SIGNIFICANCE OF ORGANIC DETRITUS IN THE DIET OF LARVAL LAMPREYS IN THE GREAT-LAKES BASIN

Larval sea lamprey (Petromyzon marinus) and northern brook lamprey (Ic hthyomyzon fossor) were collected monthly from three streams in the Up per Peninsula of Michigan from May 1992 through May 1993 and larval se a lampreys were collected during summer months from sites throughout t he Great Lakes basin. Organic detritus made up most of the diet ash-fr ee-dry-mass (AFDM) throughout the year, averaging 97.79%, with algae ( 2.12%) and bacteria (0.09%) making up the remainder of the diet AFDM. Assimilation efficiency for AFDM averaged 72% during warmer months and 53% during cooler months (annual mean = 61%). Gut fullness (amount of AFDM in the anterior one-tenth of the intestine) was low (mean = 0.10 mg diet AFDM . g(-1) ammocoete). There were no significant difference s in these measures between ammocoetes collected from the Upper Penins ula and those collected throughout the Great Lakes basin. From a labor atory-determined relationship between gut fullness and feeding rate, f eeding rate in the field was estimated to be extremely slow, ranging f rom 4.2 to 5.5 mg diet AFDM . g(-1) ammocoete . d(-1). These observati ons indicate that larval lampreys efficiently utilize a diet of organi c detritus during warmer months when stream temperatures and food qual ity are more favorable for feeding, digestion, and growth.