No. 35–White Shrimp Season

Note: The following is a timely reprise of a column from 2003 with additional information.

It won’t 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 ppt 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, though salmon are longer lived, the spring spawn is the culmination of the one year 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 spawning season progresses, the SC Department of Natural Resources sends biologists offshore on test drags for shrimp. When it has been determined that a large majority of the shrimp have completed spawning, the roe shrimp season is opened.

The primary factor determining the size of that harvest and the ensuing fall harvest is water temperatures during winter. Shrimp aren’t able to tolerate temperatures below 50 degrees for any appreciable length of time. Anything over a day or two begins to take a toll on the stock.

Mortality rates of up to 90% have been recorded due to winter kill. Depending on the severity of the phenomena, it may take one to two years for the population to fully rebound.

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 recover from winter kill and other weather related die-offs. Eggs are fertilized as they emerge past a spermatophore (sperm packet) that was placed in the strategic location by the male. The eggs are thought to sink to the bottom where, if not consumed beforehand, they will begin hatching in twelve to twenty-four hours.

Newly hatching shrimp are among the tiniest of morsels in the rich planktonic soup that our waters become each spring. After roughly a two week period and eleven larval stages, the young shrimp, which now actually look like shrimp, reach their first post larval stage and a grand length of 3 to 4 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 in place through the ebbing tide. Thus, with the cycles of the tides they are brought progressively closer to their summer home in the estuary.

The tiny shrimp (redundant, I know) first move into the upper reaches of small tidal creek systems where they hide from predators amid the marsh grass and feed primarily on detritus and small organisms found on and in the mud bottom.

As they grow they expand their range to the larger creeks where those spawned earliest reach a length of 2 to 3 inches by early August. Recreational shrimpers begin see 46-50 count (number of tails to make a pound) shrimp in their nets by mid September with October and November showing the best catches. With favorable weather conditions shrimp may be taken well into December.

There is one factor can bring the inshore recreational season to an abrupt and untimely halt. Some may recall last year’s season pretty much ending in August as the rains of brush-by tropical weather systems impacted the area. A rapid drop in salinity resulting from too much rain in a short period will flush the shrimp to offshore waters. An extended period of rain will have the same effect though more slowly.

Water temperature is the ultimate season closer in most years. It’s time to start thinking about hanging the nets up as it drops into the mid 60s.

Again a reminder to cast netters to carry a pair of nail clippers to free gill hung fish relatively unharmed. The trout or flounder you save could be on your plate in a couple of years.

Note: You can support your local shrimpers and have a wonderful time doing so during the October 13 & 14 Shrimp Festival in historic downtown Beaufort. A week later you may join the Estuarium and our many friends in the Old Village of Port Royal for our 3rd. Annual Oktoberfest on the 21st.