Vertical Migration Of Blue-Crab Callinectes-Sapidus Megalopae - Implications For Transport In Estuaries
The sandbar shark, Carcharhinus plumbeus, is the most common large coastal shark in Virginia waters and is an important component of recreational and commercial fisheries along the east coast of the United States. Sandbar shark demographic analyses, using known and estimated life history parameters, including fishing mortality (F) at ages and levels estimated in a recent stock assessment, were used to estimate potential population growth and exploitation. Life history tables were constructed by using best estimates of natural mortality (M) of O.11 or 0.07 for maximum ages of 30 or 60 yr, respectively. Natality was fixed at 2.1 female pups/yr. Fishing mortality (F=0.05, 0.10, 0.15, 0.20, or 0.25) was simulated to begin at age 8, 10, 15, 20, or 29. The annual population growth rate was highest under a ''best-case'' scenario of M=0.05 (1/2 best estimate) and maximum age of 30 yr, but was only 11.9%/yr. At M=0.11 for all ages, the population increase rate was 6.4%/yr, and the generation time was about 20 years. At higher juvenile mortality rates, the population growth rate decreased to 2.6%/yr. Adding fishing mortality at immature ages caused the population to decline unless F levels were <0.10 and 0.05 at maximum age = 30 and 60, respectively. It is apparent that sandbar shark populations will decline under any substantial fishing mortality on immature ages and that mature fish can be exploited only at very low levels.