The influence of sediment compositions on the Western School Prawn Metapenaeus dalli in a temperate Australian estuary

The composition of the sediment is known to influence the distribution of estuarine biota, as it can provide both habitat and, either directly or indirectly, a food source for invertebrates and fish. While the flora and fauna of estuaries are often well studied, less focus is typically placed on elucidating the composition of the sediment. Thus, this study characterised and classified the sediments of the nearshore and offshore waters of the Swan-Canning Estuary. Multivariate statistical analyses indicated that the composition of the sediment from 20 nearshore and 16 offshore sites, in two seasons, differed significantly among water depths, seasons and sites, with seasonal changes only being detected in offshore waters. Differences in sediment composition between water depths were particularly marked due to higher contributions of particulate organic matter and finer inorganic grain sizes present in offshore waters. A quantitative classification of the sediment composition using the CLUSTER and SIMPROF procedures identified 11 significantly different sediment types, which were used to produce a comprehensive map of the sediment types present within the Swan- Canning Estuary. A decision tree was then constructed, which provides a set of quantitative thresholds to allow for the sediment type to which any new sample (site) from within the estuary belongs to be identified. As well as a marked decline in the abundance of the Western School Prawn (Metapenaeus dalli) in the Swan-Canning Estuary since the 1980s, anecdotal evidence from recreational fishers also indicated that there had been a spatial shift in their distribution within the estuary, away from the upper reaches. In the past, migration into these areas was thought to be driven by an increase in salinity. Declining rainfall has resulted in this change in salinity occurring earlier and it is hypothesised that a change in sediment composition may be responsible for the shift in the abundance of M. dalli away from these upper reaches. To test this theory, the density of M. dalli was recorded monthly between October 2013 and August 2014 at 36 sites representing each of the 11 sediment types. The results demonstrated that the density of M. dalli differed among sediment types in summer, but not winter, perhaps due to the homogeneity of the sediment types in the latter season. No difference in the average length of M. dalli among sediment types was detected in either season, suggesting that the prawns are not spatially distributed by size classes within the estuary. Laboratory experiments, conducted under controlled conditions, were used to investigate whether M. dalli exhibited a preference for one of two sediment types one from the upper reaches of the spatial extent of the study (i.e. Garratt Road Bridge) and one from the lower reaches (i.e. Dalkeith). The results demonstrated that prawns did exhibit a preference for both nearshore and offshore sediments that contained a lower percentage contribution of larger grain sizes and/or a higher percentage contribution of finer grain sizes (i.e. those at Dalkeith). These experiments also revealed that emergence and activity rates of M. dalli are strongly related to photoperiod, with individuals preferring to remain buried during daylight hours and become active during darkness. Visual observations also indicated that the prawns were able to bury more rapidly in finer than coarse sediments, thus reducing their length of exposure to predators. In summary, this study has provided evidence to show that the composition of the sediment does influence M. dalli and that this species exhibits a preference for finer sediments in the lower reaches of the estuary, thus supporting the hypothesis that a change in the sediment composition may be responsible for the shift in spatial distribution of M. dalli in the Swan-Canning Estuary.