Estuaries as juvenile habitats for lutjanid and serranid fishes in tropical northeastern Australia

Sheaves, Marcus John (1995) Estuaries as juvenile habitats for lutjanid and serranid fishes in tropical northeastern Australia. PhD thesis, James Cook University.

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A number of lutjanid and serranid fishes are thought to utilise tropical estuaries as juvenile habitats. However, little detailed biological or life-history information exists for any species, and the species compositions of these families inhabiting tropical estuaries, are poorly known. During this study, the species compositions and life-histories of lutjanid and serranid fishes inhabiting estuaries along the north-eastern coast of tropical Australia were investigated. Using fish-traps, estuary faunas were compared to those inhabiting near-shore reefs. Additional samples were donated by anglers and collected from estuaries by angling. The reproductive statuses, and size and age structures of two serranids (Epinephelus coioides and E. malabaricus) and two lutjanids (Lutjanus russelli and L. argentimaculatus), in estuaries were compared to those of the same species from offshore. Fish-traps were also used to obtain data on the distribution and abundance by size of L. russelli, E. coioides and E. malabaricus from three estuaries - Cattle, Barramundi and Alligator Creeks - over a two year period.

Far fewer species of lutjanids and serranids were trapped from estuaries than from nearshore reefs. While fish-trap and angling collections from estuaries produced 9 species of serranids and 5 species of lutjanids, most were collected in low numbers. Only two serranids (Epinephelus coioides and E. malabaricus) and two lutjanids (Lutjanus russelli and L. argentimaculatus) were common in either trap or angling catches. All fish of each of these species from estuaries were found to be much smaller and younger than the largest and oldest fishes of these species from offshore waters. Furthermore, all fish of each species from estuaries were found to be in pre-reproductive condition. This implies that the estuarine populations of these species consist of juveniles, and that they undergo migrations to offshore adult habitats. Thus they possess three distinct life-history stages (pelagic larvae, estuarine juveniles, offshore adults) that correspond to major habitat shifts.

L. russelli were common in all estuaries and distributed throughout the three estuaries studied in detail. The probability of capturing L. russelli was similar in seaward areas of Cattle, Barramundi and Alligator Creeks, remained similar upstream areas of Barramundi Creek, but fell markedly in upstream parts of Cattle Creek. Spatial differences in the size of L. russelli (both within and between estuaries) were small, however, there was a strong pattern of seasonal change in the size of L. russelli in all estuaries. This seasonal pattern was apparently a product of the interaction between recruitment, mortality and migration. Studies in Alligator Creek showed that L. russelli demonstrated a strong preference for structurally complex habitats provided by fallen timbers and mangrove roots.

The numbers of both species of Epinephelus in trap catches declined in upstream areas of the three estuaries considered in detail. However, this reduction was much more marked for E. coioides than E. malabaricus, and while in downstream areas, the numbers of E. coioides were similar to or greater than those of E. malabaricus, in upstream areas E. malabaricus dominated. This suggests that the two species differ in their abilities to access or remain in upstream areas of estuaries. There was a strong negative correlation between the maximum deviation of salinity from 'normal' seawater levels and catches of both species of Epinephelus, suggesting that long-term salinity variation may be important in determining the distribution and abundance of Epinephelus spp. within estuaries.

The occupation of specific habitats during particular periods of development must be considered in the development of management strategies for these fishes. The use of estuaries as juvenile nursery grounds underlines the importance of maintaining the quality of estuarine habitats. More data from localities further afield (both within Australia and overseas) are needed to determine if the reproductive patterns found here apply to these species in other areas or to related species.

Item ID: 24125
Item Type: Thesis (PhD)
Keywords: abundance; Alligator Creek; distribution; estuaries; fish traps; fishes; habitat preferences; monthly variation
Date Deposited: 13 Dec 2012 02:33
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960802 Coastal and Estuarine Flora, Fauna and Biodiversity @ 100%
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