Ecological Role of Grizzly Bay as Habitat for Resident Fishes

Project Chief: Larry Brown

Shallow-water habitats (shoals) are believed to provide important nursery areas for young fishes in Suisun, Grizzly, and Honker Bays; however, recent drifter studies in Honker Bay and Grizzly Bay suggest that water residence times in the shoal areas are very short. Given these results, there are four alternative hypotheses regarding the possible benefits of shallow water habitats to young fishes in Grizzly and Honker Bays: (1) no benefits accrue because organisms cannot maintain position there; (2) benefits accrue due to random transport onto the shoals by the tides; (3) benefits accrue due to active position maintenance in shoal areas; (4) benefits accrue by selective movement (or, transportation by tidal currents) between Honker Bay (on flood tides) and Grizzly Bay (on ebb tides). Ongoing USGS projects, including deployment of hydrodynamic instruments in the area of Grizzly Bay, provide a unique opportunity to evaluate the importance of shoals to zooplankton and young fishes.
This study has the following objectives:

1. Determine if zooplankton and larval/juvenile fishes maintain abundance in the SWH in Grizzly Bay.
2. Determine if zooplankton and larval fish are passively transported by hydrodynamic forces onto (or off of) shoals, or if they exhibit behaviors that enable them to maintain position in the shoals.
3. If larval/juvenile fish are able to maintain position in the shoals, determine if they accrue a benefit, defined as an increased growth rate or better condition.
4. Determine the species and life stages of zooplankton consumed by larval/juvenile fishes and determine fish feeding rates.
5. Determine if Suisun (or Montezuma) Slough provide a conduit for the input or export of organisms to Grizzly Bay.

Data to address these objectives are collected by intensive sampling of zooplankton and small fishes during two sampling cruises, one during a spring tide and one during a neap tide. Sampling occurs over the entire tidal cycle (about 30 hours) at approximately hourly intervals. Sampling is done at key hydrodynamic sites using nets towed at one or more depths to determine patterns of longitudinal and vertical movement of organisms. Patterns in fish densities then are combined with hydrodynamic data collected concurrently to determine patterns of fish movement. These data also are used to construct a bioenergetics model of fish production in shoal habitat.

Contact Information

Larry Brown
Office phone: 916-278-3000

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