Tagging Shrimp to Explore Impact of Warm Winter
Tag Along! This post is part of a series about the world of animal tagging and tracking technology. See earlier animal tagging posts here.
On a clear, sunny day in late January, the R/V Silver Crescent headed out of Charleston Harbor on calm seas to pilot a new experiment. Commercial shrimp trawlers dotted the horizon, an unusual sight for the time of year. The projected high was 72 degrees Fahrenheit, about 15 degrees higher than the historic average.
The warm weather this winter has made for a number of perfect days on the water – and it hasn’t gone unnoticed by marine life.
This January, shrimp stuck around South Carolina waters later than normal due to warmer water temperatures. South Carolina’s shrimp populations follow a complex but cyclical pattern, and in late fall, dropping water temperatures typically prompt mature white shrimp to migrate as far south as Cape Canaveral, Florida.
The delayed migration has been good news for commercial shrimpers. SCDNR typically closes shrimp season in mid-January as adult shrimp migrate south and juvenile shrimp move out of estuaries, but this season saw a two-part closure, with some state waters remaining open until February 6.
“We tried to close the current season in a way that provided protection to the overwintering shrimp still inside, while allowing commercial shrimpers to benefit from the unusual presence of these valuable late-season large shrimp offshore,” said SCDNR Office of Fisheries Management director Mel Bell.
At the same time, SCDNR biologists were curious about the warm weather’s impacts on shrimp migration patterns over the coming months. To learn more, our crustacean lab recently took to the Atlantic to try something unusual: working offshore to catch and tag large shrimp.
If you spend time birdwatching or angling, you might be familiar with tagged fish or banded birds – methods by which biologists mark individual animals. Over time, as these individuals are recaptured/resighted and reported by biologists and members of the public, a fascinating story about their lifespan and travel comes into view.
Conventional fish tags are too large for shrimp – so fisheries scientists devised a better solution: streamer tags. These small plastic ribbons can be threaded through a shrimp’s muscle without harming the animal. They’re widely used by researchers studying crustaceans such as prawns, lobsters, and shrimp.
Within three miles of the coast of Charleston, SCDNR researchers aboard the R/V Silver Crescent set out a trawl net and reeled in mature white and brown shrimp (as well as fish, whelks, squid, and even a young horseshoe crab, which they released). Staff then measured, tagged, and monitored the shrimp in a tank to ensure they remained in good health. Finally, the shrimp were returned to the ocean. The hope is that tagged shrimp will then find their ways into commercial trawl nets offshore over the coming months, and shrimpers will note and report their finds to SCDNR.
In the end, the day brought far fewer shrimp than the team hoped to catch – but the SCDNR scientists are confident that these collection and tagging methods can be used to look at possible changes in shrimp migration in the future. If weather conditions remain warm, the team may soon make a second tagging trip.
The odds of recapturing shrimp may seem slim, but there are promising precedents. In two similar experiments SCDNR staff conducted in the early 1970s, an average of 17% of the tagged shrimp (out of 1,871 total) were later recovered and reported within a month. In another SCDNR tagging study in 1998, a higher number of shrimp were released (4,900) and reported from as far away as St. Augustine, but with a much lower recapture rate (3.8%).
“We would like to have caught and tagged more shrimp on this trip, but we’re excited to have another tool that we can use in the future to better understand our state’s commercially important shrimp resources,” said Jeff Brunson, SCDNR crustacean biologist.