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Effects of chelae immobilization on growth and survivorship for individually and communally raised lobsters, Homarus americanus
Aquaculture, 29 (1982) 359-312 Elsevier Scientific Publishing Company,
359 Amsterdam -
Printed in The Netherlands
EFFECTS OF CHELAE IMMOBILIZATION ON GROWTH AND SURVIVORSHIP FOR INDIVIDUALLY AND COMMUNALLY RAISED LOBSTERS, HOMARUS AMERICANUS
RICHARD
A. KENDALL*,
JON C. VAN OLST and JAMES
M. CARLBERG
*Center for Marine Studies, San Diego State University, San Diego, CA 92182 (U.S.A.) Aquaculture Systems International 11211 Sorrento Valley Road, San Diego, CA 92121 (U.S.A.) (Accepted
21 November
1981)
ABSTRACT Kendall, R.A., Van Olst, J.C. and Carlberg, J.M., 1987. Effects of chelae immobilization on growth and survivorship for individually and communally raised lobsters, Homarus americanus. Aquaculture, 29: 359-372. Communal rearing systems for juvenile American lobsters, Homarus americanus, allow lower production costs than are possible in individual rearing systems, but often result in high levels of mortality due to behavioral interactions such as cannibalism. This study evaluated the effects on growth and survivorship of two methods of bilateral immobilization of chelae (chelae and dactylopodite removal) for stage IV juvenile lobsters treated initially and at monthly intervals for 6 months. Replicate groups of lobsters were cultured in communal tanks and individual holding containers. After 6 months in individual containers, mortality was highest in the lobsters in which the chelae were completely removed. Lobsters cultured individually and subjected to dactylopodite removal exhibited less mortality. However, this pattern was reversed in the mass rearing system. Cannibalism appeared to be the major source of mortality for lobsters in the mass rearing control group, whereas mortality in lobsters from which the chelae had been removed was due primarily to treatment stress and non-community based factors. Growth was not affected by immobilization of the chelae. Within the mass rearing system, growth rates for the lobsters from the two treatment groups was suppressed after the fourth month when their size and increased survivorship produced a situation where the effective bottom area per animal was smaller than that required for unlimited growth. This reduction in mortality with insignificant effect on growth required relatively little investment in man-hours; chelae removal required less than 2 min of effort for each surviving lobster. By using the chelae removal technique on communally raised juvenile lobsters, major equipment and maintenance costs inherent to individual rearing systems probably could be avoided with little or no impairment of growth.
INTRODUCTION
In recent years, researchers come the problems associated 0044~8486/82/0000-0000/$02.75
have developed with culturing o 1982
several techniques to overthe American lobster, Homarus
Elsevier Scientific
Publishing Company
360 americanus. One of the major problems remaining is the high rate of mortality that results from cannibalism when lobsters are reared communally (McLeese, 1973; Krekorian et al., 1974; Aiken and Waddy, 1977, 1978; Carlberg and Van Olst, 1977). To eliminate this problem, several systems have been developed to grow lobsters in individual containers (Lang, 1975; Van Olst et al., 1977; Aiken and Waddy, 1978). Although lobsters held individually are free from community interactions such as cannibalism, there are added expenses associated with labor, space, equipment and other production costs (Van Olst et al., 1977). In contrast, communal rearing systems have lower initial costs and require less labor for maintenance (Carlberg et al., 1979). However, lobster populations in communal rearing systems often consist of a few large dominant individuals and a relatively large number of smaller ones (Cobb and Tamm, 1974,1975; Zeitlin-Hale and Sastry, 1978). Cul ture systems employing both communal and individual rearing systems have been evaluated using computer modelsto optimize the costs of lobster production (Botsford et al., 1977; Botsford, 1977). Botsford’s model used the assumption that costs for a communal rearing system were 60% of those for an individual rearing system. Based on this assumption, for a communal rearing system to be economically superior, survivorship of lobsters to a size of 20 mm carapace length had to be only 50% of the survivorship of lobsters in a comparable individual rearing system. In communal systems the degree of antagonistic interactions can be reduced significantly and hence survivorship increased by such methods as culling dominant animals, introducing shelter, providing substrates for subordinate animals (Aiken and Waddy, 1977; Zeitlin-Hale and Sastry, 1978; Carlberg et al., 1979), using biological extracts such as ecdysterone, and surgical techniques such as eyestalk ablation (Mauviot and Castell, 1976; McCarthy, 1978; Trider et al., 1979) or using chelae immobilization techniques (Aiken and Young-Lai, 1979, 1981). Few studies have evaluated chelae immobilization and its effect on survivorship and growth of juvenile lobsters cultured in individual and communal rearing systems. For adult lobsters, the use of rubber bands and wooden or plastic pegs has been effective in immobilizing the chelae while lobsters are being held for market. Aiken and Waddy (1978) used bands to increase survivorship in older juvenile lobsters cultured communally, but this method becomes inpractical when working with the young juvenile stages. Chelae immobilization could have a variety of effects depending on the severity of the technique used, the time and energy required for the lobster to negate the effects of the treatment and the degree to which the treatment reduces antagonistic interactions among communally reared juvenile lobsters. Any method of chelae immobilization causes some physiological stress to the individual lobsters (Scarratt, 1973). This stress, depending on the severity of the treatment, could result in increased levels of mortality. In the case of dactylopodite or chelae removal techniques, the open wounds
may make the lobsters more susceptible to disease (Schapiro et al., 1974; Fisher et al., 1978). If’ the absence of functional chelae can reduce or eliminate undesirable behavioral interactions, such as cannibalism, and that reduction is not negated by the increased mortality due to the severity of the treatment or the cost of replacing the chelae, then chelae immobilization would result in a higher level of survivorship in communally reared lobsters. In this study, immobilization techniques developed by Aiken and YoungLai (1979, 1981), dactylopodite and chelae removal, were evaluated in the second phase of the three-part culture process proposed by Van Olst et al. (1975a, 1977). The three-phase system consists of: (1) communal rearing of larvae, as described by Hughes et al. (1974); (2) communal rearing of juvenile lobsters for a period of approximately 6 months; and (3) rearing to market size in an appropriate individual rearing system. Additionally, these techniques were applied to individually reared juvenile lobsters to examine the treatment’s effects on growth and mortality without the added complications of group interactions. MATERIALS
AND METHODS
The effects of chelae immobilization on growth and survival were assessed for groups of Homarus americanus held in both individual and mass rearing systems for 6 months. This study was conducted in San Diego State University’s Aquaculture Laboratory, located at the Scripps Institute of Oceanography, La Jolla, CA. Larval lobsters were reared communally to stage IV in a system described by Hu.ghes et al. (1974) and were randomly divided into three sets of two treatment conditions and a control for communal and individual culture. The three communal rearing systems were replicated for a total of nine experimental groups. A total of 240 juvenile lobsters were cultured in the communal rearing tanks, while 30 were cultured in each of the individual holding groups for a total of 1530 juvenile lobsters. In the first treatment both chelae of each lobster were removed. The claws were pulled with sufficient force that the animal autotomized its claws near the coxa. This natural process of autotomy has been described by Barnes (1974). Removal of the claws was performed initially and when the chelae had functionally regenerated as determined at subsequent surveys. These surveys were performed at about 30-day intervals for 6 months. In the second treatment, the dactylopodites (the articulated elements of the chelae) were removed by severing with a scalpel. This procedure was performed initially and at approximately 30-day intervals when the dactylopodites had regenerated. The control groups were maintained under the same conditions and surveyed as the treatment groups, except that the chelae were left intact. Survival of communally reared lobsters was determined by recording monthly the total numbers of individuals present in each group. Growth
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was estimated monthly from the increase in the average carapace lengths of 20 animals selected randomly from each of the groups. At the conclusion of the experiment all of the surviving animals in the two replicates of the three communal rearing groups were measured and total wet weights of all lobsters combined in each group were obtained. The control and treatment groups under communal conditions were held in circular fiberglass tanks with a diameter of 115.2 cm and a depth of 34 cm. Seawater at ambient temperature (16.0 + 4.0%) was supplied to each tank at an approximate flow rate of 1 l/min. An air stone was used in each tank to assure normal levels of dissolved oxygen and to increase water circulation. Five sections of a plastic matrix used commercially as biological trickle filtration medium measuring 80 X 10 X 30 cm (Carlberg et al., 1979) were used in each tank to provide refuges for juvenile lobsters. The six communal tanks were stocked at an initial density of 240 lobsters/m2, and each of the groups was fed ad libitum with frozen brine shrimp (Artemia salina) on a daily basis. The antibiotic neomycin was added to each tank after each monthly survey at a concentration of 0.05 mg/l to reduce potential bacterial disease problems. The 30 additional animals of each treatment and control group were held in individual rearing systems described in detail by Carlberg and Van Olst (1977). These individual holding systems allowed the effects of each treatment to be evaluated in the absence of complicating factors of communal interaction. Survival and growth were determined in the same manner as for the lobsters reared communally except that each individual was measured at each monthly survey. In addition, during the final 3 months of the experiment, daily molt data for these lobsters were collected to monitor two components of growth; the molt frequency and incremental gain/molt. The total numbers of man-hours required to remove the chelae or dactylopodites and to conduct the surveys were determined for each culture group. This was done as a basis for evaluating the time and costs that would be involved in these operations if they were used in a commercial lobster farm. RESULTS
Growth To analyze the growth data from the three paired experimental groups; held in the mass rearing system, the mean carapace lengths for the seven ,dates for these groups were converted into Walford lines (Ricker, 1958). Walford lines were developed to linearize growth data where growth rates decrease with time. The resulting data were subjected to analysis of covariance and Newman-Keuls multiple range tests (Zar, 1974) for differences among their slopes and intercepts, in order to determine whether there were significant differences in growth within the replicate groups of the three ex-
363
perimental conditions, These data met the requirements for normality and homoscedasticity for parametric analysis. Also, analysis of covariance indicated no significant differences between the replicates of the three experimental groups within the mass rearing system. Therefore, the data were pooled to give a larger statistical base. The resulting growth data along with the analogous data from the individual rearing system are plotted in Fig. 1. The Walford lines for these growth data are plotted in Fig. 2. Individuals in all the experimental groups in both the mass rearing and individual rearing systems were essentially the same size at the start of the experiment, approximately 4.6 mm carapace length. At the end of 6 months their sizes ranged from 15.5 mm carapace length for the individually reared lobsters from which the dactylopodites had been removed to 11.1 mm carapace length for the mass reared lobsters from which the chelae were removed. The analysis of covariance for the Walford lines associated with the pooled growth data indicated a significant difference among the six slopes. Analysis of variance and Newman-Keuls multiple range test for the final mean carapace lengths for these six groups indicated that the final sizes for lobsters raised in the mass rearing system control group and lobsters raised in the three individual rearing treatments were not significantly different (P > 0.05). However, the mean carapace length for lobsters from the dactylopodite removal group in the mass rearing system was significantly smaller and that for the mass reared lobsters from which the chelae were removed was significantly smaller still (P > 0.05). In experiments by Aiken and Young-Lai (1979, 1981), juvenile lobsters cultured under less crowded conditions showed no significant differences in growth as measured by increase in carapace length.
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Fig. 1. Mean carapace length for juvenile lobsters raised in the control treatment (e), dactylopodite removal treatment (m), and chelae removal treatment (A) in the mass rearing (closed) and individual rearing (open) systems. Fig. 2. Walford lines (mean carapace length at t + 1 versus mean carapace length at t) for juvenile lobsters raised in the control treatment (o), dactylopodite removal treatment (m), and chelae removal treatment (A) in the mass rearing (closed) and individual rearing (open) systems.
364
To examine variation in size within the treatment groups, the distributions of carapace lengths within each of the six experimental treatments for each survey date, ‘as measured by standard error values, were subjected to Bartlett’s test (Zar, 1974). No significant differences were found (P > 0.05), but the trends follow those indicated by Carlberg and Van Olst (1977). The standard error for lobsters raised in the mass rearing system was larger than the standard error for those in the individual rearing system. Also, the standard error for the animals in both treatments is smaller than for the animals in the corresponding control groups. In order to determine whether chelae immobilization had any effect on either the incremental gain/molt, molt frequency, or time from immobilization to subsequent molt, the daily molt data collected for individually reared lobsters were analyzed using single factor analysis of variance. In all cases, no differences were found between lobsters raised in the control group and lobsters raised in either of the two treatment groups (P > 0.05). Biomass measurements from the final survey indicated significant changes in the mean wet weight per lobster for the animals in the mass rearing system among the three experimental treatments. The average weight for individuals in the control group was 2.25 g, while the average weight of individuals in the groups from which the dactylopodites and chelae were removed were 1.47 g and 1.03 g respectively. This pattern was not found in the weight measurements for lobsters raised in the individual rearing system. For those animals, the average weight for the control, dactylopodite removal and chelae removal groups were 2.30, 2.32, and 2.20 g, respectively.
Survivorship The survivorship data for each of the six experimental groups from the mass rearing system were also analyzed using analysis of covariance. By assuming that lobster survivorship follows the expression Nt = NoewZf where: Nt is the survivorship at time t, No is the initial stocking density, t is the time in days, and z is a mortality factor; simple linear regression lines can be computed from En Nt versus t where the slopes of these lines represent the mortality factors (2). These regressions were subjected to analysis of covariance for differences among slopes. That analysis showed no differences in survivorship between replicate groups for lobsters raised in the mass rearing system (P > 0.05), and therefore these data were pooled. The resulting combined survivorship data expressed as percent survivorship for the mass rearing system are plotted in Fig. 3. The data in Fig. 3 illustrate the problems of individual and mass rearing system approaches to culturing juvenile H. americanus. All three of the individual rearing treatments produced higher survivorship than the treatments for the mass rearing system. Within the individual rearing system, the individuals in the control group had the highest survival (90%). The lobsters from which the chelae were removed had the lowest survival (67%), and the
365
MO
MI
M2
Survey
M3
Time,
M4
M5
M6
month
Fig. 3. Survivorship of juvenile lobsters raised in the control treatment (o), dactylopodite removal treatment (=), and chelae removal treatment (A) in the mass rearing (closed) and individual rearing (open) systems.
ones from which the dactylopodites were removed were intermediate (83%). The pattern of survival in the mass rearing system was opposite to that seen in the individual rearing system. The groups from which the chelae had been removed had the highest survival (62%) and the control groups had the lowest survival with 17%. Those from which the dactylopodites were removed showed intermediate survivorship (41%). Aiken and Young-Lai (1979, 1981) also found that the two treatments enhanced survivorship over the control, but the lobsters from which the dactylopodites were removed showed the best survivorship in both studies. The combined values for survivorship were linearized as above and tested for differences among slopes using analysis of covariance. Analysis of slopes indicated a significant difference in survivorship and the Newman-Keuls multiple range test revealed several patterns (P > 0.05). Within the individual rearing system, no differences were found among the three experimental groups. Within the mass rearing system, survivorship of lobsters raised in the control treatment was significantly less than that of lobsters in the two immobilization groups. Survivorship was not significantly different between the lobsters in these two treatment groups. Survivorship of lobsters without chelae cultured in the two types of rearing systems was not significantly different. In contrast, both the controls and those without dactylopodites had significantly lower survivorship in the mass rearing systems than in the individual rearing systems. Mortality observed for animals in the three treatment conditions in the individual rearing system was the result of such factors as handling, survey measurements, and in the cases of the two treatments, the removal of chelae or dactylopodites (z = Mt). For the lobsters raised in the three experimental groups in the mass rearing system, mortality was the sum of the mortality due to handling, treatment, etc., plus the mortality due to community behavioral interactions (z = Mt + Mb). Assuming that the mortality due to treatment, handling, etc., was similar in lobsters raised in the analogous treatment types between rearing systems, a measure of the com-
366
munity determined mortality factors (Mb) can be made by subtracting the mortality factors for the lobsters raised in the individual rearing experimental groups from the mortality factors for the lobsters raised in the mass rearing experimental groups (Mb = 2 - Aft). The resulting mortality functions for behavioral interactions (% mortality = 100 - N,,e - Mb t) are plotted in Fig. 4. By the final survey, lobsters in the chelae removal group had the lowest community determined mortality, with a value of only 7%. Those in the control groups had the high.est mortality (83%), and those raised in the dactylopodite removal group were intermediate with 49%.
100' MO
MI
M3
M2
Survey
M4
M5
M6
Time, month
Fig. 4. Mortality due to community behavioral interactions among juvenile lobsters in the control treatment (o), dactylopodite removal treatment (m), and chelae removal treatment (A) in the mass rearing system.
Time requirements To evaluate the time and costs for conducting the two treatment procedures in a commercial lobster farming operation, the man-hours expended for maintaining the mass rearing experiments were computed. The time involved for the treatments for the first six surveys is shown in Fig. 5. The time for treatment included time devoted to tank maintenance, measurement of the 20 animals selected at random, as well as the treatment time for the removal of the chelae or dactylopodites. For the control group, treatment time was relatively constant throughout the course of the experiment, indicating a constant time required in each treatment for maintenance. When this maintenance time was subtracted from the time for each of the other two treatments, the treatment time required for these procedures was obtained. This amounted to 1136 mm for the lobsters from which the dactylopodites were removed and 514 min for those from which the chelae were removed. Expressed as total time required for each surviving lobster, this amounts to 5.71 and 1.74 mm/lobster, respectively. The amount of treatment time required for each surviving animal at each survey date is plotted in Fig. 6. More treatment time was required for removal of dactyl-
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t
MO k
MI
Survey
M2
M3
M4
_
I.Or
M5
Time, month
Fig. 5. Time required (mm/survey) for cleaning, measurement, and treatment of juvenile lobsters in the control treatment (e), dactylopodite removal treatment (m), and chelae removal treatment (A) in the mass rearing system. Fig. 6. Time required (min/animal) for treatment for the juvenile lobsters in the dactylopod ite removal treatment (m) and the chelae removal treatment (A) in the mass rearing system.
opodites in the earlier months of the experiment when the lobsters were very small. The time required for this group was consistently higher than for lobsters from which the chelae were removed, but this difference was reduced in the later months. DISCUSSION
The results of these experiments showed that survivorship of juvenile lobsters raised in mass rearing systems was significantly increased by immobilizing the juvenile lobster’s chelae. This increase in survivorship was accompanied by only a slight decrease in growth, and that only in the later months.
Survivorsh ip Removal of the chelae is a severe treatment for lobsters and, as expected, causes mortalities in groups of animals held in individual systems. Removal of dactylopodites caused fewer mortalities and the untreated groups exhibited the highest survivorship. This pattern was reversed in the mass rearing systems, such that the highest survivorship was seen in the animals raised in the most severe treatment. Survivorship for the lobsters receiving either form of treatment was significantly greater than that for lobsters in which the chelae were not immobilized. Thus, the effect of these means of chelae immobilization was positive or negative depending on the type of rearing system used. In the mass rearing system, the increased mortality due to the severity of the treatments was more than offset by a decrease in adverse community behavioral interactions, such as cannibalism, and resulted in significantly higher survivorship.
368
The trends in the factors for mortality due to community interactions (Mb) within the mass rearing system parallel the severity of the treatment. For untreated lobsters raised communally, the mortality factor for behavioral interactions (Mb) was more than 10 times greater than the factor due to treatment (Mt). This reflected the fact that there was very little mortality among untreated animals in the individual rearing system and that mortality seen in the untreated lobsters in the mass rearing system was due to community behavioral interactions, such as cannibalism. For treated lobsters, the mortality factor associated with behavioral interactions (Mb) was much smaller than that associated with the treatment (Mt), indicating that the mortality was due primarily to the treatments themselves. Mortality due to community behavioral interactions for lobsters without chelae raised in mass rearing systems was reduced to near zero, and mortality for lobsters without dactylopodites was intermediate. The mechanism of this response may involve a reduction in the ability to perform dominance displays as characterized by Krekorian et al. (1974), or a reduction in the ability to attack successfully and kill a conspecific. Since the response appears to be linked to the severity of the immobilization treatment used, in order to achieve higher survivorship in mass reared lobsters, it may be necessary to reduce the treatment, for example, by increasing the time between immobilization. Growth The experimental treatments appeared to have no effect on the growth rates of juvenile lobsters. Neither of the treatments caused a shift in molt frequency, incremental gain per molt, final mean weight per lobster, or led to a synchronization of molting within an experimental group. In addition, the growth rates for animals cultured in the individual rearing groups were not significantly different from the growth rates for animals in the mass rearing control treatment. Apparently, the increased intermolt periods for animals held in communal systems observed by Cobb and Tamm (1974, 1975) did not occur, or had no effect on the overall growth of the lobsters. The growth rates measured in this study for untreated animals raised in mass rearing systems are slightly less than those presented by Carlberg et al. (1979). The data presented by Carlberg et al. (1979) represent growth for juvenile lobsters reared under optimum temperature conditions (20”(Z), or fed live, instead of frozen, brine shrimp (Artemia sulina). Both of these conditions were shown to improve growth rates (Ford et al., 1975; Carlberg and Van Olst, 1977), but neither of these conditions was included in the experiments presented here. These differences may account for the slightly depressed growth rates. Analysis of the growth rates for the three mass rearing treatments showed no significant differences, but the animals subjected to both treatment conditions were significantly smaller than the others at the final sur-
369
vey. At that time, the animals from which the chelae had been removed were significantly smaller than those from which only the dactylopodites had been removed. Overcrowding, rather than inhibition due to the presence of other lobsters as reported by Cobb and Tamm (1974, 1975), probably was the cause of this depression in growth in the final month. Growth of lobsters raised in individual and mass rearing systems also can be restricted if certain minimum space requirements are not met. Van Olst et al. (1975b) reported that the bottom area of the holding compartment is the determining factor for maintaining rapid growth in individually raised lobsters. The area required is a function of carapace length. For essentially unrestricted growth, the bottom area of the container must be at least 72 times the square of the carapace length. McLeese (1973) indicated that holding density in communal tanks can adversely affect growth, and work done by Aiken and Waddy (1978) showed that space restrictions imposed by conspecifics in communal systems act much the same as space limitation imposed by physical barriers. The lobsters in all of the individual rearing groups and the lobsters from the mass rearing untreated group had indistinguishable growth rates. These four groups never exceeded the space requirements for unrestricted growth as outlined above. However, this was not the case for either of the treated groups raised in mass rearing systems. The treated lobsters had such high survivorship that by the end of the third month, they had exceeded the space requirements described above. Overcrowding in these two groups occurred between the end of the second month and the end of the third month, but the reduction in growth was not affected until after the fourth month as indicated by growth curves. This may have been due to the presence of the biological trickle filtration media which effectively increased the space available to each lobster and hence the area required for unrestricted growth may not have been exceeded until after the fourth month. If the mortality observed for the animals held in these mass rearing treatments is relatively independent of density, then the depressed growth for the lobsters in chelae immobilization treatments may be eliminated by providing adequate space for unlimited growth. In future mass rearing experiments the initial stocking density should be lowered from the high levels used in this study until an acceptable growth rate is achieved. With the appropriate initial stocking density to optimize growth and an appropriate time between treatments to allow maximum survivorship, chelae immobiliza. tion may be a useful technique in maximizing the effectiveness of the communal rearing process.
Economics For communal rearing to be economically advantageous, Botsford’s simulation model (1977) required that survivorship in a communal system to a size of 20 mm carapace length must be at least one-half of the survivorship produced in an individual rearing system. Both the chelae removal and the
dactylopodite removal techniques fulfilled this requirement. If the added cost of the chelae immobilization treatment does not alter the predictions of the model significantly, then communal rearing systems employing chelae immobilization techniques would be preferable from an economic standpoint. The model incorporates estimates of such factors as labor, heating equipment, warehouse space and other production costs. These costs have increased since 1977 to the present time and thus the total costs for individual rearing, which is labor and equipment intensive, probably have increased more rapidly than the costs of communal rearing. These increases make use of chelae immobilization techniques in mass rearing systems even more favorable. An evaluation of the time required for the two chelae immobilization treatments indicated that removal of the chelae would be the preferred technique for lobsters of this size. The dactylopodite removal technique required much more time/animal in the early months than did chelae removal. The total time involved in dactylopodite removal was < 6 min/survivor over the 6-month experiment, whereas the chelae removal treatment required < 2 min total for each survivor. Considering the time and equipment saved in using a communal rearing system over an individual rearing system, two additional minutes of labor appears to be a minor cost. With the appropriate stocking density, elevated temperature and optimal diet, the maximum growth rates for juvenile lobsters can be achieved. With chelae removal at appropriate intervals, the survivorship of these lobsters could make communal rearing the preferred approach for raising lobsters through the first 6 months of their juvenile development. ACKNOWLEDGEMENTS
The authors appreciate the help of Ms. Martha Pool, Marie Kolb and Erma Shaputis for the preparation of this manuscript, and Mrs. Patricia Miller for drawing the figures. This work is the result of research sponsored by NOAA, Office of Sea Grant, Department of Commerce, under Grant No. 04-8-MOl-189 R/A-21. The U.S. Government is authorized to produce and distribute reprints for governmental purposes notwithstanding any copyright notation that may appear hereon. Contribution No. 53 from the San Diego State University Center for Marine Studies.
REFERENCES Aiken, D.E. and Waddy, S.L., 1977. Communal rearing of juvenile lobsters (Homarus americanus) in a culture system. In: 3rd Meeting of the I.C.E.S. Working Group on Mariculture,May lo-13 1977, Brest, France. Actes Colloq. C.N.E.X.O., 4: 283.
371 Aiken, D.E. and Waddy, S.L., 1978. Space, density and growth of the lobster (Homarus americanus). Proc. World Marie. Sot., 9: 461-467. Aiken, D.E. and Young-Lai, W.W., 1979. Cheliped ablation and immobilization: Methods for improving survival and growth of juvenile American lobsters in communal culture tanks. Proc. World Marie. Sot., 10: 159-161. Aiken, D.E. and Young-Lai, W.W., 1981. Dactylotomy, chelotomy, and dactylostasis: Methods for enhancing survival and growth of small lobsters (Homarus americanus) in communal conditions. Aquaculture, 22: 45-52. Barnes, R.D., 1974. Invertebrate Zoology. 3rd ed. W.B. Saunders Company, Philadelphia, PA, 870 pp. Botsford, L.W., 1977. Current economic status of lobster culture research. Proc. World Marie. Sot., 8: 723-740. Botsford, L.W., Van Olst, J.C., Carlberg, J.M. and Gossard, T.W., 1977. The use of mathematical modeling and simulation to evaluate aquaculture as a beneficial use of thermal effluent. In: Proceedings of the 1977 summer computer simulation conference, 18-20 July 1977. Society for Computer Simulation, Chicago, IL, pp. 405-410 Carlberg, J.M. and Van Olst, J.C., 1977. Methods for culturing the American lobster (Homarus americanus). In: 3rd Meeting of the I.C.E.S. Working Group on Mariculture, 10-13 May 1977, Brest, France. Actes Colloq. C.N.E.X.O., 4: 261-275. Carlberg, J.M., Van Olst, J.C. and Ford, R.F., 1979. Potential for communal rearing of the nephropid lobsters (Homarus spp.). Proc. World Marie. Sot., 10: 840-853. Cobb, J.S. and Tamm, G.R., 1974. Social conditions increase intermolt period in juvenile lobsters. J. Fish Res. Board Can., 32: 1941--1943. Cobb, J.S. and Tamm, G.R., 1975. Dominance status and molt order in lobsters, Homarus americanus. Mar. Behav. Physiol., 3: 119-124. Fisher, W.S., Nilson, E.H., Steenbergen, J.F. and Lightner, D.F., 1978. Microbial diseases of cultured lobsters: a review. Aquaculture, 14: 115-140. Ford, R.F., Van Olst, J.C., Carlberg, J.M., Dorband, W.R. and Johnson, R.L., 1975. Beneficial use of thermal effluent in lobster culture. Proc. World Marie. SOC., 6: 509-517. Hughes, J.T., Shleser, R.A. and Tchobanoglous, G., 1974. A rearing tank for lobster larvae and other aquatic species. Prog. Fish-Cult., 36: 129-132. Krekorian, CO., Sommerville, D.C. and Ford, R.F., 1974. Laboratory study of behavioral interactions between the American lobster Homarus americanus and the California spiny lobster Panulirus interruptus, with comparative observations on the rock crab Cancer antenna&s. U.S. Natl. Mar. Fish Serv. Fish Bull., 72: 1146-1159. Lang, F., 1975. A simple culture system for juvenile lobsters. Aquaculture, 6: 389-393. McCarthy, J.F., 1978. The influence of hormonal factors in thermal compensation during the intermolt cycle of the American lobster Homarus americanus. Camp. Biochem. Physiol. A, 59: 295-298. McLeese, D.W., 1973. Chemical communication among lobsters (Homarus americanus). J. Fish Res. Board Can., 30: 775-778. Mauviot, J.C. and Castell, J.D., 1976. Molt enhancing and growth enhancing effects of bilateral eyestalk ablation on juvenile and adult American lobsters Homarus americanus. J. Fish Res. Board Can., 33: 1922-1929. Ricker, W.E., 1958. Handbook of computations for biological statistics of fish populations. J. Fish Res. Board Can., 119: l-300. Scarratt, D.J., 1973. Claw loss and other wounds in commercially caught lobsters Homarus americanus. J. Fish Res. Board Can., 30: 1370-1373. Schapiro, H.C., Mathewson, J.H., Steenbergen, J.F., Kellogg, S., Ingram, C., Nierengarten, G. and Rabin, H., 1974. Gaffkemia in the California spiny lobster Panulirus interruptus: infection and immunization. Aquaculture, 3: 403-408.
312 Trider, D.J., Mason, E.G. and Castell, J.D., 1979. Survival and growth of juvenile American lobsters (Homarus americanus) after eyestalk ablation. J. Fish Res. Board Can., 36: 93-107. Van Olst, J.C., Carlberg, J.M. and Ford, R.F., 1975a. Effects of substrate type and other factors on the growth, survival, and cannibalism of juvenile Homarus americanus in mass rearing systems. Proc. World. Marie. Sot., 6: 261-274. Van Olst, J.C., Carlberg, J.M. and Ford, R.F., 1977. A description of intensive culture systems for the American lobster (Homarus americanus) and other cannibalistic crustaceans. Proc. World Marie. Sot., 8: 271-292. Van Olst, J.C., Carlberg, J.M. and Sommerville, D.C., 1975b. Use of thermal effluent in culturing the American lobster. San Diego State University annual report, Phase 2, 277 pp. Available from: Res. Develop. Prog., Southern Calif. Edison Co., Contract No. U2384902. Zar, J.H., 1974. Biostatistical Analysis. Prentice Hall, Englewood Cliffs, NJ, 620 pp. Zeitlin-Hale, L. and Sastry, A.N., 1978. Effects of environmental manipulation on the locomotor activity and agonistic behavior of cultured juvenile American lobsters, Homarus americanus. Mar. Biol. (Berl.), 47: 369-379.
359 Amsterdam -
Printed in The Netherlands
EFFECTS OF CHELAE IMMOBILIZATION ON GROWTH AND SURVIVORSHIP FOR INDIVIDUALLY AND COMMUNALLY RAISED LOBSTERS, HOMARUS AMERICANUS
RICHARD
A. KENDALL*,
JON C. VAN OLST and JAMES
M. CARLBERG
*Center for Marine Studies, San Diego State University, San Diego, CA 92182 (U.S.A.) Aquaculture Systems International 11211 Sorrento Valley Road, San Diego, CA 92121 (U.S.A.) (Accepted
21 November
1981)
ABSTRACT Kendall, R.A., Van Olst, J.C. and Carlberg, J.M., 1987. Effects of chelae immobilization on growth and survivorship for individually and communally raised lobsters, Homarus americanus. Aquaculture, 29: 359-372. Communal rearing systems for juvenile American lobsters, Homarus americanus, allow lower production costs than are possible in individual rearing systems, but often result in high levels of mortality due to behavioral interactions such as cannibalism. This study evaluated the effects on growth and survivorship of two methods of bilateral immobilization of chelae (chelae and dactylopodite removal) for stage IV juvenile lobsters treated initially and at monthly intervals for 6 months. Replicate groups of lobsters were cultured in communal tanks and individual holding containers. After 6 months in individual containers, mortality was highest in the lobsters in which the chelae were completely removed. Lobsters cultured individually and subjected to dactylopodite removal exhibited less mortality. However, this pattern was reversed in the mass rearing system. Cannibalism appeared to be the major source of mortality for lobsters in the mass rearing control group, whereas mortality in lobsters from which the chelae had been removed was due primarily to treatment stress and non-community based factors. Growth was not affected by immobilization of the chelae. Within the mass rearing system, growth rates for the lobsters from the two treatment groups was suppressed after the fourth month when their size and increased survivorship produced a situation where the effective bottom area per animal was smaller than that required for unlimited growth. This reduction in mortality with insignificant effect on growth required relatively little investment in man-hours; chelae removal required less than 2 min of effort for each surviving lobster. By using the chelae removal technique on communally raised juvenile lobsters, major equipment and maintenance costs inherent to individual rearing systems probably could be avoided with little or no impairment of growth.
INTRODUCTION
In recent years, researchers come the problems associated 0044~8486/82/0000-0000/$02.75
have developed with culturing o 1982
several techniques to overthe American lobster, Homarus
Elsevier Scientific
Publishing Company
360 americanus. One of the major problems remaining is the high rate of mortality that results from cannibalism when lobsters are reared communally (McLeese, 1973; Krekorian et al., 1974; Aiken and Waddy, 1977, 1978; Carlberg and Van Olst, 1977). To eliminate this problem, several systems have been developed to grow lobsters in individual containers (Lang, 1975; Van Olst et al., 1977; Aiken and Waddy, 1978). Although lobsters held individually are free from community interactions such as cannibalism, there are added expenses associated with labor, space, equipment and other production costs (Van Olst et al., 1977). In contrast, communal rearing systems have lower initial costs and require less labor for maintenance (Carlberg et al., 1979). However, lobster populations in communal rearing systems often consist of a few large dominant individuals and a relatively large number of smaller ones (Cobb and Tamm, 1974,1975; Zeitlin-Hale and Sastry, 1978). Cul ture systems employing both communal and individual rearing systems have been evaluated using computer modelsto optimize the costs of lobster production (Botsford et al., 1977; Botsford, 1977). Botsford’s model used the assumption that costs for a communal rearing system were 60% of those for an individual rearing system. Based on this assumption, for a communal rearing system to be economically superior, survivorship of lobsters to a size of 20 mm carapace length had to be only 50% of the survivorship of lobsters in a comparable individual rearing system. In communal systems the degree of antagonistic interactions can be reduced significantly and hence survivorship increased by such methods as culling dominant animals, introducing shelter, providing substrates for subordinate animals (Aiken and Waddy, 1977; Zeitlin-Hale and Sastry, 1978; Carlberg et al., 1979), using biological extracts such as ecdysterone, and surgical techniques such as eyestalk ablation (Mauviot and Castell, 1976; McCarthy, 1978; Trider et al., 1979) or using chelae immobilization techniques (Aiken and Young-Lai, 1979, 1981). Few studies have evaluated chelae immobilization and its effect on survivorship and growth of juvenile lobsters cultured in individual and communal rearing systems. For adult lobsters, the use of rubber bands and wooden or plastic pegs has been effective in immobilizing the chelae while lobsters are being held for market. Aiken and Waddy (1978) used bands to increase survivorship in older juvenile lobsters cultured communally, but this method becomes inpractical when working with the young juvenile stages. Chelae immobilization could have a variety of effects depending on the severity of the technique used, the time and energy required for the lobster to negate the effects of the treatment and the degree to which the treatment reduces antagonistic interactions among communally reared juvenile lobsters. Any method of chelae immobilization causes some physiological stress to the individual lobsters (Scarratt, 1973). This stress, depending on the severity of the treatment, could result in increased levels of mortality. In the case of dactylopodite or chelae removal techniques, the open wounds
may make the lobsters more susceptible to disease (Schapiro et al., 1974; Fisher et al., 1978). If’ the absence of functional chelae can reduce or eliminate undesirable behavioral interactions, such as cannibalism, and that reduction is not negated by the increased mortality due to the severity of the treatment or the cost of replacing the chelae, then chelae immobilization would result in a higher level of survivorship in communally reared lobsters. In this study, immobilization techniques developed by Aiken and YoungLai (1979, 1981), dactylopodite and chelae removal, were evaluated in the second phase of the three-part culture process proposed by Van Olst et al. (1975a, 1977). The three-phase system consists of: (1) communal rearing of larvae, as described by Hughes et al. (1974); (2) communal rearing of juvenile lobsters for a period of approximately 6 months; and (3) rearing to market size in an appropriate individual rearing system. Additionally, these techniques were applied to individually reared juvenile lobsters to examine the treatment’s effects on growth and mortality without the added complications of group interactions. MATERIALS
AND METHODS
The effects of chelae immobilization on growth and survival were assessed for groups of Homarus americanus held in both individual and mass rearing systems for 6 months. This study was conducted in San Diego State University’s Aquaculture Laboratory, located at the Scripps Institute of Oceanography, La Jolla, CA. Larval lobsters were reared communally to stage IV in a system described by Hu.ghes et al. (1974) and were randomly divided into three sets of two treatment conditions and a control for communal and individual culture. The three communal rearing systems were replicated for a total of nine experimental groups. A total of 240 juvenile lobsters were cultured in the communal rearing tanks, while 30 were cultured in each of the individual holding groups for a total of 1530 juvenile lobsters. In the first treatment both chelae of each lobster were removed. The claws were pulled with sufficient force that the animal autotomized its claws near the coxa. This natural process of autotomy has been described by Barnes (1974). Removal of the claws was performed initially and when the chelae had functionally regenerated as determined at subsequent surveys. These surveys were performed at about 30-day intervals for 6 months. In the second treatment, the dactylopodites (the articulated elements of the chelae) were removed by severing with a scalpel. This procedure was performed initially and at approximately 30-day intervals when the dactylopodites had regenerated. The control groups were maintained under the same conditions and surveyed as the treatment groups, except that the chelae were left intact. Survival of communally reared lobsters was determined by recording monthly the total numbers of individuals present in each group. Growth
362
was estimated monthly from the increase in the average carapace lengths of 20 animals selected randomly from each of the groups. At the conclusion of the experiment all of the surviving animals in the two replicates of the three communal rearing groups were measured and total wet weights of all lobsters combined in each group were obtained. The control and treatment groups under communal conditions were held in circular fiberglass tanks with a diameter of 115.2 cm and a depth of 34 cm. Seawater at ambient temperature (16.0 + 4.0%) was supplied to each tank at an approximate flow rate of 1 l/min. An air stone was used in each tank to assure normal levels of dissolved oxygen and to increase water circulation. Five sections of a plastic matrix used commercially as biological trickle filtration medium measuring 80 X 10 X 30 cm (Carlberg et al., 1979) were used in each tank to provide refuges for juvenile lobsters. The six communal tanks were stocked at an initial density of 240 lobsters/m2, and each of the groups was fed ad libitum with frozen brine shrimp (Artemia salina) on a daily basis. The antibiotic neomycin was added to each tank after each monthly survey at a concentration of 0.05 mg/l to reduce potential bacterial disease problems. The 30 additional animals of each treatment and control group were held in individual rearing systems described in detail by Carlberg and Van Olst (1977). These individual holding systems allowed the effects of each treatment to be evaluated in the absence of complicating factors of communal interaction. Survival and growth were determined in the same manner as for the lobsters reared communally except that each individual was measured at each monthly survey. In addition, during the final 3 months of the experiment, daily molt data for these lobsters were collected to monitor two components of growth; the molt frequency and incremental gain/molt. The total numbers of man-hours required to remove the chelae or dactylopodites and to conduct the surveys were determined for each culture group. This was done as a basis for evaluating the time and costs that would be involved in these operations if they were used in a commercial lobster farm. RESULTS
Growth To analyze the growth data from the three paired experimental groups; held in the mass rearing system, the mean carapace lengths for the seven ,dates for these groups were converted into Walford lines (Ricker, 1958). Walford lines were developed to linearize growth data where growth rates decrease with time. The resulting data were subjected to analysis of covariance and Newman-Keuls multiple range tests (Zar, 1974) for differences among their slopes and intercepts, in order to determine whether there were significant differences in growth within the replicate groups of the three ex-
363
perimental conditions, These data met the requirements for normality and homoscedasticity for parametric analysis. Also, analysis of covariance indicated no significant differences between the replicates of the three experimental groups within the mass rearing system. Therefore, the data were pooled to give a larger statistical base. The resulting growth data along with the analogous data from the individual rearing system are plotted in Fig. 1. The Walford lines for these growth data are plotted in Fig. 2. Individuals in all the experimental groups in both the mass rearing and individual rearing systems were essentially the same size at the start of the experiment, approximately 4.6 mm carapace length. At the end of 6 months their sizes ranged from 15.5 mm carapace length for the individually reared lobsters from which the dactylopodites had been removed to 11.1 mm carapace length for the mass reared lobsters from which the chelae were removed. The analysis of covariance for the Walford lines associated with the pooled growth data indicated a significant difference among the six slopes. Analysis of variance and Newman-Keuls multiple range test for the final mean carapace lengths for these six groups indicated that the final sizes for lobsters raised in the mass rearing system control group and lobsters raised in the three individual rearing treatments were not significantly different (P > 0.05). However, the mean carapace length for lobsters from the dactylopodite removal group in the mass rearing system was significantly smaller and that for the mass reared lobsters from which the chelae were removed was significantly smaller still (P > 0.05). In experiments by Aiken and Young-Lai (1979, 1981), juvenile lobsters cultured under less crowded conditions showed no significant differences in growth as measured by increase in carapace length.
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M3
Time,
M4
M5
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M6
4
Mean
6
Carapace
8
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I2
14
t, mm
Fig. 1. Mean carapace length for juvenile lobsters raised in the control treatment (e), dactylopodite removal treatment (m), and chelae removal treatment (A) in the mass rearing (closed) and individual rearing (open) systems. Fig. 2. Walford lines (mean carapace length at t + 1 versus mean carapace length at t) for juvenile lobsters raised in the control treatment (o), dactylopodite removal treatment (m), and chelae removal treatment (A) in the mass rearing (closed) and individual rearing (open) systems.
364
To examine variation in size within the treatment groups, the distributions of carapace lengths within each of the six experimental treatments for each survey date, ‘as measured by standard error values, were subjected to Bartlett’s test (Zar, 1974). No significant differences were found (P > 0.05), but the trends follow those indicated by Carlberg and Van Olst (1977). The standard error for lobsters raised in the mass rearing system was larger than the standard error for those in the individual rearing system. Also, the standard error for the animals in both treatments is smaller than for the animals in the corresponding control groups. In order to determine whether chelae immobilization had any effect on either the incremental gain/molt, molt frequency, or time from immobilization to subsequent molt, the daily molt data collected for individually reared lobsters were analyzed using single factor analysis of variance. In all cases, no differences were found between lobsters raised in the control group and lobsters raised in either of the two treatment groups (P > 0.05). Biomass measurements from the final survey indicated significant changes in the mean wet weight per lobster for the animals in the mass rearing system among the three experimental treatments. The average weight for individuals in the control group was 2.25 g, while the average weight of individuals in the groups from which the dactylopodites and chelae were removed were 1.47 g and 1.03 g respectively. This pattern was not found in the weight measurements for lobsters raised in the individual rearing system. For those animals, the average weight for the control, dactylopodite removal and chelae removal groups were 2.30, 2.32, and 2.20 g, respectively.
Survivorship The survivorship data for each of the six experimental groups from the mass rearing system were also analyzed using analysis of covariance. By assuming that lobster survivorship follows the expression Nt = NoewZf where: Nt is the survivorship at time t, No is the initial stocking density, t is the time in days, and z is a mortality factor; simple linear regression lines can be computed from En Nt versus t where the slopes of these lines represent the mortality factors (2). These regressions were subjected to analysis of covariance for differences among slopes. That analysis showed no differences in survivorship between replicate groups for lobsters raised in the mass rearing system (P > 0.05), and therefore these data were pooled. The resulting combined survivorship data expressed as percent survivorship for the mass rearing system are plotted in Fig. 3. The data in Fig. 3 illustrate the problems of individual and mass rearing system approaches to culturing juvenile H. americanus. All three of the individual rearing treatments produced higher survivorship than the treatments for the mass rearing system. Within the individual rearing system, the individuals in the control group had the highest survival (90%). The lobsters from which the chelae were removed had the lowest survival (67%), and the
365
MO
MI
M2
Survey
M3
Time,
M4
M5
M6
month
Fig. 3. Survivorship of juvenile lobsters raised in the control treatment (o), dactylopodite removal treatment (=), and chelae removal treatment (A) in the mass rearing (closed) and individual rearing (open) systems.
ones from which the dactylopodites were removed were intermediate (83%). The pattern of survival in the mass rearing system was opposite to that seen in the individual rearing system. The groups from which the chelae had been removed had the highest survival (62%) and the control groups had the lowest survival with 17%. Those from which the dactylopodites were removed showed intermediate survivorship (41%). Aiken and Young-Lai (1979, 1981) also found that the two treatments enhanced survivorship over the control, but the lobsters from which the dactylopodites were removed showed the best survivorship in both studies. The combined values for survivorship were linearized as above and tested for differences among slopes using analysis of covariance. Analysis of slopes indicated a significant difference in survivorship and the Newman-Keuls multiple range test revealed several patterns (P > 0.05). Within the individual rearing system, no differences were found among the three experimental groups. Within the mass rearing system, survivorship of lobsters raised in the control treatment was significantly less than that of lobsters in the two immobilization groups. Survivorship was not significantly different between the lobsters in these two treatment groups. Survivorship of lobsters without chelae cultured in the two types of rearing systems was not significantly different. In contrast, both the controls and those without dactylopodites had significantly lower survivorship in the mass rearing systems than in the individual rearing systems. Mortality observed for animals in the three treatment conditions in the individual rearing system was the result of such factors as handling, survey measurements, and in the cases of the two treatments, the removal of chelae or dactylopodites (z = Mt). For the lobsters raised in the three experimental groups in the mass rearing system, mortality was the sum of the mortality due to handling, treatment, etc., plus the mortality due to community behavioral interactions (z = Mt + Mb). Assuming that the mortality due to treatment, handling, etc., was similar in lobsters raised in the analogous treatment types between rearing systems, a measure of the com-
366
munity determined mortality factors (Mb) can be made by subtracting the mortality factors for the lobsters raised in the individual rearing experimental groups from the mortality factors for the lobsters raised in the mass rearing experimental groups (Mb = 2 - Aft). The resulting mortality functions for behavioral interactions (% mortality = 100 - N,,e - Mb t) are plotted in Fig. 4. By the final survey, lobsters in the chelae removal group had the lowest community determined mortality, with a value of only 7%. Those in the control groups had the high.est mortality (83%), and those raised in the dactylopodite removal group were intermediate with 49%.
100' MO
MI
M3
M2
Survey
M4
M5
M6
Time, month
Fig. 4. Mortality due to community behavioral interactions among juvenile lobsters in the control treatment (o), dactylopodite removal treatment (m), and chelae removal treatment (A) in the mass rearing system.
Time requirements To evaluate the time and costs for conducting the two treatment procedures in a commercial lobster farming operation, the man-hours expended for maintaining the mass rearing experiments were computed. The time involved for the treatments for the first six surveys is shown in Fig. 5. The time for treatment included time devoted to tank maintenance, measurement of the 20 animals selected at random, as well as the treatment time for the removal of the chelae or dactylopodites. For the control group, treatment time was relatively constant throughout the course of the experiment, indicating a constant time required in each treatment for maintenance. When this maintenance time was subtracted from the time for each of the other two treatments, the treatment time required for these procedures was obtained. This amounted to 1136 mm for the lobsters from which the dactylopodites were removed and 514 min for those from which the chelae were removed. Expressed as total time required for each surviving lobster, this amounts to 5.71 and 1.74 mm/lobster, respectively. The amount of treatment time required for each surviving animal at each survey date is plotted in Fig. 6. More treatment time was required for removal of dactyl-
367
t
MO k
MI
Survey
M2
M3
M4
_
I.Or
M5
Time, month
Fig. 5. Time required (mm/survey) for cleaning, measurement, and treatment of juvenile lobsters in the control treatment (e), dactylopodite removal treatment (m), and chelae removal treatment (A) in the mass rearing system. Fig. 6. Time required (min/animal) for treatment for the juvenile lobsters in the dactylopod ite removal treatment (m) and the chelae removal treatment (A) in the mass rearing system.
opodites in the earlier months of the experiment when the lobsters were very small. The time required for this group was consistently higher than for lobsters from which the chelae were removed, but this difference was reduced in the later months. DISCUSSION
The results of these experiments showed that survivorship of juvenile lobsters raised in mass rearing systems was significantly increased by immobilizing the juvenile lobster’s chelae. This increase in survivorship was accompanied by only a slight decrease in growth, and that only in the later months.
Survivorsh ip Removal of the chelae is a severe treatment for lobsters and, as expected, causes mortalities in groups of animals held in individual systems. Removal of dactylopodites caused fewer mortalities and the untreated groups exhibited the highest survivorship. This pattern was reversed in the mass rearing systems, such that the highest survivorship was seen in the animals raised in the most severe treatment. Survivorship for the lobsters receiving either form of treatment was significantly greater than that for lobsters in which the chelae were not immobilized. Thus, the effect of these means of chelae immobilization was positive or negative depending on the type of rearing system used. In the mass rearing system, the increased mortality due to the severity of the treatments was more than offset by a decrease in adverse community behavioral interactions, such as cannibalism, and resulted in significantly higher survivorship.
368
The trends in the factors for mortality due to community interactions (Mb) within the mass rearing system parallel the severity of the treatment. For untreated lobsters raised communally, the mortality factor for behavioral interactions (Mb) was more than 10 times greater than the factor due to treatment (Mt). This reflected the fact that there was very little mortality among untreated animals in the individual rearing system and that mortality seen in the untreated lobsters in the mass rearing system was due to community behavioral interactions, such as cannibalism. For treated lobsters, the mortality factor associated with behavioral interactions (Mb) was much smaller than that associated with the treatment (Mt), indicating that the mortality was due primarily to the treatments themselves. Mortality due to community behavioral interactions for lobsters without chelae raised in mass rearing systems was reduced to near zero, and mortality for lobsters without dactylopodites was intermediate. The mechanism of this response may involve a reduction in the ability to perform dominance displays as characterized by Krekorian et al. (1974), or a reduction in the ability to attack successfully and kill a conspecific. Since the response appears to be linked to the severity of the immobilization treatment used, in order to achieve higher survivorship in mass reared lobsters, it may be necessary to reduce the treatment, for example, by increasing the time between immobilization. Growth The experimental treatments appeared to have no effect on the growth rates of juvenile lobsters. Neither of the treatments caused a shift in molt frequency, incremental gain per molt, final mean weight per lobster, or led to a synchronization of molting within an experimental group. In addition, the growth rates for animals cultured in the individual rearing groups were not significantly different from the growth rates for animals in the mass rearing control treatment. Apparently, the increased intermolt periods for animals held in communal systems observed by Cobb and Tamm (1974, 1975) did not occur, or had no effect on the overall growth of the lobsters. The growth rates measured in this study for untreated animals raised in mass rearing systems are slightly less than those presented by Carlberg et al. (1979). The data presented by Carlberg et al. (1979) represent growth for juvenile lobsters reared under optimum temperature conditions (20”(Z), or fed live, instead of frozen, brine shrimp (Artemia sulina). Both of these conditions were shown to improve growth rates (Ford et al., 1975; Carlberg and Van Olst, 1977), but neither of these conditions was included in the experiments presented here. These differences may account for the slightly depressed growth rates. Analysis of the growth rates for the three mass rearing treatments showed no significant differences, but the animals subjected to both treatment conditions were significantly smaller than the others at the final sur-
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vey. At that time, the animals from which the chelae had been removed were significantly smaller than those from which only the dactylopodites had been removed. Overcrowding, rather than inhibition due to the presence of other lobsters as reported by Cobb and Tamm (1974, 1975), probably was the cause of this depression in growth in the final month. Growth of lobsters raised in individual and mass rearing systems also can be restricted if certain minimum space requirements are not met. Van Olst et al. (1975b) reported that the bottom area of the holding compartment is the determining factor for maintaining rapid growth in individually raised lobsters. The area required is a function of carapace length. For essentially unrestricted growth, the bottom area of the container must be at least 72 times the square of the carapace length. McLeese (1973) indicated that holding density in communal tanks can adversely affect growth, and work done by Aiken and Waddy (1978) showed that space restrictions imposed by conspecifics in communal systems act much the same as space limitation imposed by physical barriers. The lobsters in all of the individual rearing groups and the lobsters from the mass rearing untreated group had indistinguishable growth rates. These four groups never exceeded the space requirements for unrestricted growth as outlined above. However, this was not the case for either of the treated groups raised in mass rearing systems. The treated lobsters had such high survivorship that by the end of the third month, they had exceeded the space requirements described above. Overcrowding in these two groups occurred between the end of the second month and the end of the third month, but the reduction in growth was not affected until after the fourth month as indicated by growth curves. This may have been due to the presence of the biological trickle filtration media which effectively increased the space available to each lobster and hence the area required for unrestricted growth may not have been exceeded until after the fourth month. If the mortality observed for the animals held in these mass rearing treatments is relatively independent of density, then the depressed growth for the lobsters in chelae immobilization treatments may be eliminated by providing adequate space for unlimited growth. In future mass rearing experiments the initial stocking density should be lowered from the high levels used in this study until an acceptable growth rate is achieved. With the appropriate initial stocking density to optimize growth and an appropriate time between treatments to allow maximum survivorship, chelae immobiliza. tion may be a useful technique in maximizing the effectiveness of the communal rearing process.
Economics For communal rearing to be economically advantageous, Botsford’s simulation model (1977) required that survivorship in a communal system to a size of 20 mm carapace length must be at least one-half of the survivorship produced in an individual rearing system. Both the chelae removal and the
dactylopodite removal techniques fulfilled this requirement. If the added cost of the chelae immobilization treatment does not alter the predictions of the model significantly, then communal rearing systems employing chelae immobilization techniques would be preferable from an economic standpoint. The model incorporates estimates of such factors as labor, heating equipment, warehouse space and other production costs. These costs have increased since 1977 to the present time and thus the total costs for individual rearing, which is labor and equipment intensive, probably have increased more rapidly than the costs of communal rearing. These increases make use of chelae immobilization techniques in mass rearing systems even more favorable. An evaluation of the time required for the two chelae immobilization treatments indicated that removal of the chelae would be the preferred technique for lobsters of this size. The dactylopodite removal technique required much more time/animal in the early months than did chelae removal. The total time involved in dactylopodite removal was < 6 min/survivor over the 6-month experiment, whereas the chelae removal treatment required < 2 min total for each survivor. Considering the time and equipment saved in using a communal rearing system over an individual rearing system, two additional minutes of labor appears to be a minor cost. With the appropriate stocking density, elevated temperature and optimal diet, the maximum growth rates for juvenile lobsters can be achieved. With chelae removal at appropriate intervals, the survivorship of these lobsters could make communal rearing the preferred approach for raising lobsters through the first 6 months of their juvenile development. ACKNOWLEDGEMENTS
The authors appreciate the help of Ms. Martha Pool, Marie Kolb and Erma Shaputis for the preparation of this manuscript, and Mrs. Patricia Miller for drawing the figures. This work is the result of research sponsored by NOAA, Office of Sea Grant, Department of Commerce, under Grant No. 04-8-MOl-189 R/A-21. The U.S. Government is authorized to produce and distribute reprints for governmental purposes notwithstanding any copyright notation that may appear hereon. Contribution No. 53 from the San Diego State University Center for Marine Studies.
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