© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Potential bias in estimates of the size of maturity of crabs derived from trap samples
Murdoch University Centre for Fish and Fisheries Research, School of Biological Sciences and Biotechnology, Division of Science and Engineering, Murdoch University South Street, Murdoch, Western Australia 6150, Australia
*Correspondence to I. C. Potter: tel: +61 8 9360 2524; fax:+61 8 9360 6303. e-mail: i.potter{at}murdoch.edu.au.
The size at the onset of sexual maturity (SOM) of female crustaceans is typically estimated using logistic regression analysis of the proportions of mature females in sequential size classes. The validity of this approach depends on the composition of the samples reflecting accurately that present in the environment. However, catches obtained by traps, a passive fishing method, typically contain disproportionately greater numbers of large crabs, whereas those obtained using active fishing methods, such as seine-netting and otter trawling, will presumably represent far better the size composition of the population. Moreover, we demonstrate that samples of female Portunus pelagicus caught by trapping were predominantly mature, whereas those collected by seining and trawling contained numerous immature as well as mature females. Therefore, the samples of females collected by trap are clearly biased towards mature crabs. Consequently, for any size class, it would be predicted that the proportion of mature females in trap catches will be overestimated, so shifting the logistic curve fitted to the proportions of mature crabs in each size class to the left, and yielding an underestimate of the SOM. This conclusion is substantiated by the fact that the carapace width of female Portunus pelagicus, at which 50% of individuals reach maturity (SOM50), was estimated to be markedly greater when using the proportion of mature females obtained by seine-netting and otter trawling, i.e. 101.1 mm, than by trapping, i.e. 86.1 mm. Data on the size and maturity status of the deep-sea crabs Hypothalassia acerba and Chaceon bicolor are available only from trap catches. From the above data for P. pelagicus, it is considered likely that, through a greater vulnerability of mature females of these species to capture by traps, the respective SOM50s derived for female H. acerba and C. bicolor from trap samples (carapace lengths of 69.7 and 90.5 mm) will be considerable underestimates of the true values.
Keywords: crabs, seine-nets, selectivity, size at first maturity, traps, trawls
Received 27 December 2003; accepted 23 July 2004.
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The age at which fish and crustaceans are first caught should be at least as great as the age at first maturity if recruitment overfishing is to be avoided (Caddy and Mahon, 1995). Therefore, the size at which the individuals of a decapod species typically reach the onset of sexual maturity (SOM) is frequently used by fisheries managers as the basis for assigning a minimum legal size for the retention of that species (Watson, 1970). The criteria used to designate whether a female decapod is mature vary among species, and often reflect differences in the secondary sexual characteristics of those species. For example, because the abdomen of female portunid crabs becomes relatively wider and more loosely attached at the pubertal moult, this characteristic provides a sound criterion for assessing whether a female of this family has reached maturity (e.g. Van Engel, 1958; Somerton, 1981; de Lestang et al., 2003a), whereas the shape of the gonopores (Melville-Smith, 1987), or the presence of eggs under the abdomen (Levings et al., 2001), have been employed for this purpose with the females of some deep-sea species of crabs.
The size at which 50% of the females of the stock of a crab species reach the onset of sexual maturity (SOM50) is usually estimated by subjecting the proportions of mature females in sequential size classes to logistic regression analysis (e.g. Fisher, 1999; Muino et al., 1999; de Lestang et al., 2003a). The validity of this approach depends on both the immature and mature individuals of the stock in question having been sampled in an unbiased manner. As many crab fisheries employ only baited traps, scientists have often used the data obtained from the resulting catches to estimate the SOM50 (Brown and Powell, 1972; Melville-Smith, 1987; Abbe, 2002; Gardner and Williams, 2002). However, traps yield samples that are biased in terms of both the size and sex composition of the population (Williams and Hill, 1982; Zhou and Shirley, 1997; Jury et al., 2001).
Trap selectivity for decapod species typically occurs as a result of the larger individuals being more aggressive, thereby restricting the likelihood of smaller animals entering the trap (e.g. Bovbjerg, 1956; Rodhouse, 1984). In addition, the greater number of males than females typically found in the trap catches of decapods (Carroll and Winn, 1989; Potter and de Lestang, 2000) almost certainly reflects the greater aggression of males. Moreover, behavioural studies strongly indicate that, in the case of the portunid Callinectes sapidus, females are more likely to enter traps containing males once they have become mature (Jivoff and Hines, 1998). Hence, trap catches of the females of this species will almost certainly contain a disproportionately greater number of mature than immature females. Therefore, subjection of the proportions of trap-caught mature females of C. sapidus in successive size classes to logistic regression analysis would presumably underestimate the SOM. Similar biases in the estimates of SOM would be expected to result from analyses of trap catches of other crab species if there is likewise a greater likelihood of the capture of the mature females of those species than their immature females of the same size.
The commercial fishery for the blue swimmer crab (Portunus pelagicus) in Western Australia is the largest for this portunid in Australia, a total catch of 673 t being taken in 1999–2000, yielding a wholesale value of approximately $A3 million (Anon., 2002). The SOM of female P. pelagicus in Shark Bay, the location of the largest fishery for this portunid in Western Australia, has been estimated using data from samples obtained by baited traps, seine-netting and otter trawling collectively (de Lestang et al., 2003a). However, because of trap selectivity, the samples of P. pelagicus collected by baited traps in Shark Bay may have contained an atypically high proportion of mature females and, consequently, a logistic regression analysis using data from trap catches may have underestimated the SOM of the females there. In contrast, the size composition of samples of P. pelagicus collected by seine-netting and otter trawling would be far more representative of the population in Shark Bay.
The champagne crab (Hypothalassia acerba) and the crystal crab (Chaceon bicolor) are fished commercially using baited traps in water depths of about 150–360 m and 600–800 m, respectively, off the west and south coasts of Western Australia (Smith et al., in press; unpublished data). Although estimates of the SOMs can be derived from the trap catches of the females of these populations, no data are available from catches obtained using other fishing gear that could be used to determine whether vulnerability to capture by traps was greater among the mature females of these species.
This study aimed to determine whether the SOM50 estimated for female P. pelagicus in Shark Bay was significantly lower when data derived from trapping were used in preference to data from seine-netting and otter trawling collectively. If this was the case, it would demonstrate that, as with C. sapidus, the mature female P. pelagicus are more vulnerable to capture by traps than the immature females of the same size. Consequently, an SOM50 derived from trap catches will be an underestimate for species such as P. pelagicus and C. sapidus. However, the only method used for catching H. acerba and C. bicolor in Western Australia is trapping. Although it is recognized that the SOM50s derived from these trap catches may represent underestimates, we derived such values so that fisheries managers have data that, with discretion, can be used for determining, for example, the minimum legal size for capture of these species.
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Material and methods |
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Sampling of crabs
The data for P. pelagicus used in this study were extracted from the database constructed for the samples of this species collected in Shark Bay (26°S 113.5°E), at bimonthly intervals between July 1998 and May 2000 (de Lestang et al., 2003a, b). Samples were collected with a seine-net 21.5 m long with a bunt made of 3 mm mesh, a small otter trawl with a net containing a codend constructed of 25 mm mesh, and a series of crab traps consisting of either 12 or 76 mm mesh (for further details see Potter and de Lestang, 2000). The crab traps, which were the same as those used by most professional fishers in Western Australia, were ca. 630 mm high and 1000 mm in diameter. They were joined together in four lines of four, separated by a distance of 15 m. The traps were baited with fish, usually Sardinops neopilchardus, and set for ca. 24 h.
Female H. acerba were collected bimonthly between July 1999 and April 2002 from water depths of 150–350 m located at various sites between Jurien (30.3°S) and Mandurah (32.5°S) off the lower west coast of Australia. H. acerba was sampled with two types of trap typically used to catch the western rock lobster (Panulirus cygnus), and which are set in longlines. The first trap used was a rectangular trap with a steel base and timber frame that was 960 mm long x 800 mm wide x 460 mm high, and contained an entrance 270 mm in diameter x 170 mm in depth at the top. The base of this trap consisted of parallel steel rods of 5 mm diameter, situated 80 mm apart, with the sides, top, and ends of the trap being enclosed with wooden battens approximately 70 mm apart. In an attempt to catch greater numbers of small crabs, all but the entrance of the trap was later enclosed with welded galvanized rectangular aviary mesh, with dimensions of 12.7 x 25.4 mm. The other trap used was a circular cane stick trap 880 mm in diameter x 450 mm high, with an entrance of 280 mm diameter x 200 mm deep at the top. Both the trap types contained ca. 40 kg of steel as ballast. Traps were baited with fish heads, such as those of orange roughy (Hoplostethus atlanticus) or New Zealand hoki (Macruronus novaezelandiae), and a bait of cattle hide or foreleg, which was more lice-resistant than fish heads. Traps were typically set for five days.
Female C. bicolor were obtained from the catches of two commercial fishing vessels operating in water depths of ca. 800 m between Geraldton (29°S) and Fremantle (32°S) off the lower west coast of Australia. Samples were collected bimonthly between January 2000 and March 2003, using lines containing up to 100 of the plastic rock lobster traps used by recreational fishers. These traps were 675 mm long, 350 mm wide, and 475 mm high, and contained an entrance of 170 mm diameter and 200 mm deep at the top. They were made of lightweight plastic to prevent them from sinking into the soft substratum, were baited with heads of hoki or whole Australian salmon (Arripis truttaceus), and were typically set for 4–8 days.
Measurements of crabs
The carapace width (CW) of each female P. pelagicus, i.e. the distance between the tips of the two lateral spines of the carapace, was measured to the nearest 1 mm. The fact that, during the pubertal moult, the abdominal flap of female portunids changes from a triangular to an oval shape, and from being tightly to loosely fixed to the cephalothorax, was used to determine whether a female was immature or mature (Ryan, 1967; Ingles and Braum, 1989; Fisher, 1999). A record was kept of which female crabs were ovigerous.
The carapace length (CL) of each female H. acerba and C. bicolor, i.e. the distance from the midpoint between the bases of the two anterior medial horns and the posterior margin of the carapace, was measured to the nearest 1 mm. This is the typical way of measuring body size in deep-sea crabs (Levings et al., 1996; Gardner, 1997; Goshima et al., 2000), and it overcomes the problems of using the distance between the two lateral spines of the carapace, when those structures are particularly prone to wear.
As the morphology and tightness of the abdominal flap of the immature and mature females of both H. acerba and C. bicolor do not differ markedly, abdominal flap characteristics could not be used to determine the maturity status of the females of those deep-sea crabs. Moreover, as the morphology of the gonopores of immature and mature females of H. acerba does not differ markedly, which contrasts with the situation in some other species of deep-sea crab (Melville-Smith, 1987), this structure could not be used to determine the maturity status of the champagne crab. Therefore, female H. acerba were classified as having reached maturity when their ovaries were relatively large and pale yellow to pink (and shown by histology to contain yolk granule oocytes), rather than being either inconspicuous or thin and white (and shown by histology to contain oogonia and primary oocytes). As the gonopores of female C. bicolor are elliptical and compressed in immature individuals, and circular and open in mature individuals, it was possible to use this criterion to determine whether an individual crab was immature or mature.
Data analysis
The SOM50s of female P. pelagicus taken by trap, and by seining and trawling collectively, were estimated using logistic regression analysis of the proportions mature at each CW. The probability that the jth crab is mature (Pj) is
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A likelihood-ratio test (see Cerrato, 1990) was used to determine whether the SOM50s of P. pelagicus estimated from samples caught by trap, and by seining and trawling collectively, were significantly different.
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Results |
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Size composition
The 272 female P. pelagicus caught in traps in Shark Bay, of which only 2.9% were immature, ranged from 74 to 178 mm CW (Figure 1a). In contrast, the size range of the 238 female P. pelagicus caught by seining and trawling collectively, of which as many as 57% were immature, ranged from 12 to 160 mm CW (Figure 1b). The mean CWs of mature females collected by trapping (127.5 mm), and by seining and trawling collectively (125.5 mm), were not significantly different (p > 0.05).
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The CLs of the 598 female H. acerba and the 2981 female C. bicolor caught by traps ranged from 50 to 114 mm and from 34 to 148 mm, respectively (Figure 2). The size distributions of both of these species were essentially unimodal, peaking at ca. 95 and ca. 110 mm CL, respectively. The number of immature individuals in the samples of female H. acerba and C. bicolor were 20 and 186, respectively, thus contributing only 3.3% and 6.2% to the total catch of the females of these two species (Figure 2).
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Size at maturity
A logistic regression analysis of the proportions of mature female P. pelagicus in sequential CWs in trap samples yielded an SOM50 of 86.1 mm, with lower and upper 95% confidence limits of 59.7 and 97.2 mm, respectively (Figure 3a). A far greater SOM50, i.e. 101.1 mm, and a far narrower 95% confidence interval, i.e. 96.1–105.4 mm, were obtained when subjecting the proportions of mature crabs in seine and trawl samples to the same analysis (Figure 3b). The above two SOM50s were significantly different (p < 0.001). As the mean CW of ovigerous female P. pelagicus in trap catches did not differ significantly (p > 0.05) from that recorded in seine and trawl catches collectively, the size frequency data for these methods were pooled. The CWs of ovigerous females caught in Shark Bay ranged from 104 to 157 mm, with a mean and 95% confidence limits of 131.3 ± 3.4 mm (Figure 3c).
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Logistic regression analyses of the proportions of mature females of H. acerba in sequential CLs in samples collected by traps yielded an SOM50 of 69.7 mm (Figure 4a). The CLs of ovigerous females and females with egg remnants collectively ranged from 75 to 114 mm, producing a mean and 95% confidence limits of 96.6 ± 3.0 mm (Figure 4b). Subjection of the proportions of the mature females of C. bicolor in trap catches to logistic regression analysis yielded an SOM50 of 90.5 mm (Figure 5a). The CLs of ovigerous females and females with egg remnants collectively ranged from 91 to 140 mm, with a mean and 95% confidence limits of 108.2 ± 1.0 mm (Figure 5b).
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Discussion |
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The proportion of small female P. pelagicus obtained by trapping in Shark Bay is far less than that obtained by seine-netting or otter trawling in the same environment. Indeed, the percentage contribution made by female P. pelagicus with a CW <100 mm to the total catch of females was only 3.3% in trap catches, whereas it exceeded 57.6% in the combined catches obtained by seine-netting and otter trawling. The proportion of large female crabs that entered traps was also high in the case of the two species of deep-sea crabs. Therefore, the proportions of female H. acerba <70 mm CL and of C. bicolor <90 mm CL among all females of each of those species were only 3.3% and 4.4%, respectively. It should be noted that the numbers of small deep-sea crabs caught were low, even when the traps were enclosed in a smaller mesh or plastic covering and had their entrances narrowed.
The size composition data for P. pelagicus for Shark Bay, and also for the Leschenault Estuary much farther south (Potter and de Lestang, 2000; unpublished data), demonstrate that, in contrast to mature females, few immature females of this species enter traps. This finding is consistent with the implications of the results of behavioural studies on Callinectes sapidus by Jivoff and Hines (1998), which indicated that an immature female of this portunid would be more likely to avoid any male(s) of this species present in traps than would a mature female. Furthermore, male American lobster (Homarus americanus) do not allow the immature females of the same species to occupy their shelter (Salmon, 1983). If the proportion of mature females in any size category is greater in trap samples than in the environment, the SOM50 derived from such samples would underestimate this parameter as a result of the sample being biased towards those mature individuals. This conclusion is strongly supported by the fact that the estimated SOM50 of female P. pelagicus, based on data from trapping (86.1 mm CW), was significantly less than that of 101.1 mm CW estimated with the data derived from seine-netting and otter trawling, which would have caught a far more representative sample of the population. The CW of the smallest ovigerous female P. pelagicus, 104.0 mm, was substantially greater than the SOM50 of 86.1 mm, estimated from the proportions of mature females in trap catches, which, as argued above, is an underestimate.
As with P. pelagicus, the estimates of SOM50 for H. acerba and C. bicolor, 69.7 and 90.5 mm, respectively, calculated using the proportions of mature females in trap catches, were lower than the sizes of the smallest ovigerous females. Catches of those species obtained using active fishing methods are not available, so precluding investigation of the possibility that the vulnerability of the females of these species might be greater for mature than for immature crabs. Until this hypothesis can be tested for each species, the values of the SOM50s for H. acerba and C. bicolor calculated from trap catches should be considered lower "bounds" of the true SOM50s rather than accurate estimates of these crucial reproductive parameters.
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Acknowledgements |
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We are very grateful to deep-sea crab fishers for helping us obtain the crabs, and to friends, family, and colleagues for help with the sampling. We also acknowledge the helpful criticisms provided by the two referees, which hugely improved the quality of the paper. Financial support was provided by the Australian Fisheries Research and Development Corporation, and Murdoch University.
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