Mating success of female Dungeness crabs (Cancer magister) in Oregon coastal waters.
Article Type: Report
Subject: Crabs (Sexual behavior)
Courtship of animals (Research)
Authors: Dunn, Paul H.
Shanks, Alan L.
Pub Date: 08/01/2012
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 2012 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: August, 2012 Source Volume: 31 Source Issue: 3
Topic: Event Code: 310 Science & research Canadian Subject Form: Mating behaviour
Product: Product Code: 0913040 Crabs NAICS Code: 114112 Shellfish Fishing SIC Code: 0913 Shellfish
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 303011409
Full Text: ABSTRACT The Dungeness crab is an important commercial and sports fishing species in Oregon. The fishery is regulated by sex, size, and season. This study examined whether female crabs are mating despite the removal of almost all legal-size male crabs each year. Of particular concern is whether large females are finding large enough mates. Females were collected from three Oregon fishing ports, dissected, and checked for evidence of mating. Captured male and female crabs were also measured to estimate population size distributions. The majority of female crabs examined (69%) mated in the collection year, and when combined with crabs that carried sperm from previous mating encounters (females store sperm), the percent of females that would have produced viable eggs was 83%. Crabs that definitely molted during the collection year showed higher mating success (95%). The largest females examined (carapace width, 160-169 mm) showed high mating success (84% mated in the collection year, 95% could have produced viable eggs). These numbers compare favorably with a similar survey conducted in northern California, in which 69% of molting females had mated. We conclude from the data that molting females in these Oregon fishing ports are finding mates successfully, regardless of size.

KEY WORDS: Dungeness crab, Cancer magister, mating, Oregon, fertilization, fishery

INTRODUCTION

The Dungeness crab, Cancer magister Dana 1852, supports commercial and sports fisheries in the Pacific Northwest from Alaska to California. These fisheries account for thousands of jobs and millions of dollars for the economies of coastal states. The importance of maintaining a healthy fishing stock of crabs necessitates the placement of regulations to control which crabs can be fished and when. Only male crabs larger than a minimum size can be harvested, and only during a defined fishing season. In Oregon, male crabs must have a carapace width of at least 6.25 in (159 mm) when measured just anterior to the 10th lateral spine. The Oregon fishing season begins on December 1 and runs through August 14, avoiding the fall period, during which most males molt and are in poor market condition. Without any regulations directly limiting the number of large males that can be taken each year, there is the possibility that nearly all male crabs of legal size could be harvested. Indeed, the exploitation rates in some Pacific fisheries have exceeded 90% in some years (Hankin 1985, Smith & Jamieson 1989).

The mating behavior of Cancer magister involves a premating embrace, during which the male holds the female for several days before the female molts. At the time of female molting, the male holds her in the true mating embrace and deposits seminal secretions inside the 2 female reproductive tracts. These secretions later harden into a sperm plug that extends from the spermathecae into the vagina (Hartnoll 1969, Jenson et al. 1996). Although the mating process may last 1-2 wk, the period during which females molt lasts for several months, and male crabs can mate with several females each year. Female crabs typically reach maturity during their second year, at a carapace width of around 100 mm (Tasto 1983). They then typically molt once each year for the remainder of their growing period, potentially mating after each molt. After they reach a carapace width of 155 mm or larger, female Dungeness crabs typically stop molting (Hankin et al. 1985). For the male to embrace the female, he must be larger than she. If, after the fishing season, there are too few large males remaining, then it stands to reason that some of the larger females may not have an opportunity to mate (Smith & Jamieson 1989).

Two recent studies in Alaska suggested that such a problem could be occurring there. A shortage of very large potential mates was given as a possible reason for why some large Dungeness females did not extrude eggs every year (Swiney et al. 2003), and increased female fertility was given as one possible outcome of an uptick in the number of large males in areas protected from commercial fishing around Glacier Bay (Taggart et al. 2004). These studies relied on data collected from crab pots and dive--transect surveys that allowed the authors to determine the ratio of ovigerous to nonovigerous females during the sampling period. Given the fact that not all female crabs extrude eggs every year, and the nonuniform timing of egg extrusion among crabs that do, this sampling method could underestimate the number of females that mated successfully. A more direct approach to measuring fertilization success was taken by Hankin et al. (1997) in northern California. They used the presence of the sperm plug in the female reproductive tract as well as the presence of sperm in the spermathecae as evidence for mating. A later study showed that sperm plugs disintegrate at a slow enough rate that their presence alone is a reliable indicator of recent mating (Oh & Hankin 2004). Presence of sperm in the spermathecae indicates both recent and past mating success. Sperm stored in the spermathecae can be used to produce viable egg masses for at least 2.5 y, albeit with reduced fecundities compared with crabs that had recently mated after molting (Hankin et al. 1989). Hankin et al. (1997) concluded from their study that "virtually all large molting females mated ... in the intensely harvested northern California population of Dungeness crabs" (p. 667).

We present here the results of a similar survey of female Dungeness crabs conducted along the coast of Oregon. We examine the mating success of females as well as a size distribution of males. We discuss the impact of the results of this study on the Oregon Dungeness crab fishery.

MATERIALS AND METHODS

With the aid of commercial fishermen, Dungeness crabs were collected out of the 3 fishing ports of Brookings, Coos Bay, and Newport. All collections occurred in July 2009, after the regular female molting and mating season. At Brookings and Coos Bay, 14 fish traps with 12-mm mesh were connected by either 15- or 30-foot lines to commercial crab pots and allowed to soak overnight (12-18 h). At Newport, all crabs were collected using commercial crab pots (5-cm wire mesh with escape ports open). Traps and pots were baited with fresh hanging bait of tuna, salmon, or halibut and perforated bait jars containing squid or herring. Potentially mature females [> 100 mm in carapace width (CW); i.e., width anterior to the 10th lateral spine] from both the fish traps and the commercial pots were kept and transported back to the laboratory at the Oregon Institute of Marine Biology for freezing and dissecting. The CW of the first 100 males caught in the traps and pots at both Coos Bay and Brookings was also measured (not including spines), but no males were kept for further study. All males captured at Coos Bay were counted for abundance estimates. Because the collecting happened 7 mo into the commercial crabbing season, low counts of large males were expected.

[FIGURE 1 OMITTED]

In the laboratory, each female crab's CW was measured to the nearest millimeter. The hardness of the shell was tested by placing pressure on the ventral side of the carapace with both thumbs. If the shell flexed, the crab was labeled "soft"; if it did not, the crab was considered "hard." Dissections were carried out following the methods described in Hankin et al. (1997). The presence or absence of sperm plugs was noted, as well as whether the spermathecae were swollen (contained sperm).

RESULTS

Male Crab CW and Abundance

We measured 112 male crabs from the Coos Bay traps. CW ranged from 101-170 mm, with an average of 144.95 mm. We also measured 100 male crabs from Brookings. CW ranged from 150-184 mm, with an average of 161.71 mm. The size-frequency distribution of all measured males from both sites is presented in Figure 1A. We can produce an index of crab abundance by determining the number of crabs caught per trap per hour of fishing time. For male crabs caught at the Coos Bay site, abundance is estimated at an average of 1.92 males/trap/h. Figure 1B displays the size-frequency of Coos Bay males adjusted by fishing effort.

Female Crab CW and Abundance

The size of the 427 female crabs captured in this study varied from 100-168 mm. A size-frequency distribution is presented in Figure 2A. Abundance estimated by catch per unit effort at Coos Bay was 0.33 females/trap/h, whereas off Brookings it was 0.24 females/trap/h. Figure 2B shows the size-frequency of females off Coos Bay adjusted for unit effort, and Figure 2C shows the same for Brookings.

[FIGURE 2 OMITTED]

Female Mating Success and Site

Dissections provided evidence that 69% (295/427) of the captured females had sperm plugs in their reproductive tracts, indicating that these crabs mated this year. The percentage of females with swollen spermathecae but no sperm plugs (indicating mating success during a previous year) was 14%. This means that 356/427 (83%) of the females examined could produce viable eggs in the year of collection. Crabs from each of the 3 ports showed similar mating trends, and can thus be grouped together for analysis (Table 1).

Female Mating Success and Size

All females were placed into 1 of 7 size categories to determine whether large females were mating less successfully than smaller individuals. The results did not display this trend. In fact, mating success increased with female size (Table 2). Large females showed very high mating success (Fig. 3).

Female Mating Success and Shell Condition

Of the 427 crabs examined, 86 were classified as soft. The female molting season begins in the spring, and the shells of females that molted in April and May would have been hard in July when we trapped them. Only the females that had molted toward the end of the season would have still been in a soft condition. The 86 soft females showed very high fertilization success: 95% with a sperm plug and 97% including those with swollen spermathecae only. Among the 341 hard crabs, 62% contained sperm plugs and another 18% had swollen spermathecae without sperm plugs.

DISCUSSION

The results from this study are almost identical to the results from the study conducted during the late 1990s in northern California by Hankin et al. (1997). They found sperm plugs in 69% of all female crabs classified as having definitely molted. Although we did not classify our crabs in the same way, we found that 69% of all female crabs examined carried sperm plugs and had mated in the year of collection. If, however, we only consider crabs we were sure molted in the collection year (those classified as soft and all those carrying sperm plugs), we find much higher mating success (99%). The hypothesized problem of large females being unable to locate large enough males is also rejected on the basis of our data. Females with a CW larger than 150 mm mated just as successfully as smaller females, and the largest females (CW, >160 mm) had the highest mating success of all. There is also an interesting trend if one examines the percent females with swollen spermathecae by size class (Fig. 3). The larger the female crab, the greater the probability that it would have mated in the past and would have some sperm saved in its spermathecae.

The smallest size class of females in this study (CW, 100-109 mm) was included because female Dungeness crabs typically reach sexual maturity when they have a CW of around 100 mm (Tasto 1983). The lower mating success this group exhibited may represent immaturity at the time of molting. Our sample size (n = 7), however, was likely too small to achieve an accurate estimate of mating success for this size class. Although the mesh size of our fish traps was small enough to keep smaller crabs from escaping, we did not capture a large number of smaller crabs. The fishermen with whom we worked informed us that crabs often segregate by size and sex, and different groups are found at different depths during the year. One fisherman also suggested that females and small crabs of both sexes avoid close contact with large male crabs. If a large male entered the trap first, its presence there may have affected the composition of the other crabs that might enter the trap. Another possibility may be that the mesh opening on either side of the trap was more difficult to reach for smaller crabs. Anyone wishing to capture smaller size classes of crabs in the future should take these ideas into consideration. If these obstacles could be removed, small-crab data would be very useful in studying the size-frequency distribution of a crab population as well as tracking the evolution of a specific cohort of recruits.

[FIGURE 3 OMITTED]

Our data suggest that the mature molting female Dungeness crabs from the waters adjacent to 3 Oregon ports were mating successfully, regardless of size. Under the current management system, female mating success is probably not an important limiting factor affecting this fishery.

ACKNOWLEDGMENTS

We thank the Oregon Dungeness Crab Commission for funding this project and for aiding us in working with the commercial fishing fleet. We also thank Mike Lane (Coos Bay), John Terebesi (Brookings), and Al Pazar (Newport) for use of their boats, traps, and knowledge in collecting the crabs for this study. Last, we acknowledge Ashley Bulseco and Kyle Krumsick for their help collecting, freezing, and dissecting the crabs.

LITERATURE CITED

Hankin, D. G. 1985. Proposed explanations for fluctuations in abundance of Dungeness crabs: a review and critique. In: Metleff, B. R., editor. Proceedings of the Symposium on Dungeness Crab Biology and Management. Alaska Sea Grant report no. 85-3. Fairbanks, AK: Alaska Sea Grant Program. pp. 305-326.

Hankin, D. G., T. H. Butler, P. W. Wild & Q. Xue. 1997. Does intense fishing on males impair mating success of female Dungeness crabs? Can. J. Fish. Aquat. Sci. 54:655-669.

Hankin, D. G., N. Diamond & M. S. Mohr. 1985. Molt increments, annual molting probabilities and survival rates of adult female Dungeness crabs in northern California. In: Metleff, B. R., editor. Proceedings of the Symposium on Dungeness Crab Biology and Management. Alaska Sea Grant report no. 85-3. Fairbanks, AK: Alaska Sea Grant Program. pp. 189-206.

Hankin, D. G., N. Diamond, M. S. Mohr & J. Ianelli. 1989. Growth and reproductive dynamics of adult female Dungeness crabs (Cancer magister) in northern California. J. Cons. Int. Explor. Mer. 46:94-108.

Hartnoll, R. G. 1969. Mating in the Brachyura. Crustaceana 16:161-181.

Jenson, P. C., J. M. Orensanz & D. A. Armstrong. 1996. Structure of the female reproductive tract in the Dungeness crab (Cancer magister) and implications for the mating system. Biol. Bull. Mar. Biol. Lab. Woods Hole 190:336-349.

Oh, S. J. & D. G. Hankin. 2004. The sperm plug is a reliable indicator of mating success in female Dungeness crabs, Cancer magister. J. Crustac. Biol. 24:314-326.

Smith, B. D. & G. S. Jamieson. 1989. Exploitation and mortality of male Dungeness crabs (Cancer magister) near Tofino, British Columbia. Can. J. Fish. Aquat. Sci. 46:1609-1614.

Swiney, K. M., T. C. Shirley, S. J. Taggart & C. E. O'Clair. 2003. Dungeness crab, Cancer magister, do not extrude eggs annually in southeastern Alaska: an in situ study. J. Crustac. Biol. 23:280-288.

Taggart, S. J., T. C. Shirley, C. E. O'Clair & J. Mondragon. 2004. Dramatic increase in the relative abundance of large male Dungeness crabs Cancer magister following closure of commercial fishing in Glacier Bay, Alaska. Am. Fish. Soc. Syrup. 42:243-253.

Tasto, R. N. 1983. Juvenile Dungeness crab, Cancer magister, studies in the San Francisco Bay area. Fish Bull. 172:135-154.

PAUL H. DUNN * ([dagger]) AND ALAN L. SHANKS

University of Oregon, Oregon Institute of Marine Biology, PO Box 5389, Charleston, OR 97420

([dagger]) Current address: Max Planck Institute for Demographic Research, Konrad-Zuse-Str. 1, 18055 Rostock, Germany

* Corresponding author. E-mail: dunn@demogr.mpg.de

DOI: 10.2983/035.031.0330
TABLE 1.
Mating success of female Dungeness crabs from 3 ports on the
Oregon coast.

                                    Previously     With Viable
                        Recently      Mated           Sperm
                         Mated       (% with         (% with
                        (% with      Swollen       Sperm Plug
             Females     Sperm     Spermathecae    or Swollen
Port        Dissected    Plug)        Only)       Spermathecae)

Brookings         150         67             10              77
Coos Bay          102         68             16              84
Newport           175         72             14              86
Total             427         69             14              83

TABLE 2.
Mating success of female Dungeness crabs of different sizes.

                                   Previously     With Viable
                       Recently      Mated           Sperm
                        Mated       (% with         (% with
Carapace               (% with      Swollen       Sperm Plug
Width       Females     Sperm     Spermathecae    or Swollen
(mm)       Dissected    Plug)        Only)       Spermathecae)

100-109            7         43             14              57
110-119           26         58              4              62
120-129           39         56             13              69
130-139           65         74              3              77
140-149           79         67             20              87
150-159          173         71             18              89
160-169           38         84             11              95
Total            427         69             14              83
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