No. 21–White Shrimp Season
It won’ be long before the cast nets start flying in earnest. The marsh creeks are teeming with finger length whities bulking up for the migration back to the offshore waters where they were spawned early this spring.
White shrimp (Penaeus setiferus) spawn offshore because their eggs and larval offspring require full ocean salinity of 35 parts per thousand (ppt)to survive. Salinity refers to the volume of dissolved salts and other trace elements in a body of water. Think of it this way. If you poured a thousand cups of sea water into a container and allowed the water to evaporate, you would have thirty-five cups of sea salt remaining on the bottom.
As with salmon, The spring spawn is the culmination of the life cycle of the white shrimp. The roe shrimp harvest that follows the spawn can help make or break a commercial shrimper’s year. As the mating and spawning season progresses, the SC Department of Natural Resources sends biologists out to test drag for shrimp. When it has been determined that a large majority of the shrimp have completed spawning, the roe shrimp season is opened.
A female white shrimp may spawn several times and produce between 500,000 and 1,000,000 eggs. This accounts for the species’ ability to rebound rapidly from weather related die-offs. The eggs are fertilized as they emerge past a spermatophore (sperm packet) that was placed in the strategic location during mating. The eggs are thought to sink to the bottom where, if not consumed before hand, they will begin hatching in twelve to twenty-four hours.
Newly hatching shrimp are among the tiniest of morsels in the rich planktonic soup our waters become each spring. After roughly a two week period and eleven larval stages, the young shrimp, which now actually looks like a shrimp, reaches its first post larval stage and a grand length of 4 to 5 mm.
The migration from ocean to estuarine waters begins with the second post larval stage. Tidal forces are the most likely system of transport for the treacherous journey from as far as several miles offshore. The little shrimp are able to hitch a ride in the water column and move landward with the incoming tide. On slack high water they settle to the bottom where they remain through the ebb tide. Thus, with the cycles of the tides they are brought progressively closer to their summer home in the estuary.
We will continue with the lifecycle of Penaeus setiferus.