Known for their mallet-shaped heads, hammerhead sharks are one of the most easily recognized -- and favored -- shark species. Their “hammers” give them a widened-view to scan for food, and they have enhanced sensory organs that can detect electrical fields from their prey. If that doesn’t make hammerheads cool enough, they can grow to incredible sizes -- reaching 20 feet in length and weighing up to 1,000 pounds.
But, according to a new study recently published in BioScience, the very traits that they've successfully evolved for millions of years have made hammerhead sharks particularly susceptible to overexploitation. In fact, hammerhead sharks have declined by more than 90 percent in many regions due to overfishing, explain the authors.
“Some of the novel adaptations that have historically contributed to their evolutionary and ecological successes may now be maladaptive under current levels of exploitation,” says the study.
For example, hammerheads tend to migrate in large, sexually-segregrated groups that allow the males to easily locate the females -- an adaptation that makes them easy targets for fishermen -- and once they're caught, exhibit high physiological distress. More so, their life history strategies, such as slow growth and late sexual maturity, don’t allow hammerheads to bounce back quickly after exploitation.
“Hammerheads are often caught as bycatch on tuna and swordfish longlines, and their fins are very valuable in the fin trade,” says study co-author David Shiffman, a Ph.D. student and research assistant in the R.J. Dunlap Marine Conservation Program at the University of Miami. “Hammerheads have the highest recorded stress responses out of many shark species. When they get hooked, they go all-out in a fight-or-flight response for their lives. This can result in mortality in just an hour of doing that.”
To protect hammerheads from overexploitation and extinction, the study argues their specializations should be considered in fisheries management and conservation. For example, avoiding peak areas of shark abundance, improving shark handling, and adjusting fishing depth are just a few tactics fishermen can employ to reduce shark intake. With these tactics in mind, population recovery is possible, say the authors.