A Growth Model for the Deep-Sea Red Crab (Geryon Maritae) Off South West Africa/Namibia (Decapoda, Brachyura) Author(S): R
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A Growth Model for the Deep-Sea Red Crab (Geryon maritae) off South West Africa/Namibia (Decapoda, Brachyura) Author(s): R. Melville-Smith Source: Crustaceana, Vol. 56, No. 3 (May, 1989), pp. 279-292 Published by: BRILL Stable URL: http://www.jstor.org/stable/20104459 Accessed: 12/08/2009 22:31 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=bap. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected]. BRILL is collaborating with JSTOR to digitize, preserve and extend access to Crustaceana. http://www.jstor.org Crustaceana 56 (3) 1989, E. J. Brill, Leiden A GROWTH MODEL FOR THE DEEP-SEA RED CRAB (GERYONMARITAL) OFF SOUTH WEST AFRICA/NAMIBIA (DECAPODA, BRACHYURA) BY R. MELVILLE-SMITH Sea Fisheries Research Institute, Private Bag X2, Roggebaai 8012, South Africa and Zoology Department, University of Cape Town, South Africa ZUSAMMENFASSUNG aus vor Von 1979 bis 1986 wurden etwa 1660 markierte Geryon maritae kommerziellen F?ngen Namibia wiedergewonnen. Von diesen Tieren zeigten 518 Hinweise darauf, da? sie seit der Markierung gewachsen waren und sich geh?utet hatten. Die prozentualen Wachstumsraten waren von weiblicher Exemplare signifikant niedriger als die M?nnchen (P<0,01). F?r beide von zu Geschlechter ist eine Abnahme der Wachstumsraten in Abh?ngigkeit der K?rpergr??e zur verzeichnen. Die errechneten Zwischenh?utungsperioden stehen in direktem Verh?ltnis zur zu K?rpergr??e. M?nnchen neigen bis ersten Geschlechtsreife l?ngeren zu Zwischenh?utungsperioden als Weibchen. Danach scheinen sich die Weibchen seltener h?uten. Ein hypothetisches Wachstumsmodell zeigt, da? die M?nnchen etwa ab dem siebten Lebensjahr kommerziell genutzt werden k?nnen. Vollst?ndig rekrutierende Jahrg?nge treten aber erst mit Eintritt der Geschlechtsreife ab dem neunten Lebensjahr auf. Ein Vergleich mit Literaturangaben ?ber das Wachstum dreier Flachwasser-Krabbenarten zeigt, da? Geryon maritae verh?ltnism??ig langsam w?chst. INTRODUCTION It is essential to have an understanding of growth and age/size relationships, if one is to apply equilibrium yield models to the management of a fishery. In crustaceans, growth is discontinuous and each species can therefore be simplistically described by a model incorporating (i) the length of time between moult intervals and (ii) the increase in carapace size between moults. The model is also generally dependent on the animal's sex and the stage of its life or as cycle (whether larval, juvenile adult), as well changes in environmental conditions and the availability of food. Growth models of varying refinement exist for a number of decapod species, inter alia Cancer magister Dana, 1852 (Butler, 1961), Cancer pagurus L., 1758 & (Hancock Edwards, 1967), Paralithodes camtschatica (Tilesius, 1815) & & (McCaughran Powell, 1977), Homarus gammarus (L., 1758) (Conan Gundersen, 1979) and Chionoecetes bairdi Rathbun, 1924 (Donaldson et al., 1981). In this paper, a similar model is attempted for the deep-sea red crab, Geryon maritae Manning &Holthuis, 1981, which is commercially fished on the 280 R. MELVILLE-SMITH ~ ? ' continental slope off the Namibian coast from west of Cape Cross ( 21 50 S) to the Angolan border (17? 15'S). In addition to its prime objective, which was its application to the manage ment of the Namibian red crab stock, this study provided a rare opportunity to in a crustacean. examine situ growth of relatively deep-water Because of logistical problems associated with deep-water work, there exists far more on speculation than information this topic (see Roer et al., 1985). METHODS ? Tagging. During the period September 1979 to February 1984, a total of 10 246 crabs was tagged on the Namibian red crab grounds, in depths rang m. ing from 400 to 900 Floy FT2 dart tags were inserted through the epimeral suture with the toggle lodged in the branchial cavity, permitting the crab to moult without losing the tag. For each crab that was tagged, a record was made of its position and depth at release, tag number, carapace width (CW), sex, shell state, whether appen dages were missing and, in the case of females, their state of maturity based on the shape of the vulvae (Melville-Smith, 1987). The animals were released at the same depth and near the position at which they were captured, using a cage specially designed and constructed for this purpose. The grounds on which the crabs were released were fished year-round by five Japanese crab vessels. Tagged crabs caught in the traps set by these vessels were retained and returned together with details regarding their position, at depth and date recapture. ? Data were to analysis. Standard techniques used describe the average growth increment and intermoult period for crabs of varying carapace width. This study relied on tagged crabs returned by the commercial fishery and, because the gear was biased against catching small crabs, there is a gap in growth data for crabs smaller than 60 mm CW. This has necessitated extrapolation into the smaller size classes. Although crabs were tagged in several areas of the Namibian red crab grounds, there were insufficient data to permit an analysis of possible dif ferences in growth rate by area. ? Growth increments. The growth factor or percentage growth increment between successive moults was calculated by plotting the percentage CW incre ment against premoult CW. Using this method, those crabs that had moulted more than once while at large could be identified and separated from those once for further moulting growth-per-moult analysis. The relationship between pre and post moult carapace width (Hyatt growth curve) was established separately for males and females as described by Somer GROWTH MODEL FOR GERYON 281 curve ton (1980). The slope of the Hyatt growth changed near the size at which sexual maturity is attained by male crabs. The point at intersection of the resulting two linear regression lines (for juvenile and adult males) was found using a technique described by Somerton (1980), whereby a premoult size is found, which minimises the residual sum of squares pooled for both lines. ? Intermoult period. Intermoult periods have been graphically presented on for males in 5 mm CW size increments, based the length of time that crabs in the various size intervals were at large before moulting. A theoretical method (whereby the number of moults per days at large was regressed against an size) has been used to calculate equation describing the relationship between intermoult period and carapace width, and the resulting regression line has been compared with the observed data. Less emphasis has been placed on female intermoult period data, for reasons are which dealt with under results. A table showing maximum lengths of time immature female crabs were at large without moulting has been used as a very generalised means of comparing differences in intermoult period between the sexes. RESULTS ? Growth increments. Growth factors for all the male crabs that had grown are presented in fig. la and those for females in fig. lb. From fig. la, it is clear that the points lying between growth increments of 35 to 60 per cent are for male crabs that had moulted twice while at large. Double moult increments were not recorded for any female. Male crabs showing double growth increments were excluded from the data and predicted relationships between growth increment and size then fitted for male and female animals (fig. la, b). The equations for the fitted regressions (fig. la, b) are: = y -0,1167x + 31,75 for males and = y -0,2016x + 32,99 for females, where = the factor or moult increment between and y growth percentage pre = post moult carapace widths and x the premoult carapace width in mm. The r values for the equations above were 0,60 and 0,50 for males and females respectively. a The regressions in fig. la and b were compared statistically with view to combining the male and female data. Male crabs had, however, larger growth increments than females and the difference between the regressions for the were sexes was significant (P<0,01). Male and female data therefore kept separate in all further analyses. A Hyatt growth diagram for male red crab is presented in fig. 2a and des cribed in table I. The two regression equations for male growth in table I have their point of intersection, as calculated by Somerton's (1981) method, at 93 mm premoult CW. This point of intersection where growth changes, is 282 R. MELVILLE-SMITH 55 50 45* 40 < y=-0,U67x +31,75 55 60 65 70 75 80 85 90 95 XX) 105 110 115 120 125 130 135 140 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 PREMOULTCARAPACE WIDTH (mm) Fig. 1. Growth factors (percentage moult increment) for (a) males and (b) females of different are to premoult lengths.