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The effect of salinity on the survival and growth of striped mullet (Mugil cephalus) larvae in the hatchery
.4quaculture, 96 ( 199 I ) 249-254 Elsevier Science Publishers B.V., Amsterdam
249
The effect of salinity on the survival and growth of striped mullet (Mugil cephalus) larvae in the hatchery Ryan Murashige, Paul Bass, Laura Wallace, Augustin Molnar, Bryan Eastham, Vernon Sato, Clyde Tamaru and Cheng-Sheng Lee The Oceanic Institute, Makapuu Point, P.O. Box 25280, Honolulu, HI 96825, USA (Accepted 30 December
1990)
ABSTRACT Murashige, R.. Bass, P., Wallace, L., Molnar, A., Eastham, B., Sato, V., Tamaru, C. and Lee, C.-S., 199 I. The effect of salinity on the survival and growth of striped mullet (Mugil cephalus) larvae in the hatchery. Aquaculture, 96: 249-254. Newly hatched striped mullet larvae were exposed to salinities ranging between 17-18, 22-23, 27-28, and 32-35 ppt and reared in 30-l polycarbonate tanks for I5 days posthatching. In addition, newly hatched larvae were exposed to salinities ranging between 22-23 ppt and 32-35 ppt and cultured at their respective salinities in 5000-l tanks until 50 days posthatching. In the small-scale, shortterm experiments, no differences in survival (range: 30.52-4 1.OI) were detected across the salinities tested. However, larvae from the 22-23 ppt group were significantly and consistently larger (5.9? 0.8 mm total length) than those exposed to the higher salinities (5.4* 0.7 mm TL). In contrast, the largescale, long-term experiments, resulted in no significant differences in survival (29.0% f 13.6O/aand 22.01+4.6% for the 32-35 and 22-25 ppt groups, respectively) or growth (19.4& 5.0 mm TL and 19.9 + 4.6 mm TL for the 32-35 and 22-25 ppt groups, respectively). No apparent advantage or disadvantage derives from adjusting the salinity during a larval-rearing trial of striped mullet.
INTRODUCTION
Striped mullet (Mugil cephalus) juveniles and adults are known to exhibit a remarkable tolerance of salinity fluctuations. Researchers have found that the ability to cope with salinity fluctuations is also present, to a limited degree, in mullet embryos and newly hatched larvae (Sylvester et al., 1975; Lee and Menu, 1981; Walsh et al., 1989). However, long-term effects of salinity on growth of posthatched larvae and their survival to fry stages have yet to be investigated_ The effects of salinity during this period of ontogeny are crucial to the development of hatchery procedures for this species, especially in areas where the quantity and/or quality of seawater may be a limiting factor. The present study focused on the long-term effects of various salinities on 0044-8486/91/$03.50
0 199 1 Elsevier Science Publishers
B.V.
250
R. MURASHIGE ET AL.
growth and survival of posthatched mullet larvae up to the fry stage. The 30-l small-scale experiments were conducted for short-term of 15 days posthatching. The 5000-l large-scale experiments were conducted for long-term, 50 days posthatching durations. MATERIAL AND METHODS
Spawning and incubation Fertilized eggs were obtained from females induced to spawn as described by Lee et al. ( 1987). In summary, a female at the appropriate state of maturity was injected with carp pituitary homogenate (20 mg/kg body weight). This injection was followed 24 h later with a second injection of luteinizinghormone-releasing-hormone analogue (200 hg/kg body weight). Fertilized eggs were transferred from the spawning tank into 1000-l incubation cylinders provided with running seawater (35 ppt, 24-26’ C) and supplied with continuous aeration. The eggs were quantified volumetrically and transferred to their appropriate experimental tanks approximately 24 h before hatching at the lo- 12 somite stage. Small-scale, short-term experiments Two small-scale, short-term rearing trials were conducted in 30-l polycarbonate tanks during the 1987-88 mullet spawning season. The tanks were darkened on the exterior with high gloss epoxy paint. Before the stocking of fertilized eggs, the tanks were filled with 35 ppt sea water at a 25-l working volume. Fertilized eggs were stocked at 25 eggs/l. After hatching, dead eggs and larvae were counted, to obtain the initial stocking densities. Each trial was conducted using a randomized block design with four salinities representing each treatment block. Each treatment was triplicated, except for the 22-23 and 32-35 ppt treatments in the first trial. Salinities were adjusted over the initial 24-h period after hatching to the following ranges: 17-18, 22-23, 27-28, 32-35 ppt, in treatments I, II, III, and IV respectively. Temperatures were maintained at 26.5”C by placing all tanks in a water bath. A mortality count was taken twice a day throughout the 15-day posthatching experimental period. All tanks were supplied with constant aeration. No significant differences were observed in pH (range: 7.7 to 7.9)) dissolved oxygen (range: 5.8 to 6.1 mg/l), or temperature (range: 25.3 to 25.9”C) among treatments for both trials. The phytoplankton, Nannochloropsis oculata and the rotifer, Brachionus plicatilis (S-type, 11O-230 ,um lorica length), were cultured as described by Eda et al. ( 1990). The algae and rotifers were stocked into the tanks on the second day after hatching at densities of 500 000 cells/ml and 5 individuals/ ml, respectively. Algal densities were monitored and maintained throughout the course of the experiment. Rotifers were adjusted to 20 individuals/ml on
EFFECT OF SALINITY ON SURVIVAL AND GROWTH OF MULLET LARVAE
251
the fifth day posthatching and maintained at that density for the duration of the experiment. Approximately 10% of the water from each tank was changed daily and salinities were maintained at predetermined levels. After 15 days posthatching all larvae from each tank were anesthetized using MS-222 (Sigma Chemical Company, St. Louis, MO, USA) at a final concentration of 60 ppm. The larvae were then fixed in 2.5% glutaraldehyde (Eastman Kodak Company, Rochester, NY, USA), and stored at 5 ‘C as described by Oozeki and Hirano ( 1988 ) . The number of fixed larvae from each tank was counted to determine survival. Average total fixed length was determined from 20 randomly selected individuals. The salinity that produced the best survival and/or growth (22-23 ppt) was used as the treatment salinity in the large-scale experiments. Large-scale, long-term experiments Three long-term, large-scale intensive larval-rearing trials were conducted in 5000-l round fiberglass tanks. For each trial, the treatments were duplicated, Fertilized mullet eggs were obtained as described previously and stocked at the same densities as in the small-scale experiments. Each trial consisted of eggs from a single spawning. All eggs were initially placed in a salinity of 35 ppt. When approximately 80% of the hatching was completed, the salinity of the treatment tanks was adjusted to 22-25 ppt by the addition of fresh tap water over a 24-h period. This salinity was maintained over the course of the 50-day trial. Initial stocking of phytoplankton and rotifers and their maintenance was as previously described. Newly hatched Artemia nauplii were fed to the larvae from the 12th day posthatching until the end of the experiment. Artificial feed (ground shrimp feed containing 42% protein) was introduced between the 20 and 25th day posthatching and continued for the experiment’s duration. Water temperature ranged from 22.2”C to 26.O”C and was not significantly different among treatments during all three trials. This was also the case for observed pH, which ranged from 7.5 to 8.0. Only dissolved oxygen was found to be significantly higher in lower salinity treatments, with ranges of 5.4 to 7.0 mg/l in the 32-35 ppt treatment and 6.0 to 7.6 mg/l in the 22-25 ppt treatment. Growth was not determined for larvae during trial 1 because of an accident during preservation of the larvae. However, during trials 2 and 3, at approximately 5-day intervals, 20 individuals from each tank were randomly obtained and fixed in 2.5% glutaraldehyde and later measured as described previously. The number of fry in each tank was determined at the end of the rearing trials. Fifty individuals were obtained from each tank in trials 2 and 3 and dried at 60’ C. The dry weight of each fry was then determined using a Mettler analytical balance.
252
R. MURASHIGE ET AL.
Statistical analysis Differences in survival and total length among salinities and trials were tested using a two-way analysis of variance (Sokal and Rohlf, 1969) in the small-scale experiments. Differences in larval growth from the large-scale experiments were analyzed with analysis of covariance after log transformation of the data (Sokal and Rohlf, 1969). To detect differences in larval survival during the large-scale trials, the results were subjected to the t-test. RESULTS
Small-scale experiments A summary of salinity effects on larval survival and growth during the first 15 days posthatching is presented in Table 1. No detectable differences in survival occurred among the salinity treatments within each trial. However, significant (P-C 0.01) differences in survival were noted when trials were compared. In contrast, differences in growth were detected among the salinity treatments. In trial 1, the total length of the larvae obtained from the 17- 18 ppt group was significantly (P-C 0.05) larger than those of the 27-28 ppt group, but no differences could be detected between larvae from the 17- 18 ppt group and those of the other salinity groups. The larvae, however, from the 22-23 ppt treatment were significantly (P< 0.0 1) larger than those from both of the higher salinities. This trend was also observed in the second trial, however, a difference in total length was detected between 22-23 ppt and higher salinities. In addition, the larvae from trial 2 were significantly (PC 0.01) smaller than those obtained from trial 1. TABLE 1 The effects of salinity on striped mullet larvae survival and growth during the first 15 days posthatching (average + s.d. ) Salinity (ppt)
Trial I 1I-28 22-2F 27-28 32-W’= Trial 2 17-18 22-23 27-28 32-35 **Duplicated only.
Number stocked
Number harvested
Survival (%)
Total length (mm)
256f32 307 + 32 359+ 120 281 f26
95+9 116+164 139+24 86?8
37.3 f 1.4 38.64? 9.4 41.0 +8.2 30.5 +0.8
5.6 + 0.9 5.9kO.8 5.3_+ 1.0 5.4*0.7
334+52 25Of 102 349+ 105 328k56
184+65 144f94 214f85 212+32
53.4 51.9 59.6 64.7
5.OkO.8 5.1 f0.8 4.8f0.5 4.7f0.6
+ 10.4 f14.3 + 5.4 f2.0
EFFECT
OF SALINITY
ON SURVIVAL
AND
GROWTH
OF MULLET
253
LARVAE
TABLE 2 Average survival and growth during 50-day larval-rearing salinities
trials conducted
in 5000-l tanks at two
Salinity (PPt)
Stocking density (#/I)
Harvest density
Survival
Total length
Dry weight
(#/I)
(Oh)
(mm)
(g)
32-35 22-X
17.4+ 6.5 15.0* 15.1
4.5 + 2.4 3.052.4
29.0& 13.6 22.02 4.6
19.4**5.0 19.9*24.0
0.021*+0.023 0.022** 0.020
*Data from trials 2 and 3 only.
Large-scale experiments
A summary of the survival and growth obtained from the large-scale larvalrearing trials is presented in Table 2. No significant differences could be detected in survival, total length, and dry weight of 50-day-old fry reared at either salinity. Similarly, no differences were detected between growth of the larvae from both treatments over the course of the 50-day trial. DISCUSSION
Earlier investigations have demonstrated that increased tolerance of salinity fluctuations in striped mullet embryos is coincident with their ontogeny (Sylvester et al., 1975; Lee and Menu, 198 1). However, the optimal salinities that have been described (range: 30-40 ppt ) for hatching are those encountered in the sea. The early stages of development or, in hatchery terms, the incubation period, may be restricted to seawater. Salinity may not affect hatching by way of metabolic processes, but rather by affecting the buoyancy of fertilized eggs. This is suggested by an undetectable effect on the oxygen consumption of striped mullet embryos exposed to various salinities and by an increase in the range of salinities in which hatching occurs when an improved aeration system is used (Walsh et al., 1989, 199 1). An improvement in growth was detected at a lower salinity (i.e., 22-23 ppt ) during the first 15 days posthatching. The overall significance of this observation is presently unknown because this was not observed when larvae were reared at 17- 18 ppt. This is in contrast to the negative correlation observed between salinity and the resulting length and weight of larval herring, Clupea harengus (Holliday and Blaxter, 1960). In addition, the effects exhibited in the small-scale, short-duration experiments were not manifested during a complete 50-day larval-rearing trial. The difference in the larval size obtained in both small-scale trials is felt to be from improved survival, which resulted in the overall higher larval density observed in the second trial. The inverse relationship between larval density and larval growth has been well documented in other species (May, 1974; Houde, 1975, 1977; Werner and Blaxter, 1980). Apparently, other factors such as larval density, food density, nutrition, etc., exert a more profound
254
R. MURASHIGE ET AL.
influence on larval growth and survival than does salinity. As with overall survival, at the salinities tested, no apparent advantage or disadvantage to growth results from varying salinities during striped mullet larval rearing. The results of the current investigation demonstrate that, with regard to hatchery practices, a change in salinity over the ranges tested poses no detriment to mullet larvae. In cases where changes in salinity are unavoidable (e.g., rainfall) or are necessary (e.g., treatment of disease), relatively little effect on mullet larvae growth and survival will result. ACKNOWLEDGMENTS
The authors thank Suzanne Halemano for manuscript preparation. The authors are also grateful to the reproduction team of the Finfish Program at the Oceanic Institute that provided the fertilized mullet eggs for this study. Funding for this research was obtained from the United States Agency for International Development, Contract Number DAN-4 16 1-A-00-405 5-00. REFERENCES Eda, H., Murashige, R., Oozeki, Y., Hagiwara, A., Eastham, B., Bass, P., Tamaru, C.S. and Lee, C.-S., 1990. Factors affecting intensive larval rearing of striped mullet, Mugif cephafus. Aquaculture, 9 1: 28 l-294. Holliday, F.G.T. and Blaxter, J.H.S., 1960. The effects of salinity on the developing eggs and larvae ofthe herring. J. Mar. Biol. Assoc. U.K., 39: 591-603. Houde, E.D., 1975. Effects of stocking density and food density on survival, growth, and yield of laboratory-reared larvae of the sea bream, Archosurgus rhomboid& (L. ) (Sparidae). J. Fish Biol., 7: 115-127. Houde, E.D., 1977. Food concentration and stocking density effects on survival and growth of laboratory-reared larvae of bay anchovy Anchoa mitchilli and lined sole Achirus lineatus. Mar. Biol., 43: 333-341. Lee, C.-S. and Menu, B., 1981. Effects of salinity on egg development and hatching in grey mullet, Mugil cephalus L. J. Fish Biol., 19: 179-l 88. Lee, C.-S., Tamaru, C.S., Miyamoto, G.T. and Kelley, C.D., 1987. Induced spawning of grey mullet (Mugil cephalus) by LHRH-a. Aquaculture, 62: 327-336. May, R.C., 1974. Larval mortality in marine fishes and the critical period concept. In: J.H.S. Blaxter (Editor), The Early Life History of Fish. Springer, Heidelberg, pp. 3-19. Oozeki, Y. and Hirano, R., 1988. Effects of glutaraldehyde fixation on the body size of red sea bream (Pagrus major). Aquaculture, 70: 159- 167. Sokal, R.R. and Rohlf, F.J., 1969. Biometry. Freeman, San FranciscoCA, 776 pp, Sylvester, J.R., Nash, C.E. and Emberson, C.R., 1975. Salinity and oxygen tolerances of eggs and larvae of Hawaiian striped mullet, Mugil cephalus L. J. Fish Biol., 7: 62 l-629. Walsh, W.A., Swanson, C., Lee, C.-S., Banno, J.E. and Eda, H., 1989. Oxygen consumption by eggs and larvae of striped mullet, Mugil cephalus, in relation to development, salinity, and temperature. J. Fish Biol., 35: 347-358. Walsh, W.A., Swanson, C. and Lee, C.-S., 199 1. Combined effects of temperature and salinity on development and hatching of striped mullet, Mugil cephalus. Aquaculture (in press). Werner, R.G. and Blaxter, J.H.S., 1980. Growth and survival of larval herring (Clupeu harengus) in relation to prey density. Can. J. Fish. Aquat. Sci., 37: 1063-1069.
249
The effect of salinity on the survival and growth of striped mullet (Mugil cephalus) larvae in the hatchery Ryan Murashige, Paul Bass, Laura Wallace, Augustin Molnar, Bryan Eastham, Vernon Sato, Clyde Tamaru and Cheng-Sheng Lee The Oceanic Institute, Makapuu Point, P.O. Box 25280, Honolulu, HI 96825, USA (Accepted 30 December
1990)
ABSTRACT Murashige, R.. Bass, P., Wallace, L., Molnar, A., Eastham, B., Sato, V., Tamaru, C. and Lee, C.-S., 199 I. The effect of salinity on the survival and growth of striped mullet (Mugil cephalus) larvae in the hatchery. Aquaculture, 96: 249-254. Newly hatched striped mullet larvae were exposed to salinities ranging between 17-18, 22-23, 27-28, and 32-35 ppt and reared in 30-l polycarbonate tanks for I5 days posthatching. In addition, newly hatched larvae were exposed to salinities ranging between 22-23 ppt and 32-35 ppt and cultured at their respective salinities in 5000-l tanks until 50 days posthatching. In the small-scale, shortterm experiments, no differences in survival (range: 30.52-4 1.OI) were detected across the salinities tested. However, larvae from the 22-23 ppt group were significantly and consistently larger (5.9? 0.8 mm total length) than those exposed to the higher salinities (5.4* 0.7 mm TL). In contrast, the largescale, long-term experiments, resulted in no significant differences in survival (29.0% f 13.6O/aand 22.01+4.6% for the 32-35 and 22-25 ppt groups, respectively) or growth (19.4& 5.0 mm TL and 19.9 + 4.6 mm TL for the 32-35 and 22-25 ppt groups, respectively). No apparent advantage or disadvantage derives from adjusting the salinity during a larval-rearing trial of striped mullet.
INTRODUCTION
Striped mullet (Mugil cephalus) juveniles and adults are known to exhibit a remarkable tolerance of salinity fluctuations. Researchers have found that the ability to cope with salinity fluctuations is also present, to a limited degree, in mullet embryos and newly hatched larvae (Sylvester et al., 1975; Lee and Menu, 1981; Walsh et al., 1989). However, long-term effects of salinity on growth of posthatched larvae and their survival to fry stages have yet to be investigated_ The effects of salinity during this period of ontogeny are crucial to the development of hatchery procedures for this species, especially in areas where the quantity and/or quality of seawater may be a limiting factor. The present study focused on the long-term effects of various salinities on 0044-8486/91/$03.50
0 199 1 Elsevier Science Publishers
B.V.
250
R. MURASHIGE ET AL.
growth and survival of posthatched mullet larvae up to the fry stage. The 30-l small-scale experiments were conducted for short-term of 15 days posthatching. The 5000-l large-scale experiments were conducted for long-term, 50 days posthatching durations. MATERIAL AND METHODS
Spawning and incubation Fertilized eggs were obtained from females induced to spawn as described by Lee et al. ( 1987). In summary, a female at the appropriate state of maturity was injected with carp pituitary homogenate (20 mg/kg body weight). This injection was followed 24 h later with a second injection of luteinizinghormone-releasing-hormone analogue (200 hg/kg body weight). Fertilized eggs were transferred from the spawning tank into 1000-l incubation cylinders provided with running seawater (35 ppt, 24-26’ C) and supplied with continuous aeration. The eggs were quantified volumetrically and transferred to their appropriate experimental tanks approximately 24 h before hatching at the lo- 12 somite stage. Small-scale, short-term experiments Two small-scale, short-term rearing trials were conducted in 30-l polycarbonate tanks during the 1987-88 mullet spawning season. The tanks were darkened on the exterior with high gloss epoxy paint. Before the stocking of fertilized eggs, the tanks were filled with 35 ppt sea water at a 25-l working volume. Fertilized eggs were stocked at 25 eggs/l. After hatching, dead eggs and larvae were counted, to obtain the initial stocking densities. Each trial was conducted using a randomized block design with four salinities representing each treatment block. Each treatment was triplicated, except for the 22-23 and 32-35 ppt treatments in the first trial. Salinities were adjusted over the initial 24-h period after hatching to the following ranges: 17-18, 22-23, 27-28, 32-35 ppt, in treatments I, II, III, and IV respectively. Temperatures were maintained at 26.5”C by placing all tanks in a water bath. A mortality count was taken twice a day throughout the 15-day posthatching experimental period. All tanks were supplied with constant aeration. No significant differences were observed in pH (range: 7.7 to 7.9)) dissolved oxygen (range: 5.8 to 6.1 mg/l), or temperature (range: 25.3 to 25.9”C) among treatments for both trials. The phytoplankton, Nannochloropsis oculata and the rotifer, Brachionus plicatilis (S-type, 11O-230 ,um lorica length), were cultured as described by Eda et al. ( 1990). The algae and rotifers were stocked into the tanks on the second day after hatching at densities of 500 000 cells/ml and 5 individuals/ ml, respectively. Algal densities were monitored and maintained throughout the course of the experiment. Rotifers were adjusted to 20 individuals/ml on
EFFECT OF SALINITY ON SURVIVAL AND GROWTH OF MULLET LARVAE
251
the fifth day posthatching and maintained at that density for the duration of the experiment. Approximately 10% of the water from each tank was changed daily and salinities were maintained at predetermined levels. After 15 days posthatching all larvae from each tank were anesthetized using MS-222 (Sigma Chemical Company, St. Louis, MO, USA) at a final concentration of 60 ppm. The larvae were then fixed in 2.5% glutaraldehyde (Eastman Kodak Company, Rochester, NY, USA), and stored at 5 ‘C as described by Oozeki and Hirano ( 1988 ) . The number of fixed larvae from each tank was counted to determine survival. Average total fixed length was determined from 20 randomly selected individuals. The salinity that produced the best survival and/or growth (22-23 ppt) was used as the treatment salinity in the large-scale experiments. Large-scale, long-term experiments Three long-term, large-scale intensive larval-rearing trials were conducted in 5000-l round fiberglass tanks. For each trial, the treatments were duplicated, Fertilized mullet eggs were obtained as described previously and stocked at the same densities as in the small-scale experiments. Each trial consisted of eggs from a single spawning. All eggs were initially placed in a salinity of 35 ppt. When approximately 80% of the hatching was completed, the salinity of the treatment tanks was adjusted to 22-25 ppt by the addition of fresh tap water over a 24-h period. This salinity was maintained over the course of the 50-day trial. Initial stocking of phytoplankton and rotifers and their maintenance was as previously described. Newly hatched Artemia nauplii were fed to the larvae from the 12th day posthatching until the end of the experiment. Artificial feed (ground shrimp feed containing 42% protein) was introduced between the 20 and 25th day posthatching and continued for the experiment’s duration. Water temperature ranged from 22.2”C to 26.O”C and was not significantly different among treatments during all three trials. This was also the case for observed pH, which ranged from 7.5 to 8.0. Only dissolved oxygen was found to be significantly higher in lower salinity treatments, with ranges of 5.4 to 7.0 mg/l in the 32-35 ppt treatment and 6.0 to 7.6 mg/l in the 22-25 ppt treatment. Growth was not determined for larvae during trial 1 because of an accident during preservation of the larvae. However, during trials 2 and 3, at approximately 5-day intervals, 20 individuals from each tank were randomly obtained and fixed in 2.5% glutaraldehyde and later measured as described previously. The number of fry in each tank was determined at the end of the rearing trials. Fifty individuals were obtained from each tank in trials 2 and 3 and dried at 60’ C. The dry weight of each fry was then determined using a Mettler analytical balance.
252
R. MURASHIGE ET AL.
Statistical analysis Differences in survival and total length among salinities and trials were tested using a two-way analysis of variance (Sokal and Rohlf, 1969) in the small-scale experiments. Differences in larval growth from the large-scale experiments were analyzed with analysis of covariance after log transformation of the data (Sokal and Rohlf, 1969). To detect differences in larval survival during the large-scale trials, the results were subjected to the t-test. RESULTS
Small-scale experiments A summary of salinity effects on larval survival and growth during the first 15 days posthatching is presented in Table 1. No detectable differences in survival occurred among the salinity treatments within each trial. However, significant (P-C 0.01) differences in survival were noted when trials were compared. In contrast, differences in growth were detected among the salinity treatments. In trial 1, the total length of the larvae obtained from the 17- 18 ppt group was significantly (P-C 0.05) larger than those of the 27-28 ppt group, but no differences could be detected between larvae from the 17- 18 ppt group and those of the other salinity groups. The larvae, however, from the 22-23 ppt treatment were significantly (P< 0.0 1) larger than those from both of the higher salinities. This trend was also observed in the second trial, however, a difference in total length was detected between 22-23 ppt and higher salinities. In addition, the larvae from trial 2 were significantly (PC 0.01) smaller than those obtained from trial 1. TABLE 1 The effects of salinity on striped mullet larvae survival and growth during the first 15 days posthatching (average + s.d. ) Salinity (ppt)
Trial I 1I-28 22-2F 27-28 32-W’= Trial 2 17-18 22-23 27-28 32-35 **Duplicated only.
Number stocked
Number harvested
Survival (%)
Total length (mm)
256f32 307 + 32 359+ 120 281 f26
95+9 116+164 139+24 86?8
37.3 f 1.4 38.64? 9.4 41.0 +8.2 30.5 +0.8
5.6 + 0.9 5.9kO.8 5.3_+ 1.0 5.4*0.7
334+52 25Of 102 349+ 105 328k56
184+65 144f94 214f85 212+32
53.4 51.9 59.6 64.7
5.OkO.8 5.1 f0.8 4.8f0.5 4.7f0.6
+ 10.4 f14.3 + 5.4 f2.0
EFFECT
OF SALINITY
ON SURVIVAL
AND
GROWTH
OF MULLET
253
LARVAE
TABLE 2 Average survival and growth during 50-day larval-rearing salinities
trials conducted
in 5000-l tanks at two
Salinity (PPt)
Stocking density (#/I)
Harvest density
Survival
Total length
Dry weight
(#/I)
(Oh)
(mm)
(g)
32-35 22-X
17.4+ 6.5 15.0* 15.1
4.5 + 2.4 3.052.4
29.0& 13.6 22.02 4.6
19.4**5.0 19.9*24.0
0.021*+0.023 0.022** 0.020
*Data from trials 2 and 3 only.
Large-scale experiments
A summary of the survival and growth obtained from the large-scale larvalrearing trials is presented in Table 2. No significant differences could be detected in survival, total length, and dry weight of 50-day-old fry reared at either salinity. Similarly, no differences were detected between growth of the larvae from both treatments over the course of the 50-day trial. DISCUSSION
Earlier investigations have demonstrated that increased tolerance of salinity fluctuations in striped mullet embryos is coincident with their ontogeny (Sylvester et al., 1975; Lee and Menu, 198 1). However, the optimal salinities that have been described (range: 30-40 ppt ) for hatching are those encountered in the sea. The early stages of development or, in hatchery terms, the incubation period, may be restricted to seawater. Salinity may not affect hatching by way of metabolic processes, but rather by affecting the buoyancy of fertilized eggs. This is suggested by an undetectable effect on the oxygen consumption of striped mullet embryos exposed to various salinities and by an increase in the range of salinities in which hatching occurs when an improved aeration system is used (Walsh et al., 1989, 199 1). An improvement in growth was detected at a lower salinity (i.e., 22-23 ppt ) during the first 15 days posthatching. The overall significance of this observation is presently unknown because this was not observed when larvae were reared at 17- 18 ppt. This is in contrast to the negative correlation observed between salinity and the resulting length and weight of larval herring, Clupea harengus (Holliday and Blaxter, 1960). In addition, the effects exhibited in the small-scale, short-duration experiments were not manifested during a complete 50-day larval-rearing trial. The difference in the larval size obtained in both small-scale trials is felt to be from improved survival, which resulted in the overall higher larval density observed in the second trial. The inverse relationship between larval density and larval growth has been well documented in other species (May, 1974; Houde, 1975, 1977; Werner and Blaxter, 1980). Apparently, other factors such as larval density, food density, nutrition, etc., exert a more profound
254
R. MURASHIGE ET AL.
influence on larval growth and survival than does salinity. As with overall survival, at the salinities tested, no apparent advantage or disadvantage to growth results from varying salinities during striped mullet larval rearing. The results of the current investigation demonstrate that, with regard to hatchery practices, a change in salinity over the ranges tested poses no detriment to mullet larvae. In cases where changes in salinity are unavoidable (e.g., rainfall) or are necessary (e.g., treatment of disease), relatively little effect on mullet larvae growth and survival will result. ACKNOWLEDGMENTS
The authors thank Suzanne Halemano for manuscript preparation. The authors are also grateful to the reproduction team of the Finfish Program at the Oceanic Institute that provided the fertilized mullet eggs for this study. Funding for this research was obtained from the United States Agency for International Development, Contract Number DAN-4 16 1-A-00-405 5-00. REFERENCES Eda, H., Murashige, R., Oozeki, Y., Hagiwara, A., Eastham, B., Bass, P., Tamaru, C.S. and Lee, C.-S., 1990. Factors affecting intensive larval rearing of striped mullet, Mugif cephafus. Aquaculture, 9 1: 28 l-294. Holliday, F.G.T. and Blaxter, J.H.S., 1960. The effects of salinity on the developing eggs and larvae ofthe herring. J. Mar. Biol. Assoc. U.K., 39: 591-603. Houde, E.D., 1975. Effects of stocking density and food density on survival, growth, and yield of laboratory-reared larvae of the sea bream, Archosurgus rhomboid& (L. ) (Sparidae). J. Fish Biol., 7: 115-127. Houde, E.D., 1977. Food concentration and stocking density effects on survival and growth of laboratory-reared larvae of bay anchovy Anchoa mitchilli and lined sole Achirus lineatus. Mar. Biol., 43: 333-341. Lee, C.-S. and Menu, B., 1981. Effects of salinity on egg development and hatching in grey mullet, Mugil cephalus L. J. Fish Biol., 19: 179-l 88. Lee, C.-S., Tamaru, C.S., Miyamoto, G.T. and Kelley, C.D., 1987. Induced spawning of grey mullet (Mugil cephalus) by LHRH-a. Aquaculture, 62: 327-336. May, R.C., 1974. Larval mortality in marine fishes and the critical period concept. In: J.H.S. Blaxter (Editor), The Early Life History of Fish. Springer, Heidelberg, pp. 3-19. Oozeki, Y. and Hirano, R., 1988. Effects of glutaraldehyde fixation on the body size of red sea bream (Pagrus major). Aquaculture, 70: 159- 167. Sokal, R.R. and Rohlf, F.J., 1969. Biometry. Freeman, San FranciscoCA, 776 pp, Sylvester, J.R., Nash, C.E. and Emberson, C.R., 1975. Salinity and oxygen tolerances of eggs and larvae of Hawaiian striped mullet, Mugil cephalus L. J. Fish Biol., 7: 62 l-629. Walsh, W.A., Swanson, C., Lee, C.-S., Banno, J.E. and Eda, H., 1989. Oxygen consumption by eggs and larvae of striped mullet, Mugil cephalus, in relation to development, salinity, and temperature. J. Fish Biol., 35: 347-358. Walsh, W.A., Swanson, C. and Lee, C.-S., 199 1. Combined effects of temperature and salinity on development and hatching of striped mullet, Mugil cephalus. Aquaculture (in press). Werner, R.G. and Blaxter, J.H.S., 1980. Growth and survival of larval herring (Clupeu harengus) in relation to prey density. Can. J. Fish. Aquat. Sci., 37: 1063-1069.