Grunion Spawning Versus Beach Nourishment: Nursery or Burial Ground?by Susanne Lawrenz-Miller, Cabrillo Marine Museum, San Pedro, United States,
Abstract: At Cabrillo Beach in San Pedro, California, a major sand renourishment project was scheduled to commence during the annual grunion spawning season, which occurs from February through August. Since grunion lay eggs directly on the beach, massive sand placement could cause disastrous impacts on the reproductive cycle of this highly prized fish. Because little prior knowledge exists which could be used to devise a sand placement plan minimizing negative impacts upon the reproductive success of grunion, a study was begun in 1990 on the spatial and temporal spawning patterns of the grunion on this beach. The study also provides baseline data to assess the impact of the sand replenishment project on the reproductive success of the grunion in succeeding years. Clutches of eggs were counted using trench transects placed at intervals along the beach throughout the spawning season to determine densities and preferred location of spawning. Beach slope, sediment composition and thickness and temperature were monitored. The development and hatching success of eggs were quantified by microscopic analysis throughout the season. The study, extending from late April through July 1990, revealed high egg densities from April to mid May with a sharp decline thereafter. Hatching rates were very low in egg samples collected from the heavy spawning in late April and much higher in subsequent samples. Spatial distributions varied considerably from one spawning cycle to another.
Subject Headings: Beach nourishment | Sandy soils | Sand (hydraulic) | Beaches | Seasonal variations | Spatial distribution | Scheduling | Fish management | North America | California | United States
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