What happens once wetland structure and function has been restored? How quickly do birds, fish, and other creatures begin using the new habitat, and which species are first responders? How valid are our assumptions about which habitat types are best for focal species? Three speakers wrapped up the restoration session on Wednesday afternoon with considerations of those questions.
First up, Sarah Estrella of the California Department of Fish and Wildlife discussed the status of the northern salt marsh harvest mouse (Reithrodontomys raviventris halicoetes) in Suisun Marsh. Listed as endangered since 1970, the rodent is “characterized by cuteness,” reddish-brown fur, and a docile disposition. “In the past, there was thought to be a strong association between the mouse and pickleweed,” she said. “Those of us working in the marsh suggested they were not so limited in habitat.” A two-year mark-and-recapture study compared abundance, reproduction, and survival in pickleweed versus mixed vegetation (bulrush, fat hen, alkali heath, and others), and in tidal marsh versus diked wetlands. Among other results, harvest mouse densities were found to be higher in mixed vegetation than in pure pickleweed, at least in tidal areas, and higher in diked than in tidal marsh. However, tidal marshes did have higher post-winter survival. A more recent telemetry study challenged another assumption: “We had assumed they move up in elevation at high tide. Instead, they primarily remain in emergent vegetation over standing water when the tide comes in.” Estrella noted that the draft US Fish & Wildlife Service recovery plan covering the mouse calls for extensive conversion of diked wetland to tidal: “Even though diked wetland supports higher densities, it’s considered lower-quality habitat due to its artificial state. Additonally, many habitat models used in environmental documents fail to recognize the spectrum of vegetation types the mice thrive in.” Her recommendation: “Habitat management efforts should include mixed vegetation types, both tidal and diked wetlands, and areas where sea level rise can be accommodated.”
The most conspicuous inhabitants of wetlands, birds are easier to monitor than mice—and their responses can help reveal unintended consequences of restoration and inform adaptive management. Catherine Burns of the San Francisco Bay Bird Observatory reported on three studies of avian reactions to the South Bay Salt Pond Restoration Project. California gulls (Larus californicus), whose populations have increased exponentially in the South Bay, prey on the eggs and nestlings of other birds, including sensitive species. Burns reported that the breaching of Pond A6 in the Alviso complex displaced one of the largest gull colonies. That eased predation pressure on a nearby colony of Forster’s terns (Sterna forsteri), where chick survival increased. Snowy plovers (Charadrius nivosus) pose a different kind of management dilemma, since they nest on salt pannes that will be converted to tidal marsh. “The South Bay project aims to increase plover numbers while decreasing preferred breeding habitat, packing more of them into a smaller area,” Burns explained. Plovers are also vulnerable to gulls and other predators. The good news: nests in experimental plots covered with oyster shells show higher survival rates. As for one downside of restoration, Burns reported that analysis of Forster’s tern eggs confirm concerns that the process would mobilize mercury in pond sediments. From 2010 to 2011, mercury concentrations in tern eggs increased by an average 74 percent in restored ponds as compared with reference ponds, and exceeded toxicity thresholds 100 percent of the time. However, mercury levels in American avocet (Recurvirostra americana) eggs at the same sites did not change. “It’s been challenging,” she summed up. “We need to analyze the responses of a variety of waterbirds. It’s not going to be easy to sort out all these responses. We’re really learning a lot through monitoring.”
The session’s final speaker, UC Davis fish biologist James Hobbs, gave an update on how fish are using South Bay salt ponds that have been reconnected to the Bay. Bottom line: “The restored ponds are quickly used by fish”—and over 85 percent of the 30,000-plus fishes sampled were native species. Restoration has provided over 1,800 acres of new subtidal habitat. Since 2010, Hobbs and his crew have been surveying recently breached ponds at Bair Island, the Alviso/Coyote Slough complex, and Eden Landing, using trawls and hook-and-line angling. Numbers and diversity are highest at Alviso, the largest complex, and lowest at Eden Landing, breached only in 2010. Although the species mix varied among the sites, Pacific staghorn sculpins (Leptocottus armatus) were the most abundant in all three. Hobbs also reported seasonal changes in the species assemblage, with more pelagic types in winter. Overall, fish that can tolerate low dissolved oxygen are more abundant. The survey documented the highest abundance of mysid shrimp, the favored prey of the threatened longfin smelt (Spirinchus thaleichthys), in the Estuary. Leopard sharks (Triakis semifasciata) are visiting the Eden Landing pond: “The pond breaches serve as predator hotspots, which is good as long as the predators are native. Pond production is translating up the food chain, providing benefits to fish in the Bay.”