SIMoN
  Sanctuary Integrated Monitoring Network
Monitoring Project

Influence of varying tidal exchange on the fish and crab assemblages of Elkhorn Slough

Principal Investigator(s)

  • Amy Ritter
    Elkhorn Slough National Estuarine Research Reserve
  • Rikke Preisler
    University of California, Santa Cruz
  • Kerstin Wasson
    Elkhorn Slough National Estuarine Research Reserve

Funding

  • SIMoN
Start Date: April 01, 2005
End Date: September 01, 2005

Among the most important and threatened ecosystems of the coastal environment are estuaries, which constitute a transition zone between terrestrial and marine environments where fresh water mixes with sea water. Tidal flow and freshwater input into estuaries have often been restricted via the construction of dams, causeways, and dikes--practices that can significantly alter the habitats and associated biotic communities of estuaries. Although estuaries are known to provide critical habitat for a variety of fish and crustacean species, very little research has focused on how manipulation of tidal regimes and freshwater input influences these organisms.

This study investigated how assemblage structure and species distribution and abundance patterns of fishes and crabs are influenced by variation in tidal flow and freshwater input throughout shallow-water habitats in the Elkhorn Slough estuary. In April and August of 2005 we surveyed fish and crab abundance patterns throughout 18 locations in Elkhorn Slough fall into one of three categories: full tidal flow, muted tidal flow, very muted tidal flow/seasonally high freshwater input. At each of these locations we used three different methods of data collection, including beach seines and two types of minnow traps. Fish and crab abundance patterns varied across habitat types and across seasons depending upon which species were considered. The very muted/seasonally high freshwater input sites were most different in terms of species composition and abundance patterns compared to the other two flow regimes, but there were also differences between the full and the muted flow sites in regards to these factors.

Overall, the results of this study suggest that if the management goals for estuarine habitats include maintaining high diversity of fish and crab species, appropriate management strategies should consider preserving a mosaic of habitat types that vary in tidal flow and freshwater input.

Summary to Date

Fish and crab abundance patterns varied across habitat types and across seasons depending upon which species were considered. The very muted/seasonally high freshwater input sites were most different in terms of species composition and abundance patterns compared to the other two flow regimes, but there were also differences between the full and the muted flow sites in regards to these factors.

Monitoring Trends

  • The very muted (minimal flow) sites tended to be dominated by three-spine sticklebacks and long-jaw mudsuckers, and had few to no crabs.
  • The muted (low flow) and full tidal flow sites tended to be dominated by more marine-types of fish species, including topsmelt and bay and arrow gobies, as well as a few different crab species, including the non-native European green crab and the native Hemigrapsus crab.
  • The endangered species Tryonia imitator (brackish snail) and Eucyclogobius newberryi (tidewater goby) were only found at minimal flow sites.

Discussion

The results of this study are being incorporated along with the results of several other studies (not funded by SIMoN) generated by several additional collaborators into a synthesis manuscript describing how various taxa respond to variation in tidal exchange. This manuscript should be available by summer 2006. There may be an additional manuscript created examining only the fish assemblage patterns.

Study Parameters

  • Habitat
  • Abundance
  • Habitat association
  • Non-indigenous species
  • Distribution
  • Density
  • Size structure

Study Methods

Rectangular minnow traps: 0.81 m long by 0.61 m wide by 0.28 m deep steel frame traps with 1.27 cm polyethylene mesh. 3 replicate traps at each site, baited with 1-2 anchovies.

Small-mouthed plastic minnow traps: 0.43 m long by 0.23 m wide (widest diameter) plastic traps with 4.8 mm mesh and a 22 mm diameter entrance hole. 3 replicate traps at each site, baited with 1-2 anchovies.

Large-mouthed plastic minnow traps (spring only): same dimensions as small-mouthed traps except with larger entrance hole. 3 replicate traps at each site, baited with 1-2 anchovies.

Seines: 3 minute-long tows with a 25 ft long by 2 m deep bag seine, composed of a 3-mm square delta mesh. 3 replicate tows at each site

Figures and Images

Figure 1. A view of a study site adjacent to the Packard property.


Figure 2. A field assistant sets two kinds of crab traps near the southern end of the Moss Landing harbor.


Figure 3. Drs. Ritter and Lonhart drag a seine net through a very shallow section of the slough on the Packard property.


Figure 4. A colorful three-spined stickleback.