Study Maps Areas Where Fishers and False Killer Whales Overlap

posted in: March 2021 | 0
A Hawaiian false killer whale with a dorsal fin injury from fishing gear. Credit: Robin Baird/Cascadia Research

For years now, an issue that has stymied efforts to protect false killer whales from the often fatal interactions with the gear of non-longline commercial fishers has been the fact that there are no reliable figures on just how often such interactions occur.

“In Hawaiian waters there are no observer programs in nearshore fisheries, yet interactions with fisheries are likely the greatest threat to the endangered main Hawaiian Islands insular population of false killer whales,” write the authors of an article recently published in the journal Biological Conservation.

Their research, however, points the way to a possible solution. Relying on photographic records and the tracks of satellite-tagged individuals, lead author Robin Baird and his colleagues determine the areas where the animals are most likely to be found. They then overlay those areas to the maps of zones that show where and how many fish are reported to be caught by commercial, non-longline fishing vessels.

In this way, the authors are able to identify regions where fishers are most likely to encounter false killer whales, ranked according to a “fishery overlap index,” or FOI.

Here’s how they describe the metric: “For example, if there is a single vessel fishing in an area with several false killer whales, the probability of a whale overlapping in space and time when the vessel hooks a fish would be relatively high. If there were many vessels fishing in an area and only a single whale, from the perspective of the fishermen the probability of overlapping at a time when the vessel hooked a fish would be relatively low.” Also, the index takes into account the fact that there is some likelihood that false killer whales will be attracted to fishing vessels or their catch.

To determine fishing effort, the authors reviewed commercial catch reports for the years 2007 through 2017 and data on tagged false killer whales from that same period. Their analysis revealed several areas of high or very high use by the false killer whales, and also some low-density areas, with seasonal variation as well. The highest use areas were in the areas between Molokaʻi and Oahu, including the Penguin Bank, and off the northern point of Hawaiʻi Island. 

Fishery overlap indices (FOI) were then calculated for 90 of the state’s statistical areas where commercial fishermen report their catch. “These 90 areas accounted for 95.4 percent of all of the false killer whale time from satellite tag data analyses,” the paper states.

Their findings might be considered counter-intuitive by some. “There is a natural tendency to assume that the areas with the greatest amounts of fishing effort may be the areas with the highest probability of interactions occurring,” the authors write, “but from the perspective of the fishermen, this may not be the case…. While Kona is the area with the highest fishing effort, regardless of which measure of fishing effort was used (total catch, days fished, or the number of licenses), Kona was in the bottom 10 percent of the 90 areas for which FOIs were calculated.”

“Our findings have important implications for how to address depredation and bycatch of false killer whales in nearshore fisheries in Hawaiʻi,” the researchers write. Armed with this information, managers can better direct efforts to mitigate interactions between fishermen and these animals.

“Fishermen that regularly fish in areas with high FOI values could be the focus for targeted outreach efforts to aid in improving identification skills and generally raising awareness of the behavior of different species, particularly as it relates to the likelihood of depredation of catch,” they write. “For example, melon-headed whales and short-finned pilot whales, two other similar looking species, feed primarily at night and deep in the water column on squid or small fish that are unlikely to overlap with the catch of most nearshore fisheries. Knowing that these species are unlikely to depredate catch may benefit fishermen, who sometimes may pull gear or move to a different area if they think there is a high likelihood of depredation from whales nearby.”

In addition, measures that would be useful in gathering more information on interactions between fishermen and false killer whales, such as on-board observers or electronic monitoring, could be targeted to where such interactions are most likely to occur, based on the fishery overlap indices.

Next Steps?

With the interactions of the non-longline commercial fishery with false killer whales well established and possible mitigation measures identified, what is now needed to implement them?

“I think the next step has to be some targeted outreach and engagement efforts with fishermen that fish off the east side of O‘ahu, off Moloka‘i, and off Kohala in particular,” Baird said. The state Department of Land and Natural Resources has been engaged in outreach efforts for several years, supported by a grant from the National Oceanic and Atmospheric Administration, he added, “so I suspect they’ll play a major role.”

The Take Reduction Team established by NOAA to reduce fishery interactions with false killer whales, of which Baird is a member, does not include any representation from the non-longline fishery and has not considered its impacts. “Unless both those things change,” Baird said, the TRT “would not be the right group to take on these issues.”

When the TRT was formed, Baird and others advocated for the inclusion of non-longline members, but, he said, the National Marine Fisheries Service “typically doesn’t take action unless there are observed ‘takes’ of the animals (as bycatch or by hooking) involving a specific fishery.” Since there are no observers in the non-longline fishery, “we are caught in a situation where there is lots of evidence of interactions (e.g., depredation and hookings), but nothing that has triggered the fishery’s reclassifcation.” (Under the Magnuson-Stevens Act, fisheries are classified according to the likelihood that they interact with marine mammals.)

“Unless some of the non-longline fisheries are reclassified from Category III (with ‘a remote likelihood of or no’ bycatch) to a Category II (‘occasional incidental mortality and serious injury’), I don’t think any sort of electronic monitoring or observer programs will happen,” Baird said.

— Patricia Tummons

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