Weaver ei al.: Influence of nutrients from carcasses of Petromyzon marinus on growth of larval conspecifics 
143 
toperiod stimulate primary production and increase 
the metabolic demand of consumers, including young- 
of-the-year fish and macro-invertebrates (Hall, 1972; 
Gustafson-Greenwood and Moring, 1990). The death of 
sea lamprey and the nutrients from their carcasses can 
alleviate nutrient limitations of streams and increase 
productivity to the overall benefit of freshwater com¬ 
munities (Weaver et al., 2016). 
Carcasses of sea lamprey may directly benefit con- 
specifics. After hatching, larval sea lamprey burrow 
into fine substrate and reside in freshwater as deposit¬ 
feeding detritivores (Hardisty and Potter, 1971; Evans 
and Limburg, 2015). In a recent study, Weaver (2017) 
provided evidence of larval assimilation of nutrients 
from adult carcasses by documenting enrichment of 
the 13 C isotope among individual fish collected near 
carcasses. Larval sea lamprey that receive these nu¬ 
trient subsidies may grow faster than larvae without 
access to such nutrients. During summer, larvae initi¬ 
ate metamorphosis, a nontrophic period characterized 
by a series of physical and physiological changes and 
arrested somatic growth and feeding (Youson, 1980; 
Youson and Manzon, 2012). After several months, the 
juveniles (macropthalmia) migrate toward the ocean to 
begin feeding as parasites (Potter et al., 1978). Thus, 
the accrual of a surplus of lipids (i.e., energy) during 
the freshwater larval period is critical for both the de¬ 
velopmental “decision” to metamorphose and for the 
survival and migration of newly metamorphosed lam¬ 
prey (Lowe et al., 1973). 
Freshwater productivity can directly influence 
growth rates and drive population dynamics of fish 
populations. Although data that directly link produc¬ 
tivity to growth rates are absent for sea lamprey, there 
is strong evidence of a correlation. The time from the 
larval period to metamorphosis, for example, has a 
range of 2-14 years among sea lamprey populations 
in freshwater of varying productivity (Manion and 
Smith 1 ; Beamish, 1980; Potter, 1980; Purvis, 1980; 
Beamish et al. 2 ; Morkert et al., 1998; Quintella et al., 
2003). Larval growth rates and age at metamorphosis 
may also be influenced by stream productivity (Potter, 
1980; Purvis, 1980; Dawson et al., 2015). Temperature, 
stream conductivity, and dissolved solids (which are 
proxies for productivity) were found to be significant 
predictors of growth rate (Holmes, 1990; Young et al., 
1990; Griffiths et al., 2001). Among tributaries in the 
Great Lakes, the first occurrence of metamorphosis 
of sea lampreys ranges from age 2 in faster growing 
populations to age 7 in slower growing populations 
(Purvis, 1980; Morkert et al., 1998). This variability in 
1 Manion, P. J., and B. R. Smith. 1978. Biology of larval 
and metamorphosing sea lampreys, Petromyzon marinus, of 
the 1960 year class in the Big Garlic River, Michigan, Part 
II, 1966-72. Great Lakes Fish. Comm., Tech. Rep.30, 33 
p. [Available from website.] 
2 Beamish, F. W. H., B. J. Morrison, L. A. Barker, and B. 
J. Wicks. 1998. Ecology of recruitment in sea lam¬ 
prey. Great Lakes Fish. Comm., Proj. Compl. Rep. Summ., 
22 p. [Available from website.] 
the duration of the larval period and initiation of meta¬ 
morphosis may be influenced by the influx of nutrients 
from the mortality of postspawning adult sea lamprey. 
This influx, in turn, may create an alternative stable 
state that reinforces production of spawning adults and 
promotes population persistence (Kefi et al., 2016). 
We assessed the theoretical influences of carcass 
nutrients on larval sea lamprey, with the assumption 
that growth and subsequent metamorphosis of larvae 
are linked to freshwater productivity. We hypothesize 
that carcass-mediated productivity may shift the age 
at which larvae undergo metamorphosis. We used data 
and values from the literature to construct a heuristic 
model and probed the sensitivity of various life-history 
parameters on the growth and metamorphosis of lar¬ 
vae. We compared a hypothesized scenario in which 
carcass nutrients from adult sea lamprey had no effect 
on larval growth to a scenario in which increases in 
productivity mediated by carcasses increased growth. 
Finally, we explored theoretical effects of changes in 
metamorphosis on demographic (age) structure of lar¬ 
val populations. 
Materials and methods 
Population modeling 
We used data and values of life-history parameters ob¬ 
tained from scientific literature to create a determin¬ 
istic stock-recruitment model with the software STEL¬ 
LA 3 , vers. 10.0.6 (isee systems, Inc., Lebanon, NH). 
Our model had 3 major parts: 1) a recruitment model; 
2) a growth model; and 3) a nutrient feedback model 
(Fig. 1). The model was designed to capture the entire 
life history of sea lamprey beginning with larval re¬ 
cruitment in freshwater, metamorphosis and migration 
to the ocean, ocean survival, migration back to fresh¬ 
water, and subsequent death and nutrient deposition. 
We describe 2 populations: one in which larval growth 
rates are not influenced by returning adult sea lam¬ 
prey (i.e., unsubsidized populations) and one in which 
larval growth rates are influenced by returning adult 
sea lamprey (i.e., subsidized populations). 
Recruitment 
We characterized recruitment in early life stages by 
using a Ricker stock-recruitment relationship (Ricker, 
1975; Guy and Brown, 2007). A Ricker curve depicts 
an increase and then a decrease in larval recruitment 
with increasing numbers of spawning adults, a rela¬ 
tionship that is characteristic of an increasing prob¬ 
ability of nest superimposition that may be observed 
among species that construct nests (e.g., sea lamprey; 
3 Mention of trade names or commercial companies is for iden¬ 
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