Restoration of native oysters in a highly invaded estuary

Background/Question/Methods

One of the main methods used for restoration of native oyster populations is the provision of hard substrate, which is limited in California’s mostly soft-sediment estuaries. However, many non-native species also require hard substrate and may compete with native oysters for settlement space and/or overgrow oysters after settlement. A challenge for restoration in these systems is to enhance native species while discouraging the growth of non-natives. At Elkhorn Slough on California’s central coast, oysters are found higher in the intertidal zone than many non-native species, particularly sponges and tunicates, on which oysters won’t settle. In addition, most non-natives appear to do less well than oysters in locations where hard surfaces collect a layer of mud. Most tunicate species have relatively short dispersal distances; recruitment to new substrates is limited by distance from established populations. Thus, selective placement of restoration substrate could potentially limit the settlement of non-native species while still attracting oysters.

As part of a two-year restoration project, we tested the effects of tidal height, sediment, and distance from adult populations on oysters and other sessile species: placing substrates at high and low tidal elevations, in muddy vs. rocky locations, and close to and far from established populations.

Results/Conclusions

Oysters settled at both high (+0.5 ft above mean lower low water) and low (-0.5 ft) elevations. Settlement was higher at the lower elevation, but there was no difference in oyster survival by tidal height. There was a trend toward less non-native cover (which consisted mainly of bryozoans, tunicates and sponges) in the high vs. low elevation in Year 1; this difference was significant in Year 2. There were no differences in cover of oysters or non-natives at muddy vs. rocky sites, but the relative cover of oysters vs. non-natives was higher at locations far from vs. close to adult populations.

In this system, substrates placed in the higher intertidal zone may give native oysters an advantage over the non-native species. As survival of oysters was as high at the upper elevation than the lower one, a potential option for small-scale restoration projects would be to move substrates to the high zone after recruitment occurs. This would take advantage of the greater oyster recruitment that occurs in the low zone, but decrease the cover of non-native species, which presumably would not survive such a move. Future research is needed to determine the optimal tidal height that would achieve both goals.