Reproduction in the freshwater mussel Velesunio angasi in response to the release of water from Ranger Uranium Mine to Magela Creek
Technical Memorandum 49
Humphrey Christopher Laing
About the report
Biological studies of the population of freshwater mussel, Velesunio angasi, inhabiting the braided, sandy channel of the Magela Creek about the Ranger outflow pipe, were undertaken during the 1984/85 Wet season (March-May 1985). In particular, reproductive activity of female mussels was monitored to determine whether or not activity was altered by the release of water from Retention Pond No. 4 (RP4) that occurred between 12-23 March 1985. For assessment of reproductive activity, female mussels from the channel receiving RP4 waters were sampled above and below the outflow pipe prior to, during and after release. Assessment was based on: (1) examination of mussels for the presence and stages of development of the glochidial larvae that are nurtured in marsupial portions of the inner gills; and (2) determination of the amount or quantity of larvae present in the marsupia.
Prior to the RP4 release, reproductive activity of mussels was similar upstream and downstream of the outflow pipe. During the release (five days after commencement) however, an effect was observed in mussels sampled from four sites between the point of release and 120 m downstream. Over this distance, a decrease in both the proportion of females bearing embryos and larvae and in the quantity of larvae present in the marsupia was found in mussels sampled at increasing proximity to the source of discharge waters. These observations were in contrast to the higher reproductive activity of mussels sampled simultaneously, upstream of the release pipe. In particular, fewer larvae were present in the marsupia of females sampled from all downstream sites in contrast to those found in the marsupia of females sampled upstream. These patterns at downstream sites were evidence of suppression of gonadal activity and larval production during the RP4 release, whilst an unnatural occurrence of few glochidia in the marsupia suggested either abortion or unusually protracted release of larvae.
In mussels sampled after the RP4 release, the stage of larval development at the two downstream sites closest to the pipe (within 50 m), though similar among mussels from the same site, differed between the two sites, and both differed from all of the other sites sampled. Such observations, together with those made during and after release, indicate that marsupia of females from these two sites were empty for a total period of about 12 days, including a period of 8- 9 days after RP4 release. Recovery was slowest in mussels from the site closest (< 20 m) to the release point.
The decrease in reproductive activity of mussels found in downstream sites, at closer proximity to the source of RP4 release, was correlated with increasing electrical conductivity. Elevated water temperatures (up to 0.5° C) were found at the first two sites sampled downstream of the release pipe. Warmer waters, however, should have the effect of increasing the intensity of larval production.
The constituent(s) in RP4 waters responsible for suppression of reproductive activity in V. angasi was not identified. Early indications are that the same reduced reproductive activity was also present in the mussel population resident in RP4. The digestive glands of mussels native to RP4 and of creek mussels sampled within 50 m downstream of the release point during the RP4 release, were coloured a dark red. This colouration and an orange-red pigment present in fluids secreted by the gland were unique to these mussels, not having been observed among mussels from other waterbodies of Magela Creek.
It is argued that any suppression of reproductive activity per se caused by RP4 (or other mine waste-water) releases is unlikely to result to any significant degree in diminishing stocks of mussels downstream of the point source of release. However, such releases could adversely affect newly-metamorphosed juvenile mussels that are recruited downstream of this point during the Wet season should this life-stage also prove to be sensitive to Ranger mine waters. The suitability of reproductive activity in V. angasi as a biological monitor of possible adverse effects upon the biota in future releases of Ranger waste waters is advocated.
Associated studies of the population of V. angasi in the Magela Creek channel during the 1984/85 Wet season included estimation of mussel density and determination of environmental influences of larval production. A mean density estimate of 7.1 mussels (> 0-year old)/m of creek length (all braids included) was derived for a 50 m reach of creek sampled intensively downstream of the release pipe. It is likely that the shell-free dry weight of V. angasi in the Magela Creek channel exceeds that of all other benthic macroinvertebrates. Larval production, as measured by gravidity (ie % of all females sampled at any one time bearing embryos and larvae in the marsupia), was found to be significantly correlated with water temperature.