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Study goes below the surface to find fishery impacts on dolphins

Scarring shows that impacts on animals in Maui Nui may be worse than thought

Dolphins with scars on their dorsal fins, mouth lines and bodies were studied using underwater imagery in order to detect possible interactions with fisheries and fishing gear in the Maui Nui region. — National Marine Sancuary Foundation photo

Using dorsal fin, mouth line and underwater body imagery, a first for this type of study, researchers found that impacts of fishing activity on Maui Nui dolphins may be more of a widespread problem than initially thought.

Researchers at the Pacific Whale Foundation completed a study using new methods to evaluate the impact of local fishery interactions on dolphin populations throughout Maui, Molokai, Lanai and Kahoolawe.

“Dolphins play a key role in maintaining healthy oceans, which are needed to regulate the global climate, and act as early indicators of unhealthy oceans,” said Pacific Whale Foundation chief scientist and principal investigator Jens Currie on Friday. “Understanding how sensitive dolphin populations are to human activities is needed to allow for proper conservation efforts.”

Currie co-authored a recent study along with Abigail Machernis, Stephanie Stack, Grace Olson and Florence Sullivan.

This study used the nonprofit foundation’s long-term historic data on bottlenose and spotted dolphins collected over a 24-year period from 1996-2020.

Some fishing gear interactions can result in serious injury or mortality from entanglement or ingestion of gear, causing a decline in dolphin populations. — National Marine Sancuary Foundation photo

“Studies like this one, which aim to determine the extent of a particular threat, in this case, fisheries interactions, will help us determine where priority action is needed to conserve Hawaii’s dolphins,” Currie said.

Every image in the nonprofit’s photo ID catalog was also reviewed to identify dolphins with scars on dorsal fins, mouth lines and bodies that may indicate past interactions with fisheries and fishing gear in the Maui Nui region.

The objective was to determine if underwater body images, in addition to the traditionally used dorsal fin and mouth line images, increased rates of finding evidence linked to fisheries.

Most population monitoring is based on above-water images of dorsal fins, which limits scar detections to the dorsal fin and a small area of the body around it, Currie explained.

This new approach involves combining individual assessments of dolphins’ dorsal fins, mouth lines and bodies into a single assessment, which increased scar detection rates between 40 to 50 percent, he said.

“Using underwater imagery provided us with a full view of the dolphin’s body to look for evidence of fisheries interactions,” he added. “Without the underwater body images, we are assuming the scars detected on the dorsal fins are indicative of the entire body, which we found was not the case and results in underestimation of fisheries-related scar detections.”

Machernis, a research biologist with the foundation and lead author of the paper, examined data collected from 255 bottlenose dolphins and 374 spotted dolphins.

“Using this new technique, we found that 27 percent of bottlenose dolphins and 13 percent pantropical spotted dolphins in Maui showed signs of previous fisheries interactions,” Currie told The Maui News.

Using underwater photo analysis was “a game changer,” Machernis said in a news release.

“Most research literature that examines fisheries interactions look primarily at dorsal fins and we wanted to use all the data we have collected to examine as much of the dolphins’ body as possible for evidence of fishing-gear related scars,” she said. “We highly recommend researchers interested in examining the threat of fisheries interactions to dolphins make a concerted effort in the field to collect above-water mouth line and body shots, in addition to underwater footage.”

Worldwide, interactions with fisheries resulting in lethal or nonlethal consequences is one of the leading conservation concerns for cetaceans.

According to the foundation, the full extent of the issue is hard to assess because most entanglements are never observed. However, the International Whaling Commission reported that more than 300,000 whales and dolphins die annually due to entanglement in marine debris, which can have a “devastating, long-term conservation impact” on those populations that are already threatened.

The number of deaths in Hawaii specifically is “very hard to accurately estimate,” Currie said, because most whales and dolphins that die at sea will sink to the bottom and never be retrieved; very few end up stranded on beaches.

Still, the Hawaiian Islands Humpback Whale National Marine Sanctuary, in conjunction with community partners, responded to 21 humpback whale entanglement reports in 2018 throughout Hawaii, he noted.

“These responses were to humpback whales still entangled in gear and do not represent the whales and dolphins that have become entangled and die at sea,” he said.

Past and ongoing research confirms that direct interactions between cetaceans and fishing gear usually happen when the animals unintentionally swim into gear, becoming entangled or entrapped, or when animals intentionally try to remove fish captured in gear, a behavior known as depredating, and become hooked and/or entangled as a result.

Some fishing gear interactions can result in serious injury or mortality from entanglement or ingestion of gear, according to the foundation.

These types of interactions happen globally and are “likely to increase” due to the potential for continued human activity in those habitats, Machernis said.

“Avoiding initial interactions will be the biggest mitigating factor,” Currie said. “Given this, additional outreach efforts that highlight the best practices to avoid interactions with dolphins when fishing and recommend appropriate measures to take if a dolphin interacts with active gear are needed.”

Some examples to help reduce risks include reeling in line if dolphins appear in the area while fishing or changing locations if dolphins show interest in the bait.

Foundation researchers concluded that the level of impacts from fishery interactions on cetaceans are “of great concern” and there is a major need to identify which species interact with which fisheries and the locations these interactions occur so officials can work with fishers to find sustainable solutions.

The organization recently published a paper citing a downward trend in population numbers.

“Further research is needed to determine whether the observed rates of fisheries interactions are impacting Maui’s dolphin populations, given that the observed scars on otherwise healthy animals are the result of nonlethal interactions with fishing gear,” Currie said. “We recently co-authored a paper that reported a decline in Maui’s bottlenose dolphin population. These two papers together highlight that we need to determine if this is a lethal or nonlethal threat and better understand how our activities are affecting dolphins.”

All Pacific Whale Foundation publications are freely available at PacificWhale.org/research/publications.

To learn more or make a contribution to support the foundation’s dolphin research, visit PacificWhale.org/pacific-whale-foundation/.

* Dakota Grossman can be reached at dgrossman@mauinews.com.

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