In lower Chesapeake Bay, a 172,235 ha marine protected area and corridor (MPAC) was recently established to protect blue crab adult females either en route to or at the spawning grounds during the reproductive period. The MPAC was justified due to a recent substantial decline in spawning stock biomass. It was situated in waters deeper than 10 m throughout the lower bay due to the high abundances of adult females in this zone, and it was an expansion of a historical spawning sanctuary near the bay mouth to include northward extensions (upper and lower MPACs). We examined spatial dynamics of the blue crab spawning stock in relation to the MPAC through analyses of trawl survey data (abundances of adult females and egg-bearing females from 1989-1997 and 1995-1997, respectively) partitioned by water depth, time (month and year), and spatial zone (upper MPAC, lower MPAC, MPAC Historical Sanctuary) during the reproductive period (June-September). Adult female abundance peaked at 6-14 m water depths. Consequently, nearly half of all adult females in the lower bay were deeper than 10 m, and therefore protected by the MPAC during the reproductive period, whereas the historical sanctuary protected about 1/3 that of the MPAC. All MPAC segments were utilized by adult females at different times of the spawning season, without consistent use of any particular segment. In contrast, abundance patterns of egg-bearing females were consistent and did not differ by developmental stage of the eggs. Peak abundances of egg-bearing females shifted from the northern to southern portions of the MPAC as the spawning season progressed. Differences in distribution of adult females and egg-bearing females demonstrated the importance of the expanded MPAC to the conservation of the spawning stock, which requires an extensive area to cover seasonal and yearly alterations in distribution. The expanded MPAC is much more effective than the historical sanctuary at protecting a consistent fraction of the blue crab spawning stock over the full spawning season and every year. Both the lower MPAC and historical sanctuary contained high abundances of adult females and egg-bearing females, and these segments therefore potentially function as corridors and spawning grounds. In contrast, whereas adult females were equally abundant in all MPAC segments, egg-bearing females were rarely common in the upper MPAC segment. Hence, the upper MPAC serves primarily as a corridor for females migrating to spawn or hatch their egg masses in the lower MPAC and historical sanctuary. The MPAC protects a major fraction of the spawning stock and spawning grounds both seasonally and yearly, and it encompasses a dispersal corridor for adult females in the deeper waters of Chesapeake Bay. The MPAC therefore serves as a foundation for long-term protection of the blue crab spawning stock, and should be utilized concurrently with complementary management measures to conserve the blue crab population in Chesapeake Bay. Furthermore, the MPAC for the blue crab in Chesapeake Bay may serve as a model system for investigating the value of marine protected areas for exploited marine populations with ontogenetically disjunct stages in the life cycle that encompass diverse habitats.
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