Four sperm whale strandings point to potential human causes

Four sperm whales that stranded separately on southeastern U.S. coastlines between 2020-22 were emaciated and malnourished, with ingested fishing gear and marine debris found in two of them, according to a new study that compared the four cases.

The investigations, intended to better understand the causes behind whale deaths and to inform future marine mammal management decisions, were reported in a study published April 9 in the journal Diseases of Aquatic Organisms.

"Many of the issues come back to potential human influences, things people are doing in the waters that are affecting these species that have been around for millions of years," said Jennifer Bloodgood, assistant professor of practice in the Department of Public and Ecosystem Health in the College of Veterinary Medicine and the paper's lead author.

The four whales were each found separately, three in Florida and one in Alabama, between 2020 and 2022. Three males, age 12, 17 and 28, were discovered and one female, age 8. Endangered sperm whales are known to live over 60 years.

Sperm whales are the largest toothed predator, with diets consisting of squid, sharks, skates and fish found in deep waters. Females become fully mature at around 30 years of age when they can grow to 35 feet long. Males reach physical maturity at around 50 years and can grow to more than 50 feet long. They are listed as endangered under the U.S. Endangered Species Act, with main threats including entanglement in fishing gear, vessel strikes, pollution and climate change.

The whales stranded alive but died soon after. In many strandings, bodies have already decomposed and are no longer viable for tissue analysis by the time they are discovered, but the live strandings in these cases allowed the researchers to conduct a thorough necropsy, histopathology and infectious disease and biotoxin testing on each after they died.

Emaciation is common among stranded whales, which suggests more study is needed to understand the contributing factors, according to the study. Analyses of these strandings pointed to next steps in the research.

Stomach contents revealed the whales were eating their normal prey, mostly certain species of squid, with two of the whales containing well over 1,000 squid beaks.

The size of squid could be estimated from their beak sizes, which showed that overall, on average, the ingested squid were smaller than average, a trend that has been documented in the past.

"A follow-up question to this study is whether there is something going on with the prey, where squid are smaller than they used to be and therefore not as nutritious?" Bloodgood said. She added that more research is needed to understand if climate change is playing a role in squid size.

Along with less nutritious prey, inefficient foraging may also play a role, Bloodgood said. Sperm whales are known to dive very deep for prey, in areas without light. They are the only large whales to echolocate, meaning they use internal sonar to detect their surroundings and food. Human activities like seismic surveys for oil in the Gulf of Mexico and shipping may be disrupting their echolocation. "It begs the question of whether sperm whales are having to work harder and expend more energy to find prey that may not be as nutritious," Bloodgood said.

Examinations of one whale's ear bones revealed no damage, according to the study.

One whale had a length of fishing gear wrapped around its mandible, a bolus of trawl net in its esophagus, and more fishing gear and other plastic debris was found in its stomach, while another whale had sections of gill net, trawl net and long-line fishing gear in its stomach. Long-line gear consists of a main line with connected perpendicular branch lines, with a hook at each end. The segment in this whale had a minimum of 480 branch lines.

"Derelict gear should not be out there in the first place, and there are new and better fishing technologies that could prevent entanglements and ingestion of fishing gear," Bloodgood said. "We're the ones causing many of these issues and many of them should be entirely preventable."

Bloodgood previously worked as a research veterinarian at the Alabama Marine Mammal Stranding Network at the Dauphin Island Sea Lab.

Funding was provided through the Dauphin Island Sea Lab, the Gulf World Marine Institute, the Harbor Branch Oceanographic Institute and the Florida Fish and Wildlife Southwest Field Laboratory.