Fish culling reduces tapeworm burden in Arctic charr by increasing parasite mortality rather than by reducing density‐dependent transmission

Abstract

1. Two common Dibothriocephalus (formerly Diphyllobothrium) tapeworm species were significantly reduced by experimental culling of their fish host Arctic charr (Salvelinus alpinus) in a subarctic lake.2. Between 1984 and 1991, funnel traps were used to cull ~35 metric tons of Arctic charr, reducing charr density by ~80%. As charr densities decreased, tapeworm prevalence and then intensity also declined over the following three decades, with D. dendriticus (formerly dendriticum) responding faster than D. ditremus (formerly ditremum). The two main hypotheses for how culling a host can decrease parasitism are reductions in parasite transmission due to reduced host density and reductions in parasite survival through increases in host mortality rates.3. We found little evidence that charr density was the main driver for reduced parasite transmission. Instead, decreased survivorship in charr, initially, through fishing-induced changes in charr age structure, and later through increased predation rates by brown trout, led to increased parasite mortality. Although brown trout, which increased significantly after fish culling, are also hosts, they are often too big for the final host birds to eat, thus becoming parasite sinks. 4. Synthesis and applications. Fish populations with heavy parasite burdens constitute a management problem. Our results show how fish culling can indirectly reduce transmitted parasites through increased parasite mortality. Managing overcrowded fish populations by culling can produce two desirable outcomes: an increase in fish growth rates and reduced parasite burdens.

Factors determining parasite abundance in European perch, Perca fluviatilis, European whitefish, Coregonus lavaretus, and Arctic charr, Salvelinus alpinus, in an oligotrophic lake, southern Norway

“…The abundance of D. dendriticus exhibited seasonal variations, with fewer specimen found in the fall. This is also unusual, as the parasite lives for several years in fish (Halvorsen & Andersen 1984), and other studies show no significant seasonal variations in its abundance (Henriksen et al 2019). As the first intermediate host of D. dendriticus are copepods (Halvorsen 1966;Marcogliese 1995;Scholz et al 2009), seasonal trends may be linked to new infections and fluctuations in copepod abundance.…”

Temporal stability of polymorphic Arctic charr parasite communities reflects sustained divergent trophic niches

“…Previous fish studies have shown relatively stable infection of trophic‐transmitted parasite taxa through time (e.g., Kennedy, 2001 ; Kuhn et al, 2016 ). Changed transmission rates may have been caused by alterations in the food‐web structure, for example, copepod transmitted parasites (Henriksen et al, 2019 ; Lopez & Duffy, 2021 ) related to human disturbances. In Loch Rannoch, one indirect cause of change could be the introduction of an alien fish species (e.g., crucian carp: Fraser & Adams, 1997 ) by fisherman and the increase in benthivore competitors (e.g., brown trout and perch), which may have altered the inter‐specific competition between fish species and changed predator–prey relationships (e.g., Britton et al, 2010 ; Gregersen et al, 2006 ; Klemetsen et al, 2003 ), and thereby changed transmission rates and routes of parasites to the three Arctic charr morphs.…”

Climate warming is predicted to enhance the negative effects of harvesting on high‐latitude lake fish

“…Lake Takvatn is located about 300 km north of the Arctic Circle in northern Norway, has an area of 15 km 2 and is situated 215 m above sea level. Data on Arctic charr have been collected yearly since the early 1980s (Amundsen et al, ) and include individual age, length, weight, maturation status, sex and fecundity data (Amundsen et al, ; Henriksen et al, ). Parameters used for our model are listed in Table S1 in Appendix , and data from Lake Takvatn charr are visualized in Figure S1 in Appendix .…”

“…Data used for model parameterization is available through the Dryad Digital Repository at https://doi.org/10.5061/dryad.bd10668 (Henriksen et al, ). Fish culling reduces tapeworm burden in Arctic charr by increasing parasite mortality rather than by reducing density‐dependent transmission).…”