Monterey Bay National Marine Sanctuary






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Changing Habitats and Associated Fish Assemblages of Elkhorn Slough

Fishes are relatively well-understood inhabitants of Elkhorn Slough and as both predator and prey, they play a critical role in the slough ecosystem. The slough provides habitat for a variety of ecologically, commercially, and recreationally important species, including year-round residents and many marine species from nearshore waters that enter the slough to feed, mate, and spawn. Several early studies on the slough's fish populations provide baseline information essential to long-term monitoring of its fish assemblages, their response to potential environmental changes, and their contribution to the nearshore fishery resources of Monterey Bay. More recent studies reveal that human impacts on Elkhorn Slough have fundamentally changed available fish habitat, resulting in changes to fish assemblages and an overall decline in diversity of both fishes and their prey.

The main channel of the slough extends in-land 7 km from the bay. Depth and width vary considerably with tidal height, but narrow and become shallower as one goes inland. Extensive mudflats fringe the main channel, and a network of tidal creeks meanders through the adjacent salt marshes.

Surfperches are the most diverse (14 species) and abundant group of fishes in Elkhorn Slough. (©1996 Monterey Bay Aquarium)

Increased rates of erosion due to changes in water current velocity in the slough—initially caused by the construction of the entrance to Moss Landing Harbor in 1947—have caused significant changes to the vegetated salt marshes, mudflats, main channel, and tidal creeks. For example, Long Canyon has increased 62 percent in width from 1980 to 1987 and con-tinues to erode. Rubis Creek has increased 8 percent in width during this time. The main channel also has broadened and deepened. Mudflat habitat that harbors dense assemblages of infaunal (living in the sediment) and epifaunal (living above the sediment) invertebrates likely is diminishing.

Distribution and abundance of fishes from six sites (Figure 1) surveyed monthly in the mid-1970s to 1980 and repeated twice in the 1990s indicate that changes have occurred. In the 1970s fishes were more abundant in the main channel than in the tidal creeks. Both abundance and species diversity were highest near the slough mouth. The upper reaches of the main channel featured intermediate levels of diversity and abundance, and the tidal creeks had comparable diversity but the lowest mean abundance. Surveys in the 1990s documented abundances that were >70 percent lower than levels in the 1970s at the deep channel sites. Conversely, 1990s abundance was two to four times higher at most sites farther up the slough.

Overall species diversity appears to have decreased throughout the slough in the past twenty years. While the deep main channel stations once had higher numbers of species than the shallow upper slough stations, diversity is now similar and relatively low at all sites.

In the 1970s the fish assemblages differed considerably among stations, mainly by depth and distance from the ocean. Twenty years later, however, these geographical differences have disappeared. Fish assemblages in the lower main channel are mostly unchang-ed, but assemblages in the tidal creeks now resemble those of the lower slough. These changes in fish assemblages coincide with the continued erosion and scouring of the slough during the last twenty years, which has resulted in a geomorphology of the tidal creeks that is now more similar to the main channel.

Figure 1: Fish sampling stations (•) in Elkhorn Slough.

Likewise, prey diversity, known from diet studies of key fish species in the main channel and tidal creeks, was noticeably lower in the 1990s than in the 1970s, with reduced use of infaunal worms and mollusks. For example, the English sole had the most diverse diet (forty-four prey taxa, including many infaunal worms) in the 1970s, but ate only fourteen prey items in the 1990s. Infaunal prey was largely replaced with epifaunal and mobile crustacea. Diets of fishes in estuaries therefore reflect changes in prey availability. Prey availability in the main channel has changed over the past twenty years, with an overall decrease in benthic invertebrates, especially infaunal worms, and an increased relative abundance of epifaunal crustacea.

As the main channel and tidal creeks of the slough continue to broaden and deepen, the species composition of fish predators and invertebrate prey apparently is being altered. From our interpretation of the studies on fishes of Elkhorn Slough from the 1970s to the present, it is obvious that changes in habitat significantly influence the distribution, abundance, and trophic patterns of the slough's fish assemblages.

For more information on the natural resources of Elkhorn Slough, visit this web site:

--Mary M. Yoklavich
NOAA/NMFS Southwest Fisheries Science Center

--Gregor M. Cailliet
Moss Landing Marine Laboratories (MLML)

(The authors relied in part on the MLML M.S. theses of Dion Oxman and Dave Lindquist.)

Citizen Monitoring Programs Provide Trend Data for Sanctuary Waters and Watersheds

Volunteers are an important part of water quality monitoring. (©Coastal Watershed Council)

Over the last year the number of citizen monitoring groups within Sanctuary boundaries has nearly doubled. With the creation of the Monterey Bay Sanctuary Citizen Watershed Monitoring Network, volunteer groups are coming together to discuss program design, data management options, and resources needed to provide better quality control for data being collected by volunteer groups. Much of the water quality data collected to date have shown that many of the streams emptying into the Sanctuary are achieving water quality objectives set by the Regional Water Quality Control Board (RWQCB). There has been documentation of some contaminants, however. Monitoring done through the Urban Watch Program has identified the presence of many urban contaminants discharging from storm drains into the Sanctuary, and ocean monitoring conducted by Surfrider Foundation has documented unsafe fecal coliform levels.

Fecal Coliform Monitoring at Ocean Bathing Areas
Surfrider's Blue Water Task Force collects weekly data at up to twenty-four swimming and surfing sites along the Santa Cruz County coastline. The samples are tested for the presence of fecal coliform bacteria. The safe water contact level is 200 bacteria per 100 ml. In 216 samples taken, Surfrider recorded thirty samples (13 percent) that met or exceeded state standards for swimmable waters and thus reflected unsafe conditions.

Local Streams Meeting Water Quality Beneficial Uses
The Coastal Watershed Council monitors salmonid streams in Santa Cruz, San Mateo, and Monterey Counties. Data collected by the programs are compared to the water quality objectives for dissolved oxygen, pH, turbidity, conductivity, and temperature set by the RWQCB. Over the last three years, data from the stream monitoring programs have shown that, of the seven stream systems regularly monitored, the majority are meeting the water quality objectives for cold water fisheries. Incidences when water quality objectives were not met accounted for less than 2 percent of 1,071 sampling events. Failing parameters included temperature, pH, dissolved oxygen, and turbidity. Natural factors usually played a role in objectives not being met.

Urban Watch Program Identifies Detergents as Primary Urban Contaminant
The Urban Watch Monitoring Program monitors storm drain outfalls for common urban runoff pollutants including detergents, phenols, copper, chlorine, ammonia, oil sheen, and sewage. Over the program's last two years, detergent has been the primary pollutant

regularly identified in sampling events. In 1997, during 43 sampling events, detergents were present in 20 (46 percent) of the samples. In 1998, during 76 sampling events, detergents were present in 75 percent of the samples. The next most common contaminant was phenols, occurring in 6 percent of samples in 1997 and 11 percent of samples in 1998. Sewage was not recorded in any samples and oil sheen was present in 10 percent of samples in 1997 and 11 percent in 1998. Chlorine and ammonia did not occur in 1997 and occurred in 10 percent and 18 percent, respectively, of samples in 1998.

--Donna Meyers
Coastal Watershed Council

Pescadero Marsh: An Expedition

Wishing we could continue the tradition of explorers like Lewis and Clark, this expedition was conducted on land and over water to survey Pescadero Marsh for the presence of invasive European green crabs (see related articles in the Exotic Species section). This initial foray was intended to begin a more comprehensive study of this important wetland area, as part of the Sanctuary's ongoing ecosystem monitoring program.

We embarked for Pescadero Marsh on an average October day. At 9:00 a.m. we stopped by the ranger station to let them know that "hunters," with appropriate permits, would be on the marsh, then weaved through five school buses in the parking lot off Highway 1 to reach our remote location. The rangers were happy to see our two-man expedition as, to their knowledge, no searches for green crabs had been conducted in the marsh.

Most scientists and resource managers are familiar with the critical shorebird and fish habitats of Morro Bay, Elkhorn Slough, and San Francisco Bay, but there is relatively little scientific information on Pescadero Marsh. This preliminary survey was a first step towards remedying that situation. Seeing the most enthusiastic teachers leading students from the buses, we wondered why they are wiser than scientists about visiting this estuary. We put on a good show by tying sixteen crab traps to our kayaks, then set the traps up a salinity gradient in the marsh towards Butano Creek.

While letting our traps "soak," we explored some of the surrounding upland habitats. Later in the afternoon we returned, in our kayaks, to the marsh to compare the Pescadero and Butano Creek arms of the estuary.

Pescadero Marsh is a highly modified system with a history of diking for creation of agricultural land. The California Department of Parks and Recreation is buying back historical marshes and initiating restoration. This will enhance habitat for the roughly 250 taxa of plants, 250 species of birds, twenty species of fish, and thirty-three species of amphibians and reptiles that have been known to use the marsh.

In our short visit up the more fresh water system of Pescadero Creek we saw close to twenty endangered western pond turtles, Kingfishers, Black Crown Night Herons, and fifteen other species of bird. The highlight of our survey to the Butano Creek side of the marsh was watching a rather cheeky Peregrine Falcon hunt off the bow of our kayaks. It first dove on a Snowy Egret, then switched its chase to Mallard Ducks before settling on a Killdeer. Pescadero Marsh is an important wildlife habitat and a truly beautiful place, with easy access.

We are happy to report that we did not catch any green crabs.

--Andrew DeVogelaere and William J. Douros,
Monterey Bay National Marine Sanctuary

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Last modified on: March 31, 2000