Northern brook lamprey (Ichthyomyzon fossor) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

This species is affected by ongoing lampricide applications conducted by Canadian and American agents of the sea lamprey management program in the Great Lakes basin. These applications reduce populations of sea lamprey; however, other lamprey species are similarly vulnerable to the chemical (King and Gabel 1985). Some streams with northern brook lamprey that have been infested by sea lamprey and subsequently treated with lampricide have undergone significant reductions, or extirpations, of populations of native lampreys. Larval northern brook lamprey are 25% less susceptible to the lampricide than sea lamprey larvae, but this difference is insufficient to allow for control of sea lamprey without impacting native lampreys (King and Gabel 1985). Barriers to sea lamprey migration offer some refuge in the upper reaches of streams that often support northern brook lamprey, as these portions of the stream are not exposed to the chemical applications. Barriers can also serve as a threat to these lamprey, potentially limiting gene flow (Schreiber and Engelhorn 1998). However, this threat is likely minor due to the limited migration of northern brook lamprey.

Fluctuating water levels are suspected to cause ammocoete mortality, due to low water levels exposing larval burrows (Bailey 1959), and flooding conditions causing excessive movement (Potter 1980b). Renaud et al. (1995) has also listed pollution (specifically, an herbicide called atrazine) as a possible contributor to ammocoete mortality. The Yamaska River in Quebec, which once had a high density of northern brook lamprey (Vladykov 1952), was found, 40 years later, to not have ammocoetes of any species (Renaud et al. 1995). Renaud et al. (1995) speculated that phytoplankton levels were reduced by this chemical; thereby limiting food availability for the ammocoetes.

The removal of riparian vegetation is also thought to contribute to lamprey decline (Fortin et al. 2005). This trend, which often accompanies agricultural and suburban development, increases sediment load in a stream and decreases shade and natural filtering of fertilizers and pesticides. The extent of all of the above listed threats has not been quantified.

The introduction of the American brook lamprey to streams along the north shore of Lake Superior (D. Cuddy, pers. observ.) may pose an additional threat to northern brook lamprey in this region. American brook lamprey and northern brook lamprey rarely co-exist (Becker 1983), and this introduction and subsequent range increase may result in competition to the detriment of the less common northern brook lamprey. However, there is no documentation of northern brook lamprey extirpation caused by the introduction of other lamprey species.

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