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Justin The Effect of Hatchery Cell Size on Growth of Juvenile Blue Crabs, Callinectes sapidus Rathbun
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Results and Discussion
Cell Area, Trial 1 In the experiment's first trial, the system was plagued early on with high levels of ammonia (Figure 4 - Click to view) because of Biofilter lag due to the change of water. High levels of ammonia are toxic and subsequently inhibit growth, thus no significant data had been collected. A comparison can be made between the Trial 1 molt numbers (Figure 6 - Click to view) and the Trial 2 molt numbers (Figure 7 - Click to view), which inversely correlate with the level of ammonia in the respective systems (Figure 4 and Figure 5, respectively - Click to view). In high ammonia levels, few molts were observed, which indicated that their growth was stunted. Under conditions such as these, the blue crabs have a difficult time achieving the number of molts necessary for proper growth and health. This is a profound illustration of part of the problem for the crab population in the Chesapeake Bay watershed, an extremely polluted watershed with worse water quality than that of Trial 1 (Chesapeake Bay Foundation). Table 3: Cell Area versus Crab Size Significance P < 0.05 for significance
 and Figure 9 (right)) Figures 8 and 9 (Click to view) In the second trial, a sufficient amount of data for valid statistical analysis was collected; this was attributed to the improvement in water quality due to stable Biofilter. Analysis showed that cell area has no significant effect (P < 0.05) upon any aspect of juvenile blue crab growth analyzed: wet weight, molt increment, or molt frequency (Table 3, see above). Thus, one of three conclusions can be made: 1) cell area has no significant effect on the growth of juvenile blue crabs, 2) juvenile blue crab growth inhibition points were not reached, or 3) the range of cage sizes used was not broad enough to detect a difference between treatments. In other words, because the crabs were small in comparison to the area of the cage, such as in Figure 8, the crabs may not have experienced any stress from the walls of the cage and thus not have been inhibited by the size of the cell. Future research could be done to substantiate which of these three conclusions is correct by either keeping the same-sized cells with larger juveniles, or by using the same-sized juveniles with smaller cells, as shown in Figure 9. Although some questions remain as to what impact, if any, further cell area reduction has on the growth of juvenile blue crabs, it has been concluded that cells at least 90% larger (smallest cell area÷average crab area) in area than the juveniles do not promote any growth inhibition. |
Table 4: Cell Depth versus Crab Size Significance
P < 0.05 for significance
Cell Depth Initial analysis of the data yielded no sign of a significant effect (P < 0.05) of cell depth upon any aspect of juvenile blue crab growth except total molts and the time between molts two and three (Table 4, see above). Post hoc analysis revealed that there was a significant difference between the total molts of shallow and medium depths and between shallow and deep depths, and that there was a significant difference between the times to reach molt three of medium and deep depths. The deep depth had both less total molts and less time between molts two and three than the shallow depth. Although this seems to contradict itself at first, since greater molt frequency would seemingly yield more molts, a closer observation of the situation reveals that this is not so. Since the deep-depth crabs molted more frequently, they also hit a larger size and their third molt faster. As crabs grow larger, molt frequency decreases, so the deep-depth crabs basically stopped growing, according to the data of the experiment, since their fourth molt was never recorded. However, since this only indicates a slight difference in stage of life and not a significant difference in actual size, there is no significant effect of cell depth upon the growth of juvenile blue crabs at least 17% (average crab volume÷average shallow depth volume) of the volume of water. In the same manner as the data for the effect of cell area on crab growth, this information could be key in setting up an economically viable separate-cell hatchery system in which the crabs grow uninhibited but still allowing for maximum organism capacity. Further study could be continued on the effect of reduced cell depths on juvenile blue crab growth. Significance Although cell size, both the area and depth aspects, doesn't seem to have a significant effect upon the growth of juvenile blue crabs, this research has still made an important contribution to knowledge about the blue crab and for the development of an economically viable separate-cell juvenile blue crab hatchery system. The BCARC (Blue Crab Advanced Research Consortium) now knows that small cells can be used with no loss of crab growth, so with maximum, uninhibited crab growth coupled with maximum organism capacity, the hatchery system can be effectively used to restock the declining blue crab populations within the Chesapeake Bay. This research project will also indirectly help to revive and sustain the blue crab-related economic activity that is essential to the Chesapeake Bay region by increasing the available blue crab supply. Thus, this research on The Effect of Hatchery Cell Size on the growth of juvenile blue crabs will ecologically help the Chesapeake Bay and economically help humans. |
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