Life History Traits and Migratory Behavior of Grey Mullet Mugil cephalus in The Tansui River
Date Issued
2010
Date
2010
Author(s)
Tzeng, Ming-Yen
Abstract
To understand the life history traits and the migratory behavior of grey mullets in the Tansui River System, 826 grey mullets were collected during different seasons from 34 sampling sites located on the river mouth of Tansui River to the upper streams, Xindian and Keelung Streams, from April, 2008 to December, 2009. At the same time, the environmental factors such as salinity and algal and benthos densities were also measured. The morphological traits and the monthly changes in GSI of each fish were also measured. The otolith (sagitta) was extracted for age determination and temporal changes of Sr/Ca ratio by Electron microprobe analyzer (EPMA) for identifying the migratory environmental history of the fish. In addition to this, 68 mullets were collected off the coast (salinity: 33 PSU) of Badoutzu in Keelung, north of Taiwan, to validate the relationship between otolith marginal Sr/Ca ratio and environmental salinity. The results indicated that:
(1) relative marginal distance (RMD) demonstrated that the year ring of the otolith was completely formed in a period of one year. Age determination by the otolith year rings showed that the age structure in the lower reach of the Tanshui River during winter was composed primary of individuals greater than 3 years old with few less than 1 year old. On the other hand, individuals aged 2 to 3 years old were mostly pooled in Xidian and Keelung Streams. This phenomenon indicated that older mullets migrated to the river mouth for spawning during winter, while the larvae or juveniles recruited into the river during spring, to grow in the middle or upper tributaries of the river until maturity.
(2) Otolith Sr/Ca ratio had a positive relationship with environmental salinity, hence, otolith Sr/Ca ratio could be used to trace back the migratory environmental history of fish. Otolith Sr/Ca ratio was significantly different among seawater, estuary, and freshwater (ANOVA, p < 0.05). Mean otolith Sr/Ca ratio greater than 6.383×10-3 indicated that the fish inhabited environment whose salinity was more than 33 PSU. On the other hand, mean otolith Sr/Ca ratio less than 3.283×10-3 indicated that the fish inhabited freshwater. Depending on the Sr/Ca ratio of the primordium, the environment in which the larvae inhabited can be classified into Type 1 (Seawater type), Type 2 (Estuary type), and Type 3 (Freshwater type), respectively.
(3) Adult fish more than 1-year old migrated to the high salinity environment during winter, but went back to the river for feeding during summer. In addition, the environment in which the larvae inhabited influenced the habitat use of the adult fish, that is, Type 2 (Estuary type) and Type 3 (Freshwater type) individuals preferred the initial environment until sexual maturity. In general, mullets in the Tanshui River System gradually migrated to the middle and lower stream of the river as they grow.
(4) Monthly changes of gonadosomatic index (GSI) showed peak value (8 %) during October, but this value was extremely low, compared to the GSI (20 %) of the spawning group in Northeastern Taiwan. The egg cells of the mullets captured during the spawning season was in vitellogenic stage, and not in final maturation stage. Hence, the probability of mullets spawning in the river was low.
(5) The relationship between sex ratio, environmental salinity, and the food density showed that there were more female fish in the nutrient-rich environment, whereas in the nutrient-poor, more male fish were found. This phenomenon indicated that the habitat use of the mullets was influenced by sex.
In conclusion, the larvae or juveniles of mullets were wildly distributed in the Tansui River System, growing in the river until sexual maturity. As the fish matured, they migrated to the sea for spawning. Evidences from GSI and egg histology indicated that the possibility of mullets spawning in the river was extremely low. In addition to this, female fish preferred to food-rich environment for growth.
Subjects
Mugil cephalus
otolith Sr/Ca ratio
migratory behavior
habitat use
spawn
Tansui River
Type
thesis
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