Experimental grow-out of the Omani abalone Haliotis mariae wood, 1828, in land-based tanks in Mirbat, oman.
KEY WORDS: abalone, Haliotis mariae, growth, land-based tanks
The Omani abalone Haliotis mariae, locally known as sufailah, is a marine gastropod mollusc in the family Haliotidae. The species is endemic off the Arabian Sea coast of southern Oman (Stirn & Al-Hashmi 1996). It inhabits inter- and subtidal rocky substrates down to a depth of 20 m (Bosch & Bosch 1982, Johnson et al. 1992). Abalone shelter among rocks and in crevices and are attached by a muscular foot to the substratum during daytime, moving slowly nocturnally to graze on marine algae (Al-Rashdi & Iwao 2008). In Omani waters, H. mariae are concentrated mainly along the 100-kin coastal strip of the Dhofar Governorate in the area between Mirbat and Hasik.
Abalone are caught by free-diving divers and are air-dried, after boiling in water, and sold. Abalone is one of the premium seafood delicacies in Oman and has the highest value per kilogram of all Omani fisheries products. In 2011, the price of abalone reached 65 OR/kg meat (US$170). The Omani abalone fishery has been active since the 1950s and reached a maximum annual production of 116 t fresh meat in 1990. In 1991, the Ministry of Agriculture and Fisheries Wealth introduced regulatory measures to reduce the abalone fishing season to 1 mo combined with the introduction of a minimum legal shell length size limit of 90 ram. Catches started to decline and ranged from 29-56 t/y (mean, 44 t/y) from 1991 through 2007. Despite protection efforts, the Omani abalone stock is dangerously overexploited because of intense fishing; illegal harvest by fishing out of the permitted season; the harvest of small, immature specimens, and a fishery practice of overturning boulders and thus destroying the abalone habitat (Al-Hafidh 2006). In addition to these anthropogenic impacts, there are other natural factors attributable to environmental changes, such as red tide and predator abundance, that affect the species negatively. To prevent critical reduction of the abalone stock, the fishery was banned from 2008 to 2010.
Because of the high value and demand of abalone products in local and international markets, and the decline of wild stocks, the Ministry of Agriculture and Fisheries in cooperation with the Japanese International Cooperation Agency in 1993 initiated a series of abalone aquaculture projects. An aquaculture station was constructed at Mirbat, located in the southern part of Oman in the Dhofar region (Fig. 1). The station has a land-based culture system with filtered seawater pumped from offshore. Initial results showed a potential to spawn Omani abalone artificially, and the first juveniles were produced successfully in 1996. The results of the research were presented in several internal and unpublished reports, and were presented poorly in available peer-reviewed literature (Al-Rashdi 2002, Benny et al. 2003, Al-Rashdi & Iwao 2008).
Several projects funded by the Directorate General of Fisheries Research have been implemented in Mirbat to improve culture technique and hatchery production, and to study the growth rate of Omani abalone under different culture conditions. We report the activities conducted for grow-out experiments with the Omani abalone Haliotis mariae at the Mirbat aquaculture facility from January 2001 to December 2011.
MATERIALS AND METHODS
Detailed descriptions of broodstock conditioning, induction of spawning and fertilization, larval settlement, and the maintenance of small juveniles of Haliotis mariae during 1999 and 2000 were described by Al-Rashdi and Iwao (2008). In the current study, the grow-out of juvenile and adult abalone continued in indoors rectangular 450-L fiberglass tanks for 5 y, from January 2001 to December 2005. A new experimental trial on abalone grow-out was conducted from December 2005 to November 2009. Spawning was initiated in December 2005, when the veligers were collected and stocked in settlement tanks provided with diatom plates and attachment substrates for nursery rearing. Early juveniles were placed in tanks and maintained in the same conditions as in the first experiment. In the third experiment, abalone were produced in December 2008 and were reared to December 2011.
A flow-through seawater system was used with water from offshore, and was filtered through sand filters and pumped directly to the tanks. Water temperature, salinity, pH, and dissolved oxygen were measured daily to study the rearing regime and its influence on growth rate. Halved plastic tubes were placed at the bottom of the tanks as shelters. Larvae and early juveniles were fed daily with diatoms; late juveniles and adults were fed with artificial food and the green algae Ulva fasciata.
During the first experiment, from 2001 to 2005, 80 specimens were selected randomly each month, and shell length and total wet weight were measured. This task was repeated for 40 specimens in the second experiment (2006 to 2009) and the 60 specimens in the third experiment (2009 to 2011). The number of abalone decreased during the experiments, and from January 2004 to December 2005, only 50 individuals were analyzed monthly. During the second trial (2006 to 2009), the measured individuals were not sexed. Yearly increments in length and weight were calculated for both sexes and for males and females separately.
Reproductive status was identified using a visual gonad maturity index (GI). The GI stages were 0, no gonad development visible; 1, immature, gonad developing but still not prominent; 2, mature, 25% of the hepatopancreas or liver is covered by the gonad; and 3, fully mature, 50% or more of hepatopancreas is covered by the gonad. A mean GI was calculated as the average of individual gonad maturity stages each month, and a percent of mature males and females were used to study maturation processes in the tanks.
The relationship between the length (L) and weight (W) of abalone was expressed by the power regression equation
W = a[L.sup.b],
where W is the total wet weight, L is the shell length, and a and b are the constants to be determined. Length--weight relationships were calculated for males, females, and both sexes combined using a least-squares fit, and regressions were compared using analyses of covariance (ANCOVAs).
To estimate the length at first maturity ([L.sub.m50]), at which 50% of females and males reach sexual maturity, the specimens were grouped by sex in I-ram-length classes, and the fraction of mature males and females (GI stage 3) was calculated. A cumulative percent of mature abalone against length classes was plotted, and the logistic equation was fitted to find length at first maturity:
P = 100/exp (-a x (L - [L.sub.m50]))'
where P is the proportion of mature females or males in each 1-mm-length class, L is the midclass length, [L.sub.m50] is length at first maturity, and a is a constant. Nonlinear least-squares fitting with Microsoft Excel Solver was used to obtain the best fit of [L.sub.m50] and a.
To study abalone growth as a relationship between shell length and age, the nonseasonal von Bertalanffy growth function was used:
[L.sub.t] = [L.sub.inf](1 - [e.sup.k(t-t0)]),
where Lt is shell length of the abalone at age t, [L.sub.inf] is the asymptotic length, k is the instantaneous growth coefficient, and [t.sub.o] is the hypothetical age at which length is equal to 0.
Water Quality Parameters
The water parameters in the tank reflected the ambient condition of the Arabian Sea. The mean monthly water temperature, dissolved oxygen, salinity, and pH in the rearing tanks during 2010 are presented in Figure 2. Water temperature showed a distinct seasonal cycle, with high values from around 27-28[degrees]C during April and May and October and November, decreasing to 20-22[degrees]C from July to September each year. This decline was associated with the southwest monsoon period, from June to October, when strong southwest winds induce a reverse of the current system and cause upwelling of cold and low-oxygen deep waters. At this time, the coast is subject to a seasonal rough sea and rainy season.
Salinity was relatively constant at approximately 37-38 ppt, with a small decrease during the monsoon period (July to September), which corresponds to the season of rain and fog. As a result of more wind mixing of the surface water layer, the concentration of dissolved oxygen increased (>6 mL/L) during the southwest monsoon, pH was mostly stable except for a slight increase during the summer monsoon season.
Grow-out in 2001 to 2005
The first experiment on the grow-out of the Omani abalone Haliotis mariae was conducted from December 1999 to December 2005. A detailed description of the hatchery, seed production, and abalone growth between 1999 and 2000 was published previously (Al-Rashdi & Iwao 2008). Starting January 24, 2001, 80 hatchery-bred juveniles were selected randomly and measured. Abalone shell length ranged from 42.0-67.5 mm (mean, 53.3 mm; SD, 4.9 mm) and wet weight was 6.7-35.1 g (mean, 20.1 g; SD, 6.3 g). The mean monthly abalone length and weight (with SDs) from January 2001 to December 2005 are presented in Figure 3; mean annual sizes are shown in Table 1.
After 3 y of rearing, in December 2003 (at the age of 4 y), the length of the abalone ranged from 69.1-98.5 mm, the average size of males was 81.7 [+ or -] 5.0 mm and 88.9 [+ or -] 15.3 g, and the average size of females was 78.9 [+ or -] 4.9 mm and 80.1 [+ or -] 15.1 g. Males grew faster than females, and ANCOVAs showed significant differences between sexes (F = 7.97, P = 0.006).
In December 2005, the abalone had attained a mean length of 88.7 [+ or -] 4.7 mm (males) and 88.2 [+ or -] 4.9 mm (females), with a mean weight of 112.5 [+ or -] 19.5 g and 104.9 [+ or -] 18.6 g, respectively. The largest abalone reached 98.6 mm and weighed 166.2 g.
The average growth increments were greatest in length during the first year of life (52.9 mm/y on December 2000), whereas weight increments were greatest during year 4 (28.2 g/y on December 2003; Table 2). Males generally, but not always, have larger increments in length and weight compared with females.
During the grow-out period, some decrease in growth was registered. Sometimes this was related to random selection of specimens for measurement, but in most cases it occurred during the southwest monsoon period (June to September), when the water temperature decreased to 17-20[degrees]C. Some specimens lose weight significantly during the summer. For example, the mean weight of the abalone decreased 0.8 g from June to July 2001, 2.4 g from May to June 2003, 5.3 g from July to August 2004, and 7.0 g from July to August 2005.
Maturation of the abalone in tanks was monitored, and some individuals reached maturity and were used as broodstock for spawning inducement experiments. The gonad maturity stages and GI changed during the rearing period, but there was no defined reproduction period noted in the tanks. Mature (stage 2) and fully mature (stage 3) males and females occurred year-round, with slight peaks in May and November.
Grow-out in 2006 to 2009
New egg hatching and larval rearing began during December 2005, and this experimental trial of abalone grow-out lasted 4 y (to November 2009). As in the first experiment, the growth rate was very fast during the first year and, by December 2006, juveniles had attained a mean length of 55.8 [+ or -] 4.10 mm, which corresponds to a monthly increment of 4.65 mm. After 4 y of rearing, in November 2009, mean abalone size was 86.6 [+ or -] 5.3 mm and 109.2 [+ or -] 16.7 g. In this case, the growth rate was greater than in the first experiment (2000 to 2005), and ANCOVAs showed a significant difference between the 2 experiments at a 5% confidence level (F = 7.18, P = 0.03).
As in the first experiment, the length increments were greatest from larva to juvenile stage, but the weight increment reached a maximum value (43.0 g/y) during the third year of culture (Table 2).
Grow-out in 2010 to 2011
Spawning was conducted from December 2008 to February 2009, and the overall grow-out period lasted 3 y (to December 2011). After 1 y of rearing (December 2010), the mean length of the abalone was 53.18 [+ or -] 4.72 mm and the mean weight was 21.62 [+ or -] 6.19 g (Fig. 3). Average growth rates were 4.43 mm/mo and 1.80 g/mo. By the end of the experiment in December 2011, males had reached a mean length of 84.0 [+ or -] 5.5 mm and a mean weight of 89.3 [+ or -] 14.8 g, and females had a mean length of 86.1 [+ or -] 4.7 mm and a mean weight of 95.4 [+ or -] 17.3 g. The growth rate was very similar to previous experiments (2006 to 2009), and ANCOVAs did not show any significant differences. Also there was no significant difference found in the growth of males and females (F = 2.57, P = 0.109). As in the first 2 experiments, the length increments were greatest during the first year of larva and juvenile growth, and weight increments reached a maximum 1 value (38.5 g/y) during the third year of rearing, as noted in the second experiment (2006 to 2009).
As in the previous experiments, some periods of decreased abalone growth were observed that usually coincided with a decrease in water temperature in tanks during the summer monsoon (Fig. 3).
Hatchery-bred abalone started to mature at very small sizes. Males and females with well-developing gonads were noted in tanks after 1 y of cultivation. The percent of individuals with gonads at stage 2 and stage 3 was slightly higher in April to May 2010, and the first peak of mature and fully mature males and females was observed from October 2010 to January 2011 (at about 2 y; Fig. 4). The next peaks of mature individuals occurred April to May and in November to December 2011.
To estimate a length-weight relationship, all data were used, including broodstock (6,203 measurements), and the parameters of the power regression equations were calculated for all combined material and for males and females separately (Table 3). The relationships demonstrate that males were slightly heavier than females of the same length. The regression equations were tested for equality using ANCOVA, which that slopes differed significantly between sexes (F = 4.86, P = 0.027). A relatively high [R.sup.2] value indicates excellent goodness of fit in power regression models.
Length at First Maturity
The smallest female with a mature gonad (GI stage 3) had a shell length of 53.3 mm. The smallest male with a fully mature testis was 49.9 mm. The portion of mature females in 1-mm-length classes was calculated, and the cumulative percentages were used to estimate length at first maturity ([L.sub.m50]) with the logistic formula (Fig. 5). The same procedures were used to calculate [L.sub.m50] for males. The length at first maturity for Haliotis mariae was estimated as 65.2 mm for males and 67.1 mm for females, indicating that males mature when they are slightly smaller than females.
The data on the growth of abalone from first and second experiments were used to calculate parameters in the von Bertalanffy growth formula. The values of the asymptotic length ([L.sub.inf]) in both experiments were similar--92.6 mm and 95.3 mm--and the growth coefficient value (k) was greater in the second experiment (0.65/y and 0.80/y, respectively; Table 4).
The growth pattern of Haliotis mariae was very similar during the 3 experimental trials, which were conducted over 6 y (2000 to 2005), 4 y (2006 to 2009), and 3 y (2009 to 2011). An increased growth rate was seen during the first year of life, with a slower continuous growth occurring during the adult years. Larvae and early juveniles had very fast growth rates, reaching an average 53-56 mm in shell length and 19-25 g in total wet weight after 1 y. Better results were obtained during the second and third experiments, when at the end of 3 y of rearing, abalone had attained a mean length of 83-85 mm and a mean weight of 89-93 g. The largest raised individual after 6 y was 99.0 mm in length and weighed 166.6 g. Differences among growth rates of abalone in the 3 experiments may be explained by improvements to maintenance techniques. There are many factors that influence abalone growth, including environmental conditions and feeding conditions. Several experiments conducted in Mirbat showed significant differences in growth rates using different photoperiod regimes and different artificial and natural feeds.
The length increments were found to be greatest during the first year of the culture, and weight gains reached maximum values during year 3 (second and third trials) and year 4 (first trial) of rearing. After 4 y of cultivation, abalone growth increments in length and weight decreased significantly.
The growth of the Haliotis mariae under experimental conditions was compared with its growth in nature estimated by Sanders (1982) and Siddeek and Johnson (1993, 1997) using length-frequency distribution analysis (LFDA). The comparison of von Bertalanffy growth parameters showed that the asymptotic length, calculated from the experimental data ([L.sub.inf] = 92.6-95.3 mm), was significantly lower than that calculated by different methods of LFDA (Table 4). This can be explained by the relatively short period of the experiments when abalone did not reach the maximum age and, size as in wild populations. For example, the maximum shell length of abalone in broodstock was 133.8 mm and the maximum weight was 371.4 g. The calculated growth rate coefficient (k = 0.65) in the first experiment was lower, but in third experiment it appeared higher (k = 0.80) than in most calculations from LFDA data, which indicates faster growth rates of the abalone in tanks compared with those in the wild.
The slope of regressions and regression coefficients (b) close to 3.2 in the length-weight relationships indicates positive allometric growth for both sexes of abalone. A study of the length weight relationship of Omani abalone on fishery data was conducted by Al-Hafidh (2006). In different locations, the coefficient a ranged from 0.000035-0.00005 and coefficient b ranged from 3.23-3.31. A comparison of the length-weight relationship between wild and cultured abalone showed good agreement, but in some sites the b values were slightly higher in the wild, which can be explained larger sizes of measured abalone from fishery.
The length at first maturity was estimated for males at 65.2 mm and for females at 67.1 mm; therefore, in culture conditions, the abalone mature at 2-3 y of age. Shepherd et al. (1995) and Johnson et al. (1992) indicated that the size of sexual maturity for Haliotis mariae ranged from 65-76 mm; A1-Hafidh (2006) reported that Omani abalone reach sexual maturity at length more than 60 mm. The age at first spawning has been suggested to be in the third year (2+ cohort) (Sanders 1982).
It is known that, in nature, abalone have a well-defined spawning period that begins in November and continues to April (Sanders 1982, Savidge et al. 1986, Siddeek & Johnson 1993, Al-Hafidh 2006). In contrast, during the first experimental trial (2001 to 2005), mature males and females (gonad stages 2 and 3) occurred in tanks year-round, with slight peaks in May and November. During the experimental grow-out of 2010 to 2011, 2 maturation peaks were observed: the first in April to May and the second in November to December. Therefore, mature males and females can be selected for spawning inducement experiments not only from the wild in November to January, but also from cultured individuals in April to May, and perhaps be in other months, which makes it possible for seed production almost year-round.
Results of the study showed the feasibility of rearing Omani abalone, Haliotis mariae, in land-based farms during a 3-4-y cycle from eggs to commercial size for commercial production and wild stock enhancement. However, several challenges remain, such as developing more effective hatching techniques, and increasing survival and settlement rates during larval development, and growth rates during the culture period.
We are grateful to Dr. Hammed Al-Oufi, Undersecretary Ministry of Agriculture and Fisheries Wealth; Dr. Saoud Al-Habsi, Director General of Fisheries Research; and Salem Al
Ghassani for the opportunity provided to carry out the abalone projects, and for financial support. We are also thankful to the technical staff at Mirbat, including Mr. Basheer and Mr. Salem Kahoom, who participated in laboratory work related to the project. We also thank the staff of Fisheries Research Center in Salalah who assisted in our work.
Al-Hafidh, A. S. 2006. Assessment and management of abalone Haliotis mariae, 1828 Wood fishery in the Omani water. PhD diss., University of Hull, International Fisheries Institute.
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Al-Rashdi, K. M. & T. Iwao. 2008. Abalone, Haliotis mariae (Wood, 1828), hatchery and seed production trials in Oman. Agr. Mar. Sci. 13:53-63.
Benny, A., A. L. Khalfan, A. R. Rashdi, A. L. Mushikhi & M. Balkheir. 2003. Laboratory observation on the spawning, larval settlement and early growth of the Omani abalone Haliotis mariae Wood. In: Proceedings of the National Seminar on Marine Biodiversity as a Source of Food and Medicine, 2003. SDMRI research publication. pp. 133-136.
Bosch, D. & E. Bosch. 1982. Seashells of Oman. London: Longman. 206 pp. Fishery statistics book for the Sultanate of Oman. 2011. Fisheries Statistic & Information Department. Ministry of Agriculture and Fisheries Wealth. Muscat. 260 pp.
Johnson, D. W., A. Al-Harassy & M. Al-Harthy. 1992. The Sultanate of Oman abalone fishery. In: S. A. Shepherd, M. J. Tegner & S. A. Guzman del Proo, editors. Abalone of the world: biology, fisheries and culture. Oxford: Fishing News Books. pp. 448-453.
Sanders, M. J. 1982. Preliminary stock assessment for the abalone taken off the southeast Coast of Oman-March 24 to April 2 1982. FAO/UNDP FI: DP/RAB/80/015/3. Rome: Food and Agriculture Organization. 48 pp.
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MOHAMMED A. BALKHAIR, * ALI R. AL-MUSHIKHI AND MIKHAIL V. CHESALIN
Ministry of Agriculture and Fisheries Wealth, Fisheries Research Center-Salalah, PO Box 33, Salalah 217, Sultanate of Oman
* Corresponding author. E-mail: email@example.com
TABLE 1. The average yearly length and weight ([+ or -] SD) of the abalone Haliotis mariae at different grow-out trials at Mirbat (Oman). Shell length (mm) Abalone 2000-2005 2006-2009 2009-2011 age (y) 1 52.9 * 55.8 [+ or -] 4.1 53.2 [+ or -] 4.7 2 64.2 [+ or -] 5.2 67.4 [+ or -] 5.1 71.8 [+ or -] 5.5 3 72.2 [+ or -] 5.3 83.4 [+ or -] 6.8 85.2 [+ or -] 5.1 4 79.8 [+ or -] 5.0 86.6 [+ or -] 5.3 5 84.5 [+ or -] 5.5 6 88.5 [+ or -] 4.9 Total weight (g) Abalone 2000-2005 2006-2009 2009-2011 age (y) 1 19.1 ([dagger]) 24.7 [+ or -] 5.5 21.6 [+ or -] 6.2 2 36.2 [+ or -] 9.9 46.2 [+ or -] 12.4 54.4 [+ or -] 11.9 3 57.6 [+ or -] 14.0 89.2 [+ or -] 15.4 92.9 [+ or -] 16.5 4 85.8 [+ or -] 15.3 109.2 [+ or -] 16.7 5 96.4 [+ or -] 18.0 6 109.2 [+ or -] 19.5 * From Al-Rashdi and lwao (2008). ([dagger]) Calculated from length-weight relationship. TABLE 2. Growth increments of Haliotis mariae during tank culture in Mirbat from January 2000 to December 2011. Abalone Grow-out Shell length age (y) period increments (mm) All Males Females Experiment 2000-2005 1 2000-2001 52.9# 2 2001-2002 11.33 11.75 10.25 3 2002-2003 8.04 8.79 7.25 4 2003-2004 7.62 8.17 7.13 5 2004-2005 4.73 5.49 4.26 6 2005-2006 4.08 3.47 4.47 Experiment 2006-2009 1 2006-2007 55.8# -- -- 2 2007-2008 11.6 -- -- 3 2008-2009 16.0 -- -- 4 2009-2010 3.2 -- -- Experiment 2009-2011 1 y 2009-2010 53.2# 2 y 2010-2011 18.6 18.06 18.86 3 y 2011-2012 13.4 12.25 14.08 Abalone Weight increments (g) age (y) All Males Females Experiment 2000-2005 1 19.1 2 17.12 17.65 16.52 3 21.44 22.46 20.74 4 28.22# 29.65# 27.65# 5 10.61 12.06 10.20 6 12.84 10.57 13.19 Experiment 2006-2009 1 24.7 -- -- 2 21.5 -- -- 3 43.0# -- -- 4 20.0 -- -- Experiment 2009-2011 1 y 21.6 2 y 32.8 28.13 33.85 3 y 38.5# 36.0# 40.2# Bold type indicates the maximum increments in the experiment. Note: The maximum increments in the experiment are indicated with #. TABLE 3. The power equations showing relationships between shell length (L) and total wet weight (W) of Haliotis mariae. Sex n Equation [R.sup.2] All 6,203 W = 0.00005[L.sub.3.2380] 0.965 Females 3,002 W = 0.00005[L.sub.3.2311] 0.967 Males 3,004 W = 0.00005[L.sub.3.2658] 0.964 TABLE 4. Comparison of growth parameters of Haliotis mariae in the von Bertalanffy formula in experimental trials in Mirbat and in natural conditions, calculated based on length-frequency data, from different studies. Condition [L.sub.inf] K [t.sub.o] Reference Experimental Current study 2000-2005 92.6 0.61 -0.06 2009-2011 95.3 0.80 0.03 Natural 119.0 0.77 Sanders (1982) Natural 137.7 0.75 0.73 Siddeek and Johnson (1993) Natural 126.6 0.97 Siddeek and Johnson, (1997) 140.0 0.67 152.8 0.58 0.55
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|Author:||Balkhair, Mohammed A.; Mushikhi, Ali R. Al-; Chesalin, Mikhail V.|
|Publication:||Journal of Shellfish Research|
|Date:||Apr 1, 2013|
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