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Research Report 9
Woodland DJ & Ward PJ
Supervising Scientist 1992
ISSN 0810-9966
ISBN 0 644 25632 X
Physico-chemical conditions, changes in fish communities and characteristics of species populations of eight permanent sandy pools along Magela Creek during the 1981 Dry season are described. Causes of mortality in each species, especially Craterocephalus marianae, were investigated.
Temperatures at the bottom of the pools ranged from 22.0-35.5° C. Dissolved oxygen concentrations (measured at 0800 - 0930h) were 0.8-6.7 mg/L at the bottom and 0.9-7.3 mg/L at the surface. During the same period: chlorophyll a concentrations were 6- 178 m g/L; chlorophyll b concentrations 1-60 m g/L; chlorophyll c 0-7 m g/L at the surface. Suspended solids values were 3-58 mg/L; conductivities were 22-61 m S cm/L at the surface and 28-62 m S cm/L at the bottom. The pH values ranged from 5.3-7.7 at the bottom and 5.5-7.4 at the surface. Most of these parameters were very stressful for fish in the pools during October. In most water samples copper, lead and uranium were present in comparatively low concentrations(<1.6 ppb); concentrations of cadmium, zinc and particularly manganese were higher (:5 120 ppb). In sediment samples concentrations of manganese, zinc and copper were comparatively high (£ 12.8 mg/kg).
The fish communities of the pools were diverse considering the apparent low heterogeneity of the habitat and the small size of the pools; most species appeared to be at high densities. Despite stressful conditions during the study period, all except one of the 20 species survived in at least a few pools - the exception was Melanotaenia nigrans. Major characteristics of the fish populations were: in biomass, Leiopotherapon unicolor and Nematalosa erebi were the dominant species; C. marianae was the most numerically abundant species at the beginning of the study period; most species were present in the pools as large juveniles or adults only, but C. marianae, Glossogobius giurus and Melanotaenia splendida occurred as fry and small juveniles as well; except for Strongy1ura krefftii and G. giurus, all species examined for reproductive characteristics were ripe in November; a few species (N. erebi, Amnialaba percoides, G. giurus, C. marianae, Glossamia aprion) spawned during the study period; condition factors were generally low by comparison with other studies, except for G. giurus, C. marianae, C. stercusmuscarum and G. aprion.
Mortality waslow(<50% of the original population) in most pools. In populations that did suffer high mortality, anoxic conditions may have been an important cause of mortality for C. marianae, A. percoides and Pingalla midgleyi. Predation by other fish was an important cause of mortality for at least one species (C. marianae); however, predation by birds was rare, especially as the Dry progressed. Condition factors and stomach fullness were low for most species, but, starvation was probably not a cause of mortality, except perhaps for L. unicolor in one pool.
These data suggest that pools might be important for the survival of P. midgleyi and C. marianae which are endemic in the Region. C. marianae might be useful as an 'indicator species' because it is particularly sensitive to heavy metals-although more data on lethal and sub-lethal tolerances are required.