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September 12, 2008

DNR biologists note decline in yellowfin tuna landings

Documented catch rates for Atlantic yellowfin tuna, an internationally managed migratory marine species distributed throughout the world's warm temperate oceans, have recently been in decline along our coast, according to landings data analyzed by state fishery managers.

Biologists with S.C. Department of Natural Resources (DNR) have observed a decline in catch rates of yellowfin tuna during the twenty year history of the S.C. Governor's Cup Billfishing Series. 

Traditionally, yellowfin tuna have made up a large part of the incidental catch during billfish tournaments ranging from a high of 494 fish landed during the Series tournaments in1999 to none landed during this year's tournaments.  "The disappearance of yellowfin tuna from the catch is of concern to fishery managers and anglers alike," said S.C. Governor's Cup Program Director Wallace Jenkins.

Yellowfin and other tunas are managed by the International Commission for the Conservation of Atlantic Tunas (ICCAT) which allocates harvest to various member countries.  Since the last population assessment was conducted in 2003, landings of yellowfin in the Atlantic Basin have declined nearly 39%.  International managers believe that overfishing of the population may be contributing to the decline in catch rates for this species.  In addition, throughout much of their range these fish are routinely harvested without regard to size or age and often before they reach sexual maturity.  In contrast, the U.S. has strict size, creel, and gear restrictions in place to protect the species from over harvest.  However, the U.S. fleet usually accounts for only five to seven percent of total annual landings.

Data collected at fishing tournaments and from sampling commercial landings in the state are routinely forwarded to federal fishery managers for use in development of national and international fishery management plans.  "Long term data sets like that of the Governor's Cup Billfishing Series and from other sources are vital to understanding the population status of not only yellowfin tuna, but other species targeted during the tournaments like marlin, sailfish, wahoo and dolphin," said Jenkins.  "State and federal fishery managers are very concerned about what appears to be a shift in population abundance of yellowfin tuna away from S.C. to more northern latitudes and theorize that this phenomenon may be due to changes in sea surface temperatures which have been rising at a rate of one percent a year since 1998," Jenkins said.  During the last decade, as catch rates of yellowfin tuna have shifted north along the eastern coast of the U.S., billfish catches in S.C. tournaments have actually increased 4-fold.  In the late 1990s, fishery managers would typically see 50-60 billfish captured and released during the S.C. tournament series; however, in 2008 this total exceeded 220 billfish.

As a result of trends taking place this summer, many anglers are questioning what can be done about this change in yellowfin tuna abundance and what it may mean for future occurrence of the species off of the coast.  The good news is that the ICCAT is currently updating the stock assessment on the species and may be able to tell if this is just a local shift in abundance or if it is indicative of a large decline in the population.  "Hopefully the results of the assessment can be used to increase restrictions on the harvest of yellowfin tuna by countries throughout the Atlantic Basin and rebuild the population to former levels," said Jenkins.

DNR protects and manages South Carolina's natural resources by making wise and balanced decisions for the benefit of the state's natural resources and its people.

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