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Shellfish Assessment Program

Red King Crab, Paralithodes camtschaticus, Size-Fecundity Relationship and Inter-Annual and Seasonal Variability in Fecundity

female red king crab brooding eggs
Figure 5.  Female red king crab brooding eggs.


Lindsey Bidder
Figure 6.  Kodiak Laboratory research technician Lindsey Bidder processing samples for fecundity estimates.


figure 7, see caption
Figure 7.  Inter-annual comparison of the size-fecundity relationship of Bristol Bay red king crab.  Only the summer data is graphed, the relationships are the same for fall data.  Circles and triangles are individual female observations.  Lines represent the relationship predicted by the best fit model and are hard to distinguish because they overlap.


figure 8, see caption
Figure 8.  Seasonal comparison of the size-fecundity relationship of Bristol Bay red king crab.  Only 2007 and 2009 pooled data are graphed, the relationship is the same for every year.  Circles are individual female observations.  Lines represent the relationship predicted by the best fit model.
 

Embryo production and its effect on population dynamics is poorly understood for crustaceans and is therefore not often included in stock assessment and management; however, such knowledge is necessary to improve management of these species. Stock assessment of Alaskan red king crab, Paralithodes camtschaticus, does not incorporate embryo production due to a lack of data.

The harvest strategy employed by the Alaska Department of Fish and Game for Bristol Bay red king crab relies on a length-based population model for stock assessment that defines male reproductive potential as mature male abundance (stratified into 5-mm carapace length (CL) size bins) multiplied by the maximum number of females with which a male in a particular size bin can mate.

Female spawning abundance, which is used as a proxy for female reproductive potential, is set equal to male reproductive potential or mature female abundance, whichever is less. This method assumes mating always occurs and unfortunately does not include any means to detect reproductive failure.

Timing of Bristol Bay red king crab mating, which occurs between larger hard-shelled males and smaller soft-shelled females, is variable and can occur from the end of January through the end of June. Red king crab females must mate annually to produce a fertilized clutch of eggs. Fertilization is external, fecundity increases with female size, and embryos are brooded approximately 10 to 12 months until hatching (Fig. 5).

Incorporation of fecundity in stock assessment and management of Bristol Bay red king crab requires an understanding of the size-fecundity relationship and its inter-annual variability. Furthermore, since eggs are lost throughout the brooding duration, efforts to relate fecundity to larval output and ultimately recruitment should estimate fecundity close to hatching or estimate rates of egg loss by looking at seasonal changes in fecundity.

Our objectives were to determine the size-fecundity relationship and understand interannual and seasonal variability in Bristol Bay red king crab fecundity by modeling data from summers 200710 and falls 200709. This study will provide critical data to incorporate into stock assessment models to improve management of the stock, and establish a baseline against which future monitoring can be used to test for environmental or fishing-induced effects on stock reproductive potential.

To compare interannual and seasonal variability in red king crab fecundity, egg clutches of females were collected from Bristol Bay, Alaska, in summer and fall. Summer samples were collected June through July 2007, 2008, 2009, and 2010 during the NMFS eastern Bering Sea bottom trawl surveys and fall samples were collected October and November 2007, 2008 and 2009 by shellfish fishery observers during the commercial fisheries. Samples were sent to the Center's Kodiak Laboratory, where they were processed and fecundity estimated (Fig. 6). Data were fit to a series of 23 models and 2 post-hoc models using maximum likelihood fitting techniques. Some of the models included a split point where the slope of the size-fecundity relationship changed above a critical point.

Fecundity increased with female size, but the slope decreased by 40% above 138-mm CL, suggesting senescence which has not been previously reported for this species. Although the size-fecundity relationship of Bristol Bay red king crabs differed among all years examined in this study except for 2007 and 2009 (Fig. 7), the variation observed is minimal (maximum of 5% difference in slope) and likely not biologically significant as it only equates to a 2% difference in the predicted fecundity of the largest sized female sampled. Fecundity varied seasonally; females smaller than 138-mm CL were 6% less fecund in the fall than summer and females larger than 138-mm CL were 10% less fecund in the fall, suggesting brood loss (Fig. 8).

Our results are a strong starting point for inclusion of embryo production in stock assessment and management, but more information is needed. It is important to note that all samples were taken during cold years in the eastern Bering Sea. It is possible that different environmental conditions affect fecundity; therefore we are cautious to apply our results to other environmental conditions. To more accurately model reproductive potential, data is needed on both temporal and spatial variation in the size-fecundity relationship, and if variation occurs, annual estimates of fecundity are needed.

By Katherine M. Swiney
 

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