)       Mean Age (y)

Islas Coronado
  Male                16    133 [+ or -] 16    28.1 [+ or -] 17
  P value
  Female *            20    131 [+ or -] 15    29.8 [+ or -] 19
  Sex combined        47    133 [+ or -] 14    31.5 [+ or -] 18
    ([dagger])
  P value
Bahia San Quintin
  Male                44    131 [+ or -] 13    27.6 [+ or -] ll
  P value
  Female              62    132 [+ or -] 12    25.4 [+ or -] 11
  P value
  Sex combined *     186    132 [+ or -] 12    27.6 [+ or -] 12
  P value
Locations combined
  Male                60    132 [+ or -] 14    28.1 [+ or -] 13
  P value
  Female              82    132 [+ or -] 12    27.8 [+ or -] 14
  P value
  Sex combined *     233    132 [+ or -] 13    28.4 [+ or -] 14
  P value

Location             [L.sub.[infinity]]            k

Islas Coronado
  Male               139 [+ or -] 6       0.297 [+ or -] 0.67
  P value            0.00                 0.66
  Female *           138                  0.306
  Sex combined       138 [+ or -] 2       0.259 [+ or -] 0.15
    ([dagger])
  P value            0.00                 0.10
Bahia San Quintin
  Male               133 [+ or -] 2       0.223 [+ or -] 0.16
  P value            0.0                  0.163
  Female             134 [+ or -] 2       0.195 [+ or -] 0.06
  P value            0.000                0.004
  Sex combined *     133 [+ or -] 1       0.199 [+ or -] 0.05
  P value            0.000                0.000
Locations combined
  Male               135 [+ or -] 2       0.195 [+ or -] 0.11
  P value            0.000                0.105
  Female             135 [+ or -] 2       0.193 [+ or -] 0.07
  P value            0.0                  0.009
  Sex combined *     134 [+ or -] 1       0.189 [+ or -] 0.05
  P value            0.000                0.001

Location                  [t.sub.0]

Islas Coronado
  Male               -0.764 [+ or -] 11.8
  P value            0.95
  Female *           -1.2
  Sex combined       -2.04 [+ or -] 3.55
    ([dagger])
  P value            0.57
Bahia San Quintin
  Male               -0.85 [+ or -] 5.8
  P value            0.885
  Female             -3.59 [+ or -] 2.9
  P value            0.226
  Sex combined *     -3.19 [+ or -] 2.5
  P value            0.206
Locations combined
  Male               -3.9 [+ or -] 5.6
  P value            0.493
  Female             -4.34 [+ or -] 3.0
  P value            0.16
  Sex combined *     -4.4 [+ or -] 2.6
  P value            0.092

The P value of each parameter is given.

* Matrix ill-conditioned; cannot compute standard error.

([dagger]) Sex combined includes unsexed organisms.
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