Shrinking Shark Size?
Gulf-Wide Decreases in the Size of Large Coastal Sharks Documented by Generations of Fishermen
Those “fish tales” of the huge one that got away so many years ago may not be an exaggeration. A group of scientists, led by Dr. Sean Powers, Senior Marine Scientist, Dauphin Island Sea Lab and Chair and Professor, Department of Marine Sciences, University of South Alabama, have examined 80 years of records from three of the oldest fishing rodeos in the United States, and have seen a marked decrease in the size of certain large coastal sharks.
In a recent paper in Marine and Coastal Fisheries: Dynamics, Management and Ecosystem Science, Dr. Powers and his colleagues found up to a 50 70% decrease in size (weight or length) of coastal sharks, particularly Tiger Sharks, Bull Sharks, and to a lesser extent, Hammerheads. These decreases occurred most starkly during the 20 years after the 1980s.
Long-term quantitative data are among the best tools for examining trends over time; unfortunately, these types of data, documenting the extent, timing and consequences of changes in shark populations, are scarce. The authors were able to examine quantitative measures of shark sizes collected annually and independently of any scientific survey by thousands of recreational fishermen over the last century by studying the records of the Alabama Deep-Sea Fishing Rodeo (ADSFR), the Mississippi Deep-Sea Fishing Rodeo (MDSRF), and the Texas Deep-Sea Fishing Rodeo (TDSFR).
Not only shark size changed over time, but also species composition. For example, winning sharks for the ADSFR were almost exclusively Tiger Sharks for the ADSFR prior to the 1990s, but only occasionally made the leaderboard post-90’s, and their weights were considerably less. Instead, smaller Blacktip Sharks and Hammerheads dominated, and even their weights were less than those seen in the 1970s and 1980s.
While anglers may be impressed with a 200 lb. Tiger Shark during the upcoming rodeo, it’s important to note that sharks of this size aren’t even reproductively mature. In the 80’s, 400-800 lb. Tiger Sharks were the norm.
Dr. Powers said,“We are incredibly fortunate to have this wealth of fishermen-generated data to demonstrate the loss of the really large sharks.” The paper also presents evidence that the decrease in large sharks over the last 25 years is likely a legacy from overharvest by the commercial long line fisheries. Because most large sharks reproduce at older ages, have long pregnancies, and produce few offspring, shark populations can take decades to recover.
Beyond the implications for the shark fishery, the paper demonstrated how traditional fishermen knowledge can be quantified and used to establish ecological baselines that predate rigorous fishery independent surveys.
Sean P. Powers, F. Joel Fodrie, Steven B. Scyphers, J. Marcus Drymon, Robert L. Shipp & Gregory W. Stunz (2013): Gulf-Wide Decreases in the Size of Large Coastal Sharks Documented by Generations of Fishermen, Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science, 5:1, 93-102