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Abstract How has parasitism changed for Alaskan salmon over the past several decades? Parasitological assessments of salmon are inconsistent across time, and though parasite data are sometimes noted when processing fillets for the market, those data are not retained for more than a few years. The landscape of parasite risk is changing for salmon, and long‐term data are needed to quantify this change. Parasitic nematodes of the family Anisakidae (anisakids) use salmonid fishes as intermediate or paratenic hosts in life cycles that terminate in marine mammal definitive hosts. Alaskan marine mammals have been protected since the 1970s, and as populations recover, the density of definitive hosts in this region has increased. To assess whether the anisakid burden has changed in salmonids over time, we used a novel data source: salmon that were caught, canned, and thermally processed for human consumption in Alaska, USA. We examined canned fillets of chum (Oncorhynchus keta,n = 42), coho (Oncorhynchus kisutch,n = 22), pink (Oncorhynchus gorbuscha,n = 62), and sockeye salmon (Oncorhynchus nerka,n = 52) processed between 1979 and 2019. We dissected each fillet and quantified the number of worms per gram of salmon tissue. Anisakid burden increased over time in chum and pink salmon, but there was no change in sockeye or coho salmon. This difference may be due to differences in the prey preferences of each species, or to differences in the parasite species detected across hosts. Canned fish serve as a window into the past, providing information that would otherwise be lost, including information on changes over time in the parasite burden of commercially, culturally, and ecologically important fish species. 

There are plenty of reasons to believe that parasite populations will respond to biodiversity loss, warming, pollution, and other forms of global change. But will global change enhance transmission, increasing the incidence of troublesome parasites that put people, livestock, and wildlife at risk? Or will parasite species decline in abundance—or even become extinct—suggesting trouble on the horizon for parasite biodiversity? Here, I explain why answers have thus far eluded us and suggest new lines of research that would advance the field. Data collected to date suggest that parasites can respond to global change with increases or decreases in abundance, depending on the driver and the parasite. The future will certainly bring outbreaks of some parasites, and these should be addressed to protect human and ecosystem health. But troublesome parasites should not consume all of our research effort, because this changing world contains many parasite species that are in trouble. 

The comparison of historical and modern food web dynamics allows ecologists to test whether the trophic connectivity we observe today is ‘normal’ in its historical context. Fish densities and abundances have changed across time, making it likely that fish trophic interactions and their trophic positions have also changed. Historical trophic data of marine fishes can now be extracted from the tissues of fluid-preserved specimens held in natural history collections via compound-specific stable isotope analysis of amino acids (CSIA-AA) of nitrogen. We conducted CSIA-AA to quantify trophic position change over the past century in 5 ecologically important fishes of Puget Sound, Washington, USA: Pacific hake Merluccius productus , walleye pollock Gadus chalcogrammus , copper rockfish Sebastes caurinus , English sole Parophrys vetulus , and Pacific herring Clupea pallasii , and examined the canonical trophic (glutamic acid) and source (phenylalanine) amino acids. For all fishes except copper rockfish, trophic position, glutamic acid, and phenylalanine values remained similar across time. For copper rockfish, glutamic acid but not phenylalanine values increased over time, indicating an increase in this species’ trophic position. The observed increase in copper rockfish trophic position may be a function of diet switching and declining prey quality rather than a consequence of rockfish consuming higher trophic level prey. This study leverages more than 100 yr of trophic data of fishes representing various feeding guilds and demonstrates that some fish species may be more trophically resilient to major environmental change than expected. Efforts should be made to identify and conserve the trophic interactions of species experiencing change. 

Long-term data allow ecologists to assess trajectories of population abundance. Without this context, it is impossible to know whether a taxon is thriving or declining to extinction. For parasites of wildlife, there are few long-term data—a gap that creates an impediment to managing parasite biodiversity and infectious threats in a changing world. We produced a century-scale time series of metazoan parasite abundance and used it to test whether parasitism is changing in Puget Sound, United States, and, if so, why. We performed parasitological dissection of fluid-preserved specimens held in natural history collections for eight fish species collected between 1880 and 2019. We found that parasite taxa using three or more obligately required host species—a group that comprised 52% of the parasite taxa we detected—declined in abundance at a rate of 10.9% per decade, whereas no change in abundance was detected for parasites using one or two obligately required host species. We tested several potential mechanisms for the decline in 3+-host parasites and found that parasite abundance was negatively correlated with sea surface temperature, diminishing at a rate of 38% for every 1 °C increase. Although the temperature effect was strong, it did not explain all variability in parasite burden, suggesting that other factors may also have contributed to the long-term declines we observed. These data document one century of climate-associated parasite decline in Puget Sound—a massive loss of biodiversity, undetected until now. 

Abstract Many of the most contentious questions that concern the ecology of helminths could be resolved with data on helminth abundance over the past few decades or centuries, but unfortunately these data are rare. A new sub-discipline – the historical ecology of parasitism – is resurrecting long-term data on the abundance of parasites, an advancement facilitated by the use of biological natural history collections. Because the world's museums hold billions of suitable specimens collected over more than a century, these potential parasitological datasets are broad in scope and finely resolved in taxonomic, temporal and spatial dimensions. Here, we set out best practices for the extraction of parasitological information from natural history collections, including how to conceive of a project, how to select specimens, how to engage curators and receive permission for proposed projects, standard operating protocols for dissections and how to manage data. Our hope is that other helminthologists will use this paper as a reference to expand their own research programmes along the dimension of time.