Identifying Food and Nutritional Resources Supporting Invasive Sea Lamprey Ammocoetes Using Isotopic Natural Abundances

Sea lamprey (Petromyzon marinus) invaded the Laurentian Great Lakes in the mid 1900s, resulting in the collapse of important commercial and recreational fish stocks until control measures were enacted in the 1960s. While the diet of the sea lamprey juvenile stage (which is parasitic on teleost fishes) has been well described, this stage constitutes only a small portion  (~1.0-1.5 yrs) of the animal’s total life of ~4-8 years. In contrast, larval sea lamprey (i.e., ammocoetes) burrow in sediments and filter feed for up to 7 years before metamorphosing and becoming parasitic juveniles. The diet of ammocoetes is both ambiguous and largely non-quantitative due to previous methods of study (i.e., predominantly gut content analysis). Through the use of natural stable isotopes (δ¹³C, δ¹⁵N, and δD) and natural radiocarbon (Δ¹⁴C) we identify and estimate the relative amounts of different autochthonous and allochthonous organic materials (OM) ingested and assimilated for growth and maintenance during different times of the ammocoete stage of the sea lamprey. Sea lamprey ammocoetes were collected from three sites on two rivers in the Lower Peninsula of Michigan in May and June of 2010 and again in October 2010. Ammocoete δ¹³C increased from ~-32‰ to ~-24‰ with increasing animal length, suggesting a shift in diet during ammocoete growth. Using isotopic mass balances, smaller ammocoetes were estimated to rely on phytoplankton for up to ~70% of their diet, but they also assimilated significant amounts of aquatic sedimentary OM and terrestrial detrital OM. The largest size classes of sea lamprey ammocoetes were unconstrained isotopically in the current study, suggesting that they assimilated another as-yet unidentified food source(s) having enriched ¹³C and ¹⁵N signatures. From prior studies on gut contents, and corroborated by the positive relationship between ammocoete C:N and animal size in the present study, larger ammocoetes ingest significantly greater amounts of low-quality aquatic detritus than smaller size classes. As a result, larger ammocoetes appear to depend to a lesser extent on fresh phytoplankton OM for their growth than their smaller counterparts. The more accurate identification and quantification of potential food sources supporting sea lamprey ammocoetes may allow for the development of alternate or more efficient control methods than those now in common use.