ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on April 22, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(6):862-872; doi:10.1093/icesjms/fsn058
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The Northwest Atlantic deep-sea red crab (Chaceon quinquedens) population before and after the onset of harvesting
1 Bigelow Laboratory for Ocean Sciences, 180 McKown Point Road, West Boothbay Harbor, ME 04575, USA
2 Northeast Fisheries Science Center, National Marine Fisheries Service, Woods Hole, MA, USA
3 School of Marine Sciences, University of Maine, Orono, ME, USA
Correspondence to R. A. Wahle: tel: +1 207 633 9659; fax: +1 207 633 9661; e-mail: rwahle{at}bigelow.org
Wahle, R. A., Bergeron, C. E., Chute, A. S., Jacobson, L. D., and Chen, Y. 2008. The Northwest Atlantic deep-sea red crab (Chaceon quinquedens) population before and after the onset of harvesting. – ICES Journal of Marine Science, 65: 862–872.The population structure of deep-sea red crab (Chaceon quinquedens) in a nearly unexploited state is compared with its condition three decades later after more than a decade of sustained harvesting. Our study is based on a camera and net trawl survey conducted in 1974, which we repeated between 2003 and 2005 on the southern New England shelf break. Although the overall biomass of red crabs was estimated to be higher than in 1974, the abundance of large males, which are targeted by the fishery, was considerably lower. In particular, the biomass of large males (
114 mm carapace width), considered in 1974 to be marketable, declined by 42%. Declines were most evident at depths and regions most accessible to the fishing fleet based in southern New England. With the change in fishery selectivity towards smaller male crabs, the abundance of currently harvestable crabs is about equal to 1974 levels. No declines were observed in the biomass of female and smaller male crabs not targeted by the fishery. Indeed, the abundance of juveniles appears considerably higher than in 1974. Perhaps, adverse effects on reproduction attributable to a reduction in the numbers of large males may be a consequence of fishing, but fishery impacts and productivity are difficult to assess because key biological information is lacking.
Keywords: Chaceon quinquedens, geryonid, harvesting impacts, virgin population
Received 26 October 2007; accepted 14 March 2008; advance access publication 22 April 2008.
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Introduction |
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It is unusual to have the opportunity to compare conditions in a harvested population with conditions in the original virgin state. For many exploited species, harvesting preceded monitoring and only qualitative historical information is available on pre-harvest conditions (Jackson et al., 2001). Quantitative historical information is often limited to catch data (Sissenwine, 1984). As a result, fisheries scientists routinely resort to modelling approaches that approximate the nature of the stock in an unexploited state based on potentially uncertain assumptions about growth, recruitment, and natural mortality (Hilborn and Walters, 1992; Silbert et al., 2006).
Problems understanding historically unexploited stocks may be exacerbated for large crustacean fisheries. Coastal fisheries for crab and lobster typically have deep historical roots, even into prehistoric times, so there is little hope of knowing the character of unexploited stocks (Wilson, 1991; Browne et al., 2001). In contrast, deep-sea fish and crustacean species support relatively new fisheries, and in a few cases, scientific surveys were conducted before appreciable harvesting started (Wenner et al., 1987; D’Onghia et al., 2005).
For the deep-sea red crab (Chaceon quinquedens), the National Marine Fisheries Service Northeast Fisheries Science Center (NMFS/NEFSC) conducted a camera and trawl survey to assess the population off the northeast coast of the USA just as a small experimental fishery was becoming established in the early 1970s (Wigley et al., 1975). Before the survey, catches were small, and deep-sea red crabs were fished only sporadically (Gerrior, 1981). In the 1980s and 1990s, fishing effort was inconsistent primarily as a consequence of variable market demand. However, a directed fishery for red crabs and consistent markets developed in the mid-1990s.
The current US fishery for red crabs has limited entry and as of 2006 consisted of 4–5 vessels 30+ m long. The fishery uses specially designed traps almost exclusively, although small catches are taken also in lobster traps. Catches are made along the continental shelf from the Canadian border (Hague Line), at the eastern end of Georges Bank, to Cape Hatteras, NC, USA, in depths ranging from 400 to 800 m. Annual US commercial landings of red crabs during the period 1982–2005 ranged from 466 t (1996) to 4000 t (2001); there was no fishery in 1994. Since 2002, when the Fishery Management Plan was implemented, landings have been stable at 
2000 t per year (NEFSC, 2006). There is no recreational fishery for the species, and red crabs in the USA are managed as a single stock. Under the management plan, a limited access fishery has been implemented, with the fishery authorized to operate with a target TAC of 2688 t, an allocation of 780 days at sea, and a trip limit of 34 t. No female red crabs are landed.
The objective of the present study was to repeat the 1974 NEFSC survey for red crabs, but using contemporary imaging technology. Our goal was to compare the conditions in the red crab stock at the beginning and end of a 30-year period, which started before there was much fishing and ended with about 15 years of sustained harvest.
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Material and methods |
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Study organism
Chaceon quinquedens is a deep-water brachyuran crab (family Geryonidae) inhabiting the edge of the continental shelf and slope from Emerald Bank, Nova Scotia, into the Gulf of Maine, south into the mid-Atlantic Bight, where it overlaps with its congener Chaceon fenneri, and into the Gulf of Mexico (Pequegnat, 1970; Williams and Wigley, 1977; Elner et al., 1987; Duggan and Lawton, 1997; Weinberg et al., 2003). Genetic differences between C. quinquedens from southern New England and the Gulf of Mexico are great enough to consider deep-sea red crabs in the two areas to be different biological populations and stocks (Weinberg et al., 2003).
Deep-sea red crabs live at depths of 200–1800 m, where temperatures are between 5 and 8°C (Wigley et al., 1975). Adult crabs are segregated incompletely by sex. Adult females generally inhabit shallower water than adult males, and juveniles tend to be deeper than adults, suggesting a deep-to-shallow migration as the crabs mature (Wigley et al., 1975). Other geryonids, including Chaceon affinis and Geryon trispinosus, show similar patterns in spatial distribution (Attrill and Hartnoll, 1991; Pinho et al., 2001). One hypothesis is that small red crabs inhabit relatively deep water to avoid competition with other crustaceans, which are more numerous shallower (Attrill et al., 1990). In contrast, the golden crab (C. fenneri) off the southeast coast of the USA does not segregate by size and depth (Wenner et al., 1987).
Information on the growth of red crabs is scarce. On the basis of very few data obtained mostly in the laboratory, red crabs are believed to require 5–6 years to attain a size of 114 mm carapace width (CW) (van Heukelem, 1983). Male red crabs are estimated to mature at 75 mm CW and to reach a maximum size of 180 mm CW. Females begin to mature somewhat smaller and reach a maximum size of 136 mm CW (McElman and Elner, 1982). The reported size of ovigerous female crabs varies between 61 and 130 mm CW (Wigley et al., 1975; Haefner, 1977, 1978; Elner et al., 1987). Ovigerous C. affinis were all >70 mm CW (Pinho et al., 2001) and C. fenneri females appear to mature at a larger size of 97 mm CW (Wenner et al., 1987).
As in other brachyuran crabs, the mating male is larger than the female and forms a protective "cage" around the female while she moults and becomes receptive to copulation. The protective and copulatory period may last as long as 2–3 weeks in red crabs, much longer than most other brachyurans (Elner et al., 1987). The minimum size of males relative to females required for successful mating is unknown. Recent evidence that the fishery may be depleting large males (Weinberg and Keith, 2003) has raised concern over the potential for fishery impacts on the reproductive ability of the population. Whether female red crabs are able to fertilize more than one clutch of eggs between moults, as can lobsters (Flight et al., 2004), is unknown.
Camera and otter trawl surveys
The 1974 NEFSC red crab survey was conducted during June on the 57-m NOAA RV "Albatross IV". The more recent survey was conducted during June and August 2003, June 2004, and June 2005 using the FVs "Hannah Boden" (29.3 m) and "Krystle James" (27.4 m), both of which participate in the deep-sea red crab fishery.
In 1974 and 2003–2005, sampling effort spanned a segment of the continental shelf break from offshore Maryland to the eastern end of Georges Bank (Figure 1). Survey results are partitioned into geographic sectors and the depth strata originally established by Wigley et al. (1975; Table 1). During the 2003–2005 survey, an effort was made to revisit sites surveyed during 1974 as often as possible (Table 1 and Figure 1).
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Both surveys consisted of camera transects from a towed benthic sled and otter trawl tows (discussed subsequently). Counts of red crabs in images from camera tows provided estimates of population density. Trawl catches provided data on sex, size composition, and maturity. Details are given below.
Camera-sled system
Photographs were used to determine the density (numbers per unit area) of red crabs and associated fauna. The photographic system used for both surveys was mounted on a benthic sled. Specifications of both the camera-sled system and the otter trawl employed in 2003–2005 were matched as closely as possible to the 1974 system (Table 2). However, the sled used in the recent survey was somewhat smaller than that used in 1974, primarily because of constraints imposed by the smaller size of the vessels. In both cases during preliminary trials in shallow water, a grid with known intervals was placed level on the seabed in front of the camera to determine the area illuminated in the image. During the image analysis, only well-lit and unobscured photographs were used.
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The camera system used in 1974 consisted of a 70 mm Nikon film camera and stroboscopic light (Wigley et al., 1975; Theroux, 1976). The camera was 1.75 m above the seabed and aimed downwards at an angle of 16° from horizontal in a vertical plane perpendicular to the sled. In that position, the camera viewed a total area of 148 m2, and the effective area sampled (illuminated) was 31.8 m2 (Theroux, 1976; Patil et al., 1979). The system was programmed to take photographs every 10 s, so at a towing speed of two knots, a photograph would be taken approximately every 10 m. A 30 min tow therefore provided approximately 180 images, representing an area of 5724 m2 of illuminated area sampled per tow.
The system used for the 2003–2005 survey consisted of a Nikon Coolpix 990 digital still camera modified with a programmable intervalometer and computer interface software designed by Engage Technologies, Ltd. The camera was protected by titanium housing and coupled to a Benthos, Inc., model 382 strobe. The camera was at a height of 1 m above the seabed, aimed downwards at an angle of 35° in a vertical plane perpendicular to the sled. In that position, the camera viewed a total area of 10 m2 and had an effective illuminated area determined to be 6.6 m2. The system was programmed to take a photograph every 15 s, which, at a towing speed of two knots, meant that a photograph was taken approximately every 14 m. A 30 min tow therefore resulted in about 120 images, representing an area of 792 m2 of illuminated area sampled per tow.
Photographs were viewed individually. For each photograph, deep-sea red crabs, other invertebrates, and fish were identified and quantified, and the bottom type was categorized. When possible, individual crabs in mating pairs were measured from the photo for an estimate of the relative size ratio of males to females observed in the copulatory embrace.
Because of the likelihood that we missed some crabs that were hidden in burrows, we consider any abundance estimates derived from these images as conservative. We assume here that this hidden fraction of crabs has not changed over the years.
We also considered the potential bias resulting from the fact that the cameras used during the two surveys sampled different areas (Table 1). To determine whether the oblique orientation of the camera relative to the seabed may have resulted in underestimates of abundance because of crabs avoiding the sled in the foreground or not being detected in the background, we conducted an analysis of images using the 2005 data. We examined a subset of 141 randomly selected photographs among all photographs containing one or more crabs. The 6.6-m2 illuminated area of each photograph was divided into equal foreground and background subareas, and the crabs in each subarea were counted. The null hypothesis that crabs were as likely to be as present in the foreground as in the background areas was tested with a simple 
2 statistic. Crabs occurred with significantly greater frequency in the background (79% of the time) than the foreground subareas (34% of the time; 2 contingency analysis, 2 = 49.06, d.f. = 1, p < 0.0001). Perhaps, therefore, crabs had an opportunity to move away from the sled before the image was taken, resulting in underestimated densities. Images from the 1974 survey were not available to conduct the same evaluation, but we suspect that this artefact might not have been as strong then, given the larger area (31.8 m2) sampled by the camera during that survey. On that basis, the proportion by which population density may have been underestimated in 1974 is likely less than that for the 2003–2005 surveys.
Otter trawl
The nets used in the two surveys were virtually identical (Table 2). Bottom trawls had a fine-mesh liner that increased retention of small crabs for a better representation of the range of population size composition. The net was towed at 1.5–2.0 knots for 30–45 min. In both surveys, the wire length-to-depth ratio varied between 1.5 and 3, depending on depth and conditions. In the recent survey, no net tows were conducted at depths >914 m because of insufficient wire on the fishing vessels used for the survey.
The trawl data were used in both surveys to prorate density estimates from the camera survey by size and sex. On a tow-by-tow basis, catch-per-30-min-tow from the recent survey correlated weakly with the density estimate from the camera tows at the same sites (r = 0.06, n = 112, p = 0.06), so camera-derived data were taken as the more reliable indicator of abundance.
Port sampling and fishery selectivity
Port samples were collected by NMFS port agents and used in the analysis of fishery selectivity. Data consisted of the sex and size of landed deep-sea red crabs. Port samples from 2004 and 2005 were assigned to a survey stratum from the camera/trawl survey based on the statistical area reported by the vessel for the location of the catch. Port sample records from statistical areas that were not covered by the survey were excluded from the analysis.
NEFSC (2006) estimated fishery selectivity curves for deep-sea red crab during recent years by comparing the length composition of landed crabs from port samples with that of crabs caught during the otter trawl survey, which were assumed to represent the population. Data from all port samples in each statistical area and year were combined by addition and binned into 5 mm size groups for subsequent analyses. All length composition data were then converted to proportions at length. Selectivity curves were fitted by linear regression with logit-transformed proportions at length. The estimated fishery selectivity curve was: sL = 1(1 + e26.86–0.2905L) where sL is the selectivity at CW L (mm).
Fishery selectivity for red crab in all regions during the period 2004–2005 is near 0% at sizes <80 mm. After 80 mm, fishery selectivity increases rapidly and is nearly 100% by 120 mm. Red crabs are estimated to be 50% selected by the fishery at 92 mm (Figure 2).
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Biomass estimates by sex and size
The biomass for both surveys was estimated using crab densities from camera tows and the proportion at sex and size composition from net tows (in Murray, 1974, for the 1974 tows) and the following sex-specific size/weight relationships (Farlow, 1980, and cited in Steimle et al., 2001):
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To calculate the red crab biomass by depth stratum within a sector, densities (number of red crabs observed in photographs divided by area of seabed examined) were multiplied by estimates of the area of seabed in each stratum. Stratum areas were deduced from Wigley et al. (1975) by dividing the reported abundance estimate by the density estimate for each stratum (Table 3).
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The depth range over which densities and standing crop are estimated is 229–914 m. No market-sized crabs were found in either survey at depths >914 m, and bottom trawl data were probably less reliable. For strata with no survey data, the mean density for that depth was used as a proxy value from which to calculate abundance and biomass.
No standard errors were available for the biomass estimate from the 1974 survey (Wigley et al., 1975). To evaluate the statistical significance of abundance and biomass estimates from the two surveys, we compared the 1974 point estimates with mean values and 95% confidence intervals for the mean values of the four survey legs conducted between 2003 and 2005 (June and August 2003, June 2004, and June 2005). Standard errors and 95% confidence intervals were calculated by treating the four survey-leg estimates as observations. This ad hoc approach was intended to characterize variability among surveys rather than variability among individual camera tows. As shown below, it sufficed to determine whether differences between the 1974 and recent estimates were statistically significant.
Biomass estimates are reported for five size categories of crab: all sizes, crabs 114 mm, male crabs, sexually mature females, and sexually mature males. Wigley et al. (1975) defined fishable crabs as crabs of both sexes 114 mm CW (considered the minimum marketable size at the time), although he did not make a distinction between male and female crabs in estimating the abundance or biomass. The red crab fishery currently harvests crabs that are smaller than what was considered profitable to processors in 1974, because there are fewer large crabs. As a result, the size groups used to compute fishable biomass differ between the historical and recent surveys. The estimated biomass of crabs 114+ mm in 1974 was, therefore, compared with that of current fishable biomass (based on the fishery selectivity curve) and with that of recent biomass of 114+ mm crabs. Using the fishery selectivity curve, fishable biomass (B) was calculated from the following equation:
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On the basis of the observations of the smallest ovigerous females in the recent survey, females 70 mm CW and larger were assumed to be mature and used in calculating sexually mature biomass and abundance. In other surveys, the minimum size of ovigerous female deep-sea red crabs ranged from 61 to 80 mm (Wigley et al., 1975; Haefner, 1977; Elner et al., 1987).
Male red crabs may be physiologically mature at sizes <40 mm (Haefner, 1977), but in camera surveys, males observed in copulatory embrace averaged 50% larger than females. This suggests that males must be considerably larger than females to mate successfully. A previous report used 75 mm as the minimum size of reproductive males (McElman and Elner, 1982). We used 75 mm CW to define mature male red crabs in this analysis, because it accounts for the apparent differences in physiological and functional maturity and because it is larger than the minimum size of maturity used here for females.
Further details of the survey methodology may be found in Wigley et al. (1975) and NEFSC (2006).
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Size and sex composition
In 1974, 795 female and 641 male crabs were collected by otter trawl (Wigley et al., 1975). Between 2003 and 2005, 4602 female and 2209 male crabs were collected. Although the overall size range and catch-per-tow of crabs from the two surveys were similar, the number of large male crabs was substantially lower in the more recent surveys than in 1974 (Figure 3). The size composition of females differed very little, the current mode falling within the same range as it did in 1974. Also, a greater proportion of smaller crabs of both sexes in the 50–80 mm size range was apparent in the 2003–2005 size distribution.
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Biomass estimates by sex and size
Total red crab biomass is estimated to have increased by 250% since 1974 (Figure 4). This increase in overall crab abundance is attributable to increased abundance of small crabs, because the estimated biomass of large crabs
114 mm declined by 27%, from 25 900 to 18 990 ± 2160 t (1 s.e.) between 1974 and 2003–2005. The reduction in the number of large crabs was attributable to a 42% decline in males 114 mm, from an estimated biomass of 23 794 t in 1974 to 13 769 ± 1334 t during 2003–2005.
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Despite the estimated decline in large males, the biomass of marketable males is roughly the same as in 1974 (36 253 ± 5459 t during 2003–2005 and 34 264 t in 1974). The point estimate for 1975 falls well within the 95% confidence interval for the recent survey (Figure 4). Despite the decrease in red crabs
114 mm CW, fishable biomass based on the current survey was similar to fishable biomass during 1974 (114 mm CW), because smaller red crabs are now accepted by processors (Figure 3). The depth distribution of red crabs appears to have shifted since 1974 (Figure 5). In the 2003–2005 survey, crabs of all sizes were estimated to be somewhat less abundant in the shallow zones than in 1974 and more abundant in the deeper zones.
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Red crab distributions by sector, combining all depths, were also somewhat different from the situation in 1974 (Figure 6). In the recent survey, total crab biomass was estimated to be higher, in all sectors except B, relative to 1974 levels (Figure 6a). The biomass of males 114+ mm was estimated to be below 1974 levels in all sectors except in D (Figure 6b). The biomass of currently fishable crabs was down in the sectors most accessible to the southern New England crab fishing fleet, B and C, but about twice 1974 levels in sectors A and D (Figure 6c).
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The depth distribution of sexually mature male and female red crabs (all sectors combined) has changed since 1974, similar to the pattern for all crabs (Figure 7a, b). Although the biomass of mature red crabs of both sexes is estimated to have declined from 1974 levels in the 320–411 m depth stratum, the biomass of mature females is currently estimated to be substantially greater in deeper water where there is less fishing activity.
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In all sectors, the biomass of mature females was estimated to be more than twice as high in 2003–2005 as in 1974 (Figure 8a and b). Differences in the mature male biomass among sectors (Figure 8b) were similar to differences for fishable males (Figure 6c). In particular, mature biomass was lower than 1974 levels in sectors B and C, but about twice as high in sectors A and B.
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In reviewing the camera images, we observed many cases in which male crabs held females in a copulatory embrace. Although it was not possible to obtain absolute measures of CW, we were able to obtain measures of the ratio of male and female CW in 10 cases. From these measurements, the mean male-to-female size ratio was 1.5 (s.d. = 0.18).
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Discussion |
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Direct comparisons of populations before and after targeted harvesting can provide insight into the nature of fishing impacts (Weinberg and Keith, 2003). This study provided an opportunity to compare the population structure of deep-sea red crabs in a near-virgin state with its condition some three decades later, after at least a decade of sustained harvesting. Declines in the biomass of large male crabs targeted by the fishery were most evident in depth zones and regions of the shelf most accessible to the fishing fleet. The spatial pattern of fishing effort as revealed from Vessel Trip Reports collected since 2001 along the southern New England and mid-Atlantic shelf break is consistent with the pattern of crab depletion (NEFSC, 2006). Spatially explicit effort data were not collected during the early years of the fishery, although it is probably safe to assume that the fishery focused first on the most accessible areas directly south of New England. Moreover, no such declines were observed in segments of the crab population not targeted by the fishery, namely, the smaller females and juveniles. The abundance of juveniles appears to have been considerably higher in recent years than in 1974.
There are, however, several issues that contribute uncertainty to our conclusions. First, we have no way to assess uncertainty around biomass estimates from the 1974 survey, other than based on variability in the 2003–2005 survey. In addition, the original sample material from the 1974 survey is no longer available for re-evaluation, and perhaps, improvements in technology, assumptions about sampled area, or unrecorded differences in procedures affected historical estimates. This situation highlights the need to preserve original visual records for camera sled surveys that may be conducted infrequently.
There is also potential bias in our estimates owing to the significant crab–escape response from the towed sled. Because crabs avoided the oncoming sled, the recent surveys may have underestimated actual population density. However, without the 1974 images, it is not possible to determine whether crab counts from that survey were subject to the same or other biases.
Although camera surveys have shortcomings, they do provide a direct estimate of population density, and potential biases may be corrected after discovery, particularly if original study material is preserved. In an earlier study of the South African deep-sea red crab, Chaceon maritae, similar camera surveys were found to give superior estimates of abundance to mark-recapture or trawling methods (Melville-Smith, 1988).
Congeners of C. quinquedens around the world have been studied, often with the goal of determining whether abundance was high enough to support a fishery. Estimates of unfished population density have been determined for the South African deep-sea red crab, C. maritae, off South Africa (Cayre et al., 1979; Beyers and Wilke, 1980; Melville-Smith, 1983), the golden crab, C. fenneri, off the southeastern USA (Wenner and Barans, 1990), G. trispinosus off Ireland (Atrill et al., 1990), and the Juan Fernández golden crab, Carpobrotus chilensis, from the Juan Fernández islands 670 km off the Chilean coast (Arana, 2000). Over a range of depths, estimates of New England red crab biomass were higher than those for the other species. Biomass estimates for the New England red crab were 62 kg ha–1 in 1974 and 155 kg ha–1 in 2003–2005, whereas estimates for C. maritae ranged from 6.9 to 83.4 kg ha–1, G. trispinosus a maximum of 9.5 kg ha–1, and C. chilensis an average of 25 kg ha–1.
Abundance of the golden crab C. fenneri, whose range overlaps that of C. quinquedens to the south, was calculated using a submersible and estimated to be 1.9 crabs ha–1 between 1986 and 1988 (Wenner and Barans, 1990), a much lower density than reported for the deep-sea red crab. Similar to C. quinquedens, there was a sporadic fishery for golden crabs in the 1970s, and then a consistent fishery with removals of several hundred tonnes per year for the past 10 years at an estimated fishing mortality of 2.0 (NMFS, 2004). In contrast to C. quinquedens, which is fished at an estimated F of 0.05, there has not been any demonstrable change in the landed crab length frequencies (Harper et al., 2000; NMFS, 2004).
The targeting and subsequent depletion of large males are common in crab and lobster fisheries and may be cause for concern to the extent that it can affect the reproductive capacity of the population (Cobb and Caddy, 1989; Hankin et al., 1997). In the Caribbean spiny lobster, Panulirus argus, for example, female fecundity declines when they mate with smaller males (MacDiarmid and Butler, 1999). In our recent survey, the males in copulatory pairs averaged 50% larger than the females.
If males are only competent to mate when they are larger than females, as observed in many other brachyurans, the opportunities for bigger females to mate may be diminished by a reduced abundance of large male red crabs. For example, populations of the lithodid crab, Haplogaster dentata, where males are targeted by the fishery, are likely to be sperm-limited (Sato and Goshima, 2006). In that case, females not guarded by a male during the moulting process may have a limited time to find a mate after moulting (Sato and Goshima, 2006).
In our case, despite the decline in larger red crabs, the abundance of smaller deep-sea red crabs has apparently increased considerably. This may be the result of strong recruitment in years before the 2003–2005 surveys. It is unlikely that the difference in the size composition of crabs from the earlier and more recent surveys was caused by differences in the selectivity of the nets, because they were virtually identical and were towed in the same manner. An alternative hypothesis for the increase in smaller crabs could be a decrease in competition and cannibalism by larger crabs. Brachyurans are notorious for engaging in cannibalistic behaviour, with direct demographic consequences (e.g. Carcinus maenas, Moksnes and van Montfrans, 1998; Paralithodes camtschaticus, Lovrich and Sainte-Marie, 1997; Calinectes sapidus, Hines and Ruiz, 1995; Cancer magister, Smith and Jamieson, 1991, and Fernandez, 1999; and Chionoecetes opilio, Sainte Marie et al., 1996, and Stevens and Swiney, 2005). An analysis by Cartes (1993) of the diet of Geryon longipes off Spain found that up to 11.7% of stomachs contained a crab of some kind, but many were unidentified, so the degree of cannibalism was unclear. Another study of both G. longipes and Chaceon mediterraneus off Spain found no crabs in the stomachs, but concluded that crustaceans were an important component of the diet (Kitsos et al., 2005).
Detailed information on growth, longevity, reproduction, and recruitment of deep-sea red crab is minimal, and the lack of basic biological information interferes with the estimation of potential fishery productivity (Hastie, 1995). Abundance of large males was lower during recent years than during 1974. However, despite uncertainties (particularly about the 1974 estimate), red crab abundance and biomass appear relatively high at present. Fishing mortality rates (F = 0.055 ± 0.008 year–1) were modest during the period 2002–2005, fishing effort was limited, and landings have been relatively constant in recent years (NEFSC, 2006). Discard mortality also appears low, of the order of 5% (Tallack, 2007), although crabs were protected in a trap when they were returned to the bottom. Although modest in absolute terms, the consequences of the current level of fishing for deep-sea red crab cannot be determined without some information about stock productivity. Therefore, basic information on growth and reproductive rates are key priorities for future research.
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Acknowledgements |
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This project was financially supported by the Northeast Consortium, the Saltonstall–Kennedy Program, Maine Sea Grant, Darden Corporation, Coastal Enterprises, Inc., and Benthic Research, Inc. NOAA’s National Undersea Research Center, University of Connecticut, and the NMFS Northeast Fisheries Science Center loaned us equipment for our recent surveys. We are grateful to J. Williams and the captains and crews of the FVs "Hanna-Boden", "Krystle-James" and "Diamond Girl" for their part in conducting the trawl and camera surveys and sampling. M. Dunnington provided valuable technical support for surveys and data analysis, and F. Serchuk provided useful editorial and technical advice. Finally, we thank A. Applegate, R. Allen, D. Boelke, C. Pickett, B. Rountree, and J. R. Weinberg for their contributions and insights, and the two anonymous reviewers for helping to improve the manuscript.
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References |
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