UC San Diego Library – Scripps Digital Collection

Sea surveys were initiated in the spring of 1965 to obtain estimates of population size, mortality rates, and growth rates of the northern California ocean shrimp population. From March 1965 through September 1968, nine surveys were conducted in a 270-square-mile area from off Mad River, California, north to Smith River, California. Four surveys were conducted in a 105-square-mile area off southern Oregon during 1967 and 1968. Estimates of the California population ranged from a low of 2.2 million pounds in the fall of 1966 to a high of 8.1 million pounds in the fall of 1967.

Survival rates, derived from natural mortality rates for ocean shrimp during their second winter of life, for the 1964 and 1965 year classes were 0.68 and 0.55 respectively. Lowest survival rates were obtained from the 1963 and 1964 year classes during their third winter of life. The highest fishing mortality rates were observed for shrimp during their third summer in the fishery. Survey data indicate a relationship between annual fishing mortality rates and total trawling hours expended by shrimp fishermen. Limited data suggest a relationship also exists between the number of spawners and 1-year-old recruits as well as the existence of a density dependent relationship between population size and survival of the incoming year class.

The Von Bertalanffy growth equation was applied to mean carapace lengths of 1964 and 1965 year class shrimp. Values for the constants L and K were very close while the values to t0 varied considerably.

I believe the spring and summer 1965 and fall 1966 surveys produced underestimates of abundance, possibly due in part to vertical migrations of shrimp during daylight in 1965 and a horizontal shift of the population north in 1966.