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Protecting whales on the US West Coast by assessing and reducing “cryptic mortality”

Many US laws and policies use “best available science” to determine when a whale or dolphin population requires protection from ship strikes or entanglement in fishing gear. These protection measures may only be triggered when the science shows that we are exceeding some allowable harm threshold, but carcasses detected on the beach reflect only the tip of the iceberg. Many forms of mortality (e.g., ship strikes or entanglement in crab gear) are inherently cryptic in nature. Following Deepwater Horizon, researchers used modeling to determine that for every dolphin or whale carcass found on the beach, an additional 50-250 disappeared at sea. We propose to apply those methods to estimate how much we could be underestimating mortality from ship strikes and entanglement in crab gear – i.e., cryptic sources of mortality that do not lend themselves to traditional fisheries observer programs.

Our analyses focused on the years 2006-2016, when both reliable abundance estimates and stranding numbers were available. Data on marine mammal strandings were collated from more than 10 members of a coast-wide marine mammal stranding response network. Data on marine mammal abundance were taken from US Stock Assessment Reports, broken down by state and seasonal movement patterns, where possible. We reviewed the literature to estimate survival rates for 10 cetacean species, which are used in combination with abundance data to estimate the expected number of deaths of each species in the region each year. The number of carcasses recovered annually, on average, by the regional stranding network as a whole is compared to the number of expected deaths to estimate the proportion of carcasses recovered. The inverse of these proportions can be used as multipliers to gauge, roughly, how much one might have to scale up known mortality to estimate total human-caused mortality in each species.

Carcass detection rates were low for all species. The highest values observed were for large whales, but <1% of Pacific white-sided dolphin carcasses were detected. Carcass detection in deep-diving, pelagic species, such as beaked whales, approached 0.

For each of the 10 species in our case studies, we will report information to NOAA’s Scientific Review Group to explore whether our findings alter the scientific community’s assessment of the conservation status of any species. Our long-term objective is for this change to be incorporated into US policy, in which the Guidelines for Assessing Marine Mammal Stocks is updated to make explicit mention of the concept of cryptic mortality and the need to develop and use multipliers to account for imperfect carcass detection rate. Once this concept is incorporated into US best practices for assessing marine mammal stocks, we can use this to set a more precautionary global standard for assessing the impacts of human activities on cetacean populations. A new US seafood trade rule, which takes effect on 1 January 2022, will require countries to ensure that their marine mammal bycatch standards are comparable in effectiveness to US standards, or countries risk losing the opportunity to export their seafood to US markets. By raising the conservation standards of US policy, we can use the purchasing power of the US market to improve global standards in fisheries management and marine conservation.