The Effect of Hatchery Cell Size on Growth of Juvenile Blue Crabs, Callinectes sapidus Rathbun
In response to declining Chesapeake Bay blue crab populations, an effort to better understand the life cycle of the blue crab has been launched in hopes of using the information to create an effective hatchery system to restore the bay's populations. Cell size significantly affects the growth of numerous crustaceans, suggesting that a similar effect could be found with blue crabs. This study examines the effect of both cell area and depth upon the growth of juvenile blue crabs.
Only during the past century has the blue crab, Callinectes sapidus Rathbun, been the subject of experimentation and study. In earlier centuries the blue crab population was abundant and healthy, faithfully providing for a huge, profitable industry. Giving no cause for special study, the biology and life cycle of the blue crab remained largely mysterious. However, in this past century the blue crab population has become less abundant, giving alarm to those that rely on this economically and ecologically important crustacean and thus sparking experimentation and study. Several projects have confirmed that the population has been greatly reduced from what it was only 200 years ago by utilizing modern dredge surveys in comparison with past crabbing figures (Lipcius and van Engel, 1990; Rugolo, et al., 1998). As a result, further studies of the blue crab were finally put into motion to understand what factors affect the blue crab and what could possibly have caused this recent decline. A common subject of study has been the relationship between blue crabs and the declining number of seagrass refuges (Heck and Thoman, 1984; Ryer, 1987; Ryer et al., 1990), as well as with shallow water refuges (Dittel, et al., 1995; Hines and Ruiz, 1995). Tidal-stream transport and migration, once mysterious aspects of the blue crab life cycle, have also been studied, and it has been found that blue crabs do in fact utilize the tides to migrate throughout estuaries (Cargo, 1958; Olmi, 1994; Tankersley et al., 1998). Blue crab prey selection and response to stimuli, a complex formula that factors in relative size, distance, contrast to the surroundings, movement, and energy usage (Hughes and Seed, 1995; Hughes and Seed, 1997; Seed and Hughes, 1997; Taylor and Eggleston, 2000; Cote et al., 2001), is yet another important, studied aspect of the blue crab life cycle. As for the quality of the water itself and its impact on blue crabs, the adverse effects of hypoxia and anoxia on blue crabs (Tankersley and Wieber, 2000; Taylor and Eggleston, 2000) have been the most common subjects of study. Beyond these few examples, there are still many more new studies that cover a broad range of items related to the blue crab, such as early development, nutrition, and metabolism.
None of these projects, however, have fully "unveiled the poorly understood, yet complex, basic biology and life cycle" of the blue crab (Zohar, 2002). Therefore, because of this uncertainty and the need for blue crab hatcheries that could restore the bay's populations, a joint, multi-state effort between Maryland, Virginia, North Carolina, and Mississippi was created in 2001 to better understand the blue crab population and to use the newly gained information to create an effective hatchery system (Zohar, 2002). Within its first year, this effort greatly advanced the current understanding of the blue crab life cycle and created a hatchery system that demonstrated the capability to produce tens of thousands of crabs (Zohar, 2002). Currently, this research is continuing so as to perfect a blue crab hatchery system, including the study of optimal conditions for juvenile blue crabs. The goal of these "optimal condition" experiments is to fine-tune a hatchery system to make it more efficient and worthwhile—in other words, to attain maximum growth per crab and a maximum yield of crabs. Ultimately, the goal is to perfect a blue crab hatchery system that is capable of restocking the Chesapeake Bay. Specifically, a separate-cell hatchery system will be used to produce adults that can be used for study or for spawning. The adult females capable of spawning could then be released into protected areas unthreatened by crabbers and spawn in the Chesapeake Bay itself. Since females can produce as many as 8 million eggs over a period of several sponges (Chesapeake Bay Program), a relatively low release count of adult females could greatly boost the crab populations in the bay.
It is common knowledge that blue crabs are cannibalistic once they reach the juvenile stages where they grow claws. This means that a single population tank for a crab hatchery would be inefficient, since the larger-sized half of the population will cannibalize the smaller-sized half of the population. The solution to this problem is to use a separate cell system, where each individual crab is in its own solitary cell, or cage. One of the single most important questions that must be addressed when developing this type of hatchery system is what impact, if any, cell size has on the growth of juvenile blue crabs. If cell size does affect the growth of the crabs, then cell size would correspond to crab size, which would dictate how large the hatchery system needs to be, and ultimately whether or not the blue crab hatchery system is a viable option for blue crab production for the restocking of the declining populations within the Chesapeake Bay. With this question in mind, the purpose of this project was to study the impact of cell size on the growth of juvenile blue crabs, Callinectes sapidus Rathbun. The impact of cell size, both area and depth, on juvenile blue crab growth were analyzed for statistical significance.
Hypotheses and Objectives
Previously, an experiment that was conducted on redclaw crayfish showed that cell size proportionally influenced crayfish size (Aflalo, 2002). Since the blue crab is also a crustacean, it was expected that The Effect of Hatchery Cell Size on wet weight, carapace width, and molt frequency (time between molts) would be significant for blue crabs and that the development of a curve modeling the ratio between crab size and cell size would be possible.
Setup and Materials
First, to construct the cages, PVC board was cut with a table saw and pieced together to the cell dimensions shown in Table 1, with the adjacent pieces interlocking. Three sets of the layout shown in Figure 1 were constructed, each placed in one tank connected to the sump.
Next, the 90-gallon tanks were rinsed with tap water, cleaned with LiquinoX soap or 12% ammonia bleach, and rinsed out with tap water again. Then the system was filled with 12% bleach at 200 parts per million (approximately 2.07 liters of 12% bleach for the 1,300-liter system), and the bleached system was run for 24 hours. Finally, the system was drained and fresh water was run through the system for 24 hours and drained.
After the system was cleaned, the tanks were assembled. With PVC pipes, every tank's drain was run into the sump. Two medium air stones were then run into each tank using plastic hose. Biofilter, two medium aeration stones, two large aeration stones, an ultraviolet filter, and the pump were all combined to create the sump, a fourth 90-gallon tank in the system. Then the inflow to each tank was set up using PVC piping and plastic hose, creating a circuit of filtered water through every tank. The physics of water equalization is used in the circuit design: the pump gives water flow into the tank and as the water level rises above that of the drain, gravity drains the overflow into the sump. The system was filled with salt water to 27 ppt, two heaters were placed in the sump with a temperature regulator set at 23°C, and the dead Biofilter was replaced with living Biofilter. Finally, the pump and air were turned on.
Next, a grid floor was propped up on PVC pipes in the tank so that the depth in the cells was about 5 cm. On top of the grid floor, a mesh floor was placed so that the crabs couldn't escape through the bottom, but air could still flow through. Then the assembled cages were placed on top of the mesh floor, and grid ceilings were placed over each block of cages. To seal the cages, extra four-inch PVC pipes and sand-filled 16-ounce plastic bottles were placed on top of the ceiling.
To set up the C6 (meaning "crab stage 6" or "6 molts into crab form since reaching megalopa form") juvenile crabs in the cells, carapace widths and wet weights were measured, the genders noted, and the crabs individually and randomly assigned to and placed in cells.
Table 2: Cell Depth Setup Parameters
First, the cells and cell stands, mesh bottoms, and grid floors were cut to the parameters described in Table 2 (see above) using pipe cutters, a utility knife, and scissors, respectively. The mesh bottoms were then hot-glued onto the bottoms of every cell.
After this, standpipes were installed in each tank to insure uniform water levels throughout the tanks. The tank water level equalizes at the mouth of the standpipe and stays there constantly, eliminating the water level variability experienced in the cell area experiment. In addition, the tanks themselves were leveled and placed at the same height for the benefit of the standpipes' operation and to further insure uniform water depth throughout and among the tanks. Once the water levels were finalized, the stands were cut so that the floor depth was 12 cm, the largest depth tested.
Next, the 90-gallon tanks were cleaned and prepared using the same method as in the area experiment, but the temperature was increased to 26°C to induce molting. Within each tank, depths were randomly assigned to each position and each tank reserved 11 positions for one depth, 11 positions for another depth, and 12 positions for the third depth, shown in Figure 2. For the deep cells, the cells were simply placed directly onto the overall grid. As shown in Figure 3, for the medium-depth cells, 5-cm-high stands and then 1-cm grid floors were placed beneath the actual cells. And for the shallow cells, 8-cm-high stands and then 1-cm grid floors were placed beneath the actual cells.
To set up the C4 juvenile crabs in the cells, carapace widths were measured with the aid of a microscope, wet weights were measured on a scale, and the crabs were individually and randomly assigned to and placed in cells. The different depths accomplished a weighted data collection for the smaller depths because the 11-cm depth tested for a significant effect in general and the closer 3-cm and 6-cm depths gave a more precise view of the significance.
The duration of this project was two to three months based upon the collected data and crab health. Trial 1 lasted from November 14, 2004, to December 9, 2004; Trial 2 lasted from December 20, 2004, to March 22, 2005. Crabs were alternately fed diced squid or Zeigler 8mm shrimp pellets ad-libitum daily. Each day, if any crabs molted the previous day, the new wet weight and carapace widths were measured. New molts that day were noted, and any dirty cells were cleaned.
At the end of the experiment, when all data was collected, the final carapace width and wet weight of every crab was measured. Then the crabs were donated to another scientist for use in a separate study.
This project began on August 22, 2005, and ended on October 17, 2005. The same procedure as that of the area experiment was used. Every day, crabs were fed Zeigler 2mm shrimp pellets ad-libitum, and one tank was measured in case any molts were missed
Cell Area The data was analyzed for a significant effect of cell area upon juvenile blue crab growth with the Analysis of Covariance (ANCOVA) procedure. The Shapiro-Wilk test was used to determine if the data was properly distributed for valid analysis. When it was found that wet weight, carapace width, and molt frequency were not usable, the logarithm of each was taken to normalize the data. Molt increment, or percent increase in carapace width, was normal, so the raw percents were analyzed. The logarithm of wet weight, the logarithm of molt frequency, and the molt increment were analyzed with the initial wet weight as the covariant while the logarithm of carapace width was analyzed with the initial carapace width as the covariant.
Cell Size This data was analyzed for a significant effect of cell depth upon juvenile blue crab growth with the Mixed Linear Model procedure. Specifically, the carapace widths and wet weights for each molt, molt increment after each molt, molt frequency, and total molts were analyzed for significant differences. The initial carapace width was used as the covariant for all data involving carapace width and initial wet weight was used as the covariant for the remaining data. For post hoc analysis, Tukey's LSD Post Hoc One-Way ANOVA was used.
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) 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) and the Trial 2 molt numbers (Figure 7), which inversely correlate with the level of ammonia in the respective systems (Figure 4 and Figure 5, respectively). 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
Cell Area, Trial 2
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
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.
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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Thanks to Dr. Yonathan Zohar, John Stubblefield, Prof. Allen Place, Odi Zmora, Andrea Findiesen, Bridgette Bystry, Sarah Grap, Eric Evans, Kimberly Gaeta, and Keri O'Neil for instruction, assistance, and the use of their labs and equipment. Also, thanks to University of Maryland in general for use of their building and all other materials. Thanks to Dr. Barney Wilson and the Baltimore Polytechnic Institute for the opportunity to conduct the research. Thanks to Mrs. Carol Costa, Mrs. Dolores Costello, Ms. Charlotte Saylor, and to all of the Ingenuity Project at Baltimore Polytechnic Institute for the opportunity to conduct the research and for their priceless guidance along the way. Finally, thanks to Gary and Lucia Tibbels for transportation and support.
More About This Resource...
This winning entry in the Museum's Young Naturalist Awards 2006 is from a Maryland 12th grader. Justin examined the effect of cell area and depth on the growth of juvenile blue crabs. His essay includes:
- an introduction to the blue crab, Callinectes sapidus Rathbun, and the recent decline in its population;
- details of the materials and methods he used to study the impact of cell size on the growth of juvenile blue crabs; and
- the results of his experiment, which indicate that cell size does not seem to have a significant effect on the growth of juvenile blue crabs.
Supplement a study of biology with an activity drawn from this winning student essay.
- Send students to this online article, or print copies of the essay for them to read.
- Working alone or in small groups, have students research and report on blue crab management and restoration efforts in the Chesapeake Bay. What methods are proving most beneficial?
OriginYoung Naturalist Awards