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University of Queensland (2010)

Impacts of Salinisation on Freshwater Fauna and Ecosystems in South-East Queensland

Rajesh Prasad

Titre : Impacts of Salinisation on Freshwater Fauna and Ecosystems in South-East Queensland

Auteur : Rajesh Prasad

Grade : Doctor of Philosophy (PhD) 2010

Université de soutenance : University of Queensland

Résumé
The issue of dryland salinisation is of major concern for Australia. While the economic impacts of salinisation on agriculture and infrastructure have received much attention, the long-term effects on freshwater ecosystems are less well known. If there is a sudden rise in salinity to levels beyond the tolerance limits of the aquatic fauna, animals will perish. This study was undertaken to assess the salinity tolerance of some of the aquatic fauna from the south-east Queensland region and to examine the impact of sublethal salinities on two well known fish species. The salinity tolerance of 10 macroinvertebrates tested ranged from 13.1 to 34.2 mScm-1. Aquatic animals are known to acclimatise to new tolerance limits if the salinity rise is gradual. Acclimation experiments conducted on two mollusc species that included Physa acuta and Corbicula sp. showed that their 72hr - LC50 value was raised by 30 and 77 % respectively. The impact of sublethal salinities were measured in term of growth and reproductive capabilities of two native Australian freshwater fishes, the Eastern Rainbowfish, Melanotaenia splendida splendida and Firetail Gudgeon, Hypseleotris galii. Both these species are commonly represented in freshwater fish assemblages and are widely distributed in the coastal drainage areas of north-eastern Australia. Eastern Rainbowfish, M. s. splendida tested during this study was found to have a direct transfer salinity tolerance of 17000 mgL-1 (25 mScm-1 – conductivity). Hypseleotris galli exhibited a similar transfer tolerance of 18700 mgL-1 (27.5 mScm-1 – conductivity). Over a period of 114 days, medium (15 mScm-1) and high (25 mScm-1) salinity treatments had an impact on the growth rates of M. s. splendida. In terms of length increase, fastest growth was achieved in the creek water control (CW), but growth in 25 mScm-1 water was only 65 % of that in CW and 67 % of that in filtered tap water (FW). Patterns of weight change followed a similar trend, with fastest growth in the CW but occurring at only 59 % of the rate in the 25 mScm-1 treatment. Growth trends for H. galii in salinity treatments paralleled those for M. s. splendida. There was a clear trend of reduced growth with increased salinity. The highest total increase in body length over 118 days was observed in FW, and the lowest in the 25 mScm-1 treatment. Length increase in the salinity treatment of 25 mScm-1 was 62 % of that in FW and 63 % of that in CW. Change in weight also showed the pattern of slowing of growth with increase of salinity. Histological examination of the gonads was used to study the effect of salinity on reproduction. In M. s. splendid females, there was a significant effect of salinity on the numbers of Stage I and Stage III oocytes. At the 35 day point, high numbers of Stage I oocytes in the 15 and 25 mScm-1 salinity treatments corresponded with low numbers of ripe, ready to ovulate Stage III oocytes. In male M. s.splendida, salinity had no statistically significant effect on sperm development. In H. galii, there was no significant effect of salinity on Stage I oocytes. However, there was a significant treatment effect on Stage II and Stage III oocyte numbers. There appeared to be a pattern of more Stage II oocytes in the 15 and 25 mScm-1 salinity treatments at the 118 day point that corresponded with low numbers of ripe, ready to ovulate, Stage III oocytes in the same treatments at the same time. The mean numbers of Stage III oocytes in 15 and 25 mScm-1 treatments were significantly different from the mean oocyte numbers in controls, at time 118 days. As far as sperm development was concerned, the results for different times and different treatments were mixed. Experiments conducted to determine the effects of salinity on spawning showed that the number of oocytes spawned by Eastern Rainbowfish in 15 and 25 mScm-1 were consistently lower than in the controls and 2.5 mScm-1. However, there were no significant differences in the mean numbers of oocytes spawned in the FW, 2.5, 15 and 25 mScm-1. One of the major findings of this study was the impact of salinity on oocyte viability. It was found that the proportion of unviable oocytes increased from the day of spawning to the day of hatching, in 15 and in 25 mScm-1, in comparison with FW and 2.5 mScm-1. The percentage of unviable oocytes in 25 mScm-1 salinity treatment was over 90 % compared to other treatments and controls. Further experiments were conducted, under three different regimes, to assess the impact of salinity on oocyte hatching. The three experimental regimes were : i. oocytes spawned in salinity treatments → hatching in salinity treatments ii. oocytes spawned in salinity treatments → hatching in filtered tap water iii. oocytes spawned in filtered tap water → hatching in salinity treatments In the salinity → salinity experiment, oocyte hatching was poorest in the salinity treatment of 25 mScm-1. In salinity → filtered water experiment, hatching was poorest in the 25 mScm-1 salinity treatment. However, when the oocytes were spawned in filtered tap water and hatched in salinity treatments (filtered water → salinity) hatching rates were almost same in treatments and controls. This result gave a clear indication that salinity impacted upon the oocytes immediately after spawning. The osmolarity measures of fish tissue and the salinity treatment solutions and control water showed that the fishes maintained their internal fluid osmolarity within a narrow range regardless of the osmolarity of the external medium. This osmolarity range was between 380 and 470 mOsmkg-1. However, the osmolarity of the oocytes was highly inconsistent especially in high salinity treatments. In the controls and the salinity treatment of 2.5 mScm-1, which had osmolarity values of approximately 50 – 150 mOsmkg-1, the osmolarity of oocytes ranged from approximately 220 – 320 mOsmkg-1. In the salinity media of 15 and 25 mScm-1, osmolarity of the oocytes ranged from 200 – 550 mOsmkg-1 Most larvae hatched seven days after the oocytes were spawned. Larval survival thereafter was very poor regardless of salinity. At day nine, two days after hatching, despite some larval mortality total survival remained above 60 % in all treatments. However, by day 10, survival was reduced to below 60 % in all treatments. Based on the results of this work, there is a clear indication that elevation of salinity would have detrimental impacts on freshwater ecosystems. The major impact would be decrease in species composition at various trophic levels in the food web. The salinity tolerance of macroinvertebrates showed that there is much variation in tolerance level of families and species. Sensitive species would perish with only small increases in salinity. If the salinity increases are gradual, many species would be able to acclimatise and tolerate salinity levels beyond their normal tolerance range. However, there would be a limit to their extent of tolerance. If the salinity increases are sudden the impact would be on a greater percentage of species. Based on past research and this study it appears that majority of freshwater fishes are more tolerant of salinity than most freshwater invertebrates, however, this study showed that sublethal salinity would still have strong deleterious impacts, regardless of their salinity tolerance limits. Decreased growth rate is a clear indication of stress. Sustained stress over prolonged periods would lead to decrease in fitness, decease in reproductive capability, decrease in competitiveness, reduced ability to find and capture prey, and increased susceptibility to disease and infections. This study showed that oocytes and larvae of fishes were strongly affected by salinity levels that were within the tolerance range of adults. The most crucial stage for the oocytes was immediately after spawning. Oocytes have no means of retaining its internal osmolarity and it was found that osmolarity of the oocytes was totally destroyed in saline solutions. This led to increase in oocyte mortality or oocyte un-viability in 15 and 25 mScm-1 salinities. Stressed and weakened adults together with low production of new recruits would have serious implications for sustaining fish populations in salinity affected ecosystems, over long term. In each subsequent generation, the population size of affected fish would shrink further, leading to complete elimination of fishes in affected areas. Although much more research is required to acquire more information on the impact of salinity on more of the freshwater fauna, this study was able to elucidate the effect of elevated salinities on some macroinvertebrates and various stages of fish’s life cycle. The methods and results obtained should be used as tools for management of freshwater ecosystems threatened by salinisation in Australia and elsewhere around the world.

Mots-clés : salinity ; macroinvertebrates ; fish ; tolerance ; sublethal ; osmolarity ; oocytes ; larvae

Présentation

Page publiée le 24 mai 2011, mise à jour le 3 juillet 2017