Research
Selective tidal-stream transport in ovigerous blue crabs
HENCH, J. L., R. B. FORWARD, JR., S. D. CARR, D. RITTSCHOF, AND R. A. LUETTICH, JR., 2004. Testing a selective tidal-stream transport model: observations of female blue crab (Callinectes sapidus) vertical migration during the spawning season. Limnology and Oceanography, 49 (5): 1857-1870.
Abstract
Female blue crabs, Callinectes sapidus, mate in estuaries and undergo a seaward spawning migration to release larvae. According to the prevailing model, females with mature embryos use nocturnal ebb tide transport (ETT) to move seaward, release larvae and then reverse to nocturnal flood-tide transport (FTT) to move back into the estuary. We tested this model by examining the vertical migratory behavior of ovigerous and post-spawned female crabs. Simultaneous physical-biological data were collected for 38-d during Aug-Sep 2002 in Bogue Sound, North Carolina, USA. Crab water column positions were determined with miniature internally recording pressure sensors. Local current and water properties were measured and crab vertical migration times relative to observed currents were used to determine ETT and FTT behavior. Surface censuses of free-swimming migrating crabs on 19 nocturnal ebbs were used to complement the intensive studies of individual crabs. The study found that: 1) the pressure sensors had a measurable but small effect on swimming, 2) females migrated during day and night ebb tides, 3) females used ETT throughout embryo development, 4) ETT corresponded to the rate of decrease in water level (hydrostatic pressure), 5) larvae were released at high tide or when water level is falling and often within several hours of sunrise, and 6) post-spawned females continued ETT and did not switch to FTT. Thus, the data did not support the prevailing ETT-FTT reversal model. Rather, females continue ETT into coastal areas releasing subsequent clutches farther seaward increasing the potential for successful larval transport to favorable offshore developmental areas.
Above photo: Daily progression of egg stage development in one of the experimental crabs. Color of egg clutch changes over time, from orange to brown to black as yolk is absorbed and eye spots develop. In the last panel, the larvae have been released.
Time series of vertical migration behavior for a single crab. Horizontal axis is time (days in 2002).
(click on image)
CARR, S. D., R. A. TANKERSLEY, J. L. HENCH, R. B. FORWARD JR., AND R. A. LUETTICH, JR., 2004. Movement patterns and trajectories of ovigerous blue crabs Callinectes sapidus during the spawning migration. Estuarine Coastal and Shelf Science, 60 (4): 567-579.
Abstract
Female blue crabs (Callinectes sapidus Rathbun) migrate from low salinity estuarine regions to high salinity regions near the ocean to release larvae. During this migration, ovigerous females use ebb-tide transport, a vertical migratory behavior in which they ascend into the water column during ebb tides, to move seaward to larval release areas. In order to determine the relationship of ebb-tide vertical migrations to local currents and the influence of these vertical migrations on the horizontal transport of blue crabs in the estuary, ovigerous females with mature embryos (~ 1 to 3 days from hatching) were tracked near Beaufort Inlet, North Carolina (USA), in July and August 2001 and 2002. Crabs were tagged and tracked using ultrasonic telemetry, and currents near the crabs were measured simultaneously with a shipboard acoustic Doppler current profiler.
During the two seasons, eight crabs were successfully tracked for periods ranging from 3.9 to 37.0 h and for distances ranging from 1.9 to 10.6 km. All crabs migrated seaward during the tracking periods. Crabs moved episodically during all tidal phases with periods of movement on the order of minutes to an hour. They moved with local currents in terms of both speed and direction during ebb tides, consistent with ebb-tide transport, and moved down-estuary (seaward) in opposition to local currents during flood tides. The percentage of time that crabs were active was higher during night ebb tides than during day ebb tides or flood tides and increased with increasing ebb-tide current speed. Mean migratory speeds were 0.11, 0.04, 0.08 and 0.02 m/s during night ebb, night flood, day ebb and day flood tides respectively, and net migratory speeds were on the order of 5 km/day. Due to the episodic nature of the crabs' movements, the total distances that crabs traveled during ebb tides ranged from 10 to 40 % of the distances that passive particles could have traveled under the same conditions.
Photo: Ovigerous blue crab fitted with an ultrasonic transmitter.
CARR, S. D., J. L. HENCH, R. A. LUETTICH, JR., R. B. FORWARD JR., and R. A. TANKERSLEY, 2005. Spatial patterns in the ovigerous Callinectes sapidus spawning migration: results from a coupled behavioral-physical model. Marine Ecology Progress Series, 294: 214-226.
Abstract
Ovigerous blue crabs Callinectes sapidus use ebb-tide transport (ETT), a vertical migratory behavior in which crabs ascend into the water column during ebb tides, to migrate from estuarine adult habitats to coastal larval release locations. In this study, we develop a detailed behavioral model of ovigerous blue crab ETT from previous laboratory and field studies and couple this model to a hydrodynamic model of the Beaufort Inlet region of North Carolina. We simulate the trajectories of migratory ovigerous crabs in the region and determine spatial patterns in migratory success, migratory speeds, the residence times of crabs in different regions of the estuary, and potential larval release locations.
Highly active crabs can start their migration from almost anywhere in the estuary and reach suitable larval release locations within a typical 4-day migratory period, whereas crabs with lower activity levels can only reach suitable larval release locations if they start their migration in the lower-mid estuary. Migratory speeds in the estuary range from < 1 to > 8 km/day. Crabs with lower activity levels are resident in the mid-upper estuary for relatively long periods of time, whereas highly active crabs are resident in the lower estuary and coastal ocean for most of the migratory period. Larval release is predicted to occur throughout the estuary and in the coastal ocean within ~ 5 km of Beaufort Inlet. Fisheries managers can use these spatial patterns to determine management strategies (e.g., spatial closures to fishing) that will protect migratory blue crab spawning stock in tidal regions effectively.