2015-11-10 - P&O Agenda - Item 7, Attach 2 - Assessment of Oyster Recruitment

and out-completing new oyster recruits for space by smothering (Walker et al. 2007; Walker
& Schlacher 2014). This can prolong the length of time for some species to become
established on sub-tidal reefs (Walker & Schlacher 2014).

Spatial differences in the recruitment of oysters throughout the study area is likely due to a
variety of factors including the supply of larvae from nearby adult populations (while not
specifically examined, this is a critical first step for successful establishment of reef
restoration), preference for settlement surface, post settlement survival (including
competition with other species such as bryozoans) and environmental factors such as the
degree of sedimentation, local currents and decreased in salinity from freshwater flows. The
greatest number of recruits occurred in areas that were generally well flushed with tidal water
and in close proximity to the mouth of the estuary.

A surprising result of this project was the lack of Sydney rock oysters (S. glomerata) recruited
to the settlement tiles, despite numerous rock oysters growing on hard engineered surfaces
(e.g. pylons, jetties and seawalls) within the Noosa Estuary (Figure 14). A variety of
environmental factors are critical for successful reproduction and recruitment of oysters, with
the failure of recruitment attributed to declining water quality and sedimentation. Ambient
environmental conditions such as salinity range are critical to the successful recruitment,
growth and survival of commercial oysters (Neil & Holliday 1988), Larval growth rates
generally highest at 23-39‰ and survival highest between 27 and 39‰ for Sydney rock
oysters (Neil & Holliday 1988). Adult Sydney rock oysters can tolerate and survive at salinity
levels from 15 to 55‰, which is within the salinity range found in Noosa estuary (Neil &
Holliday 1988). Failure of oyster recruitment in Pumicestone Passage, approximately 50 km
to the south of Noosa, has been attributed to a decline in water quality due to eutrophication
and agricultural runoff in the catchment, which causes immunosuppression and disease in
adults and disrupting reproduction and also increased sedimentation reducing the suitability
of subtidal settlement sites for larval settlement and recruitment (Diggles 2013). The Noosa
River Catchment is considered the best performing in South East Queensland (SEQ) in
terms of water quality, diversity of in-stream habitats and fish production (Sunshine Coast
Regional Council 2012) and therefore represents a location where restoration of functionally
extinct habitats (such as oyster reefs) will likely have a high chance of success (i.e. due to a
lack of other constraining factors) (Healthy Waterways 2013).

High rainfall and resulting freshwater runoff during the survey period may have been a
contributing factor to the lack of recruitment of Sydney rock oysters. It is likely that the high

Restoration of Noosa Estuary: An Assessment of Oyster Recruitment                                   16