148 
Fishery Bulletin 11 6(2) 
200 
180 
160 
>> 1 
CD 
140 
E ~ 
§ g> 
120 
100 
03 ® 
CD _ 
CO S 
80 
O 
60 
40 
20 
Metamorphosis 
- Growth feedback (“subsidized”) 
— — Baseline (“unsubsidized”) 
-i-1-1-r- 
1 2 3 4 5 6 7 8 9 10 11 12 
Age 
Figure 4 
Age at length of larval sea lamprey (Petromyzon marinus ) 
from a modeled von Bertalanffy growth curve in baseline 
unsubsidized model populations (dashed line) in which there 
are no effects of nutrients from carcasses of sea lamprey on 
the growth of larvae and changes in age at length in subsi¬ 
dized model populations (solid line). The model for subsidized 
populations manipulates growth to account for the effect (or 
growth feedback) from nutrients released from carcasses. 
Table 2 
Sensitivity (S) of the total number of returning, spawn¬ 
ing sea lamprey (Petromyzon marinus ) determined from 
stabilized model runs to a 1% increase in model param¬ 
eters. Variables include larval mortality at age (M larv ), 
mortality at metamorphosis and time of migration to 
the ocean (M met ), juvenile mortality (Mj uv ), a and p pa¬ 
rameters of the Ricker stock-recruitment relationship, 
and y and 8 logistic growth parameters of the nutrient 
feedback model. I S I values above 1.00 were deemed 
sensitive and are indicated in bold. 
Parameter 
Recruitment 
Mortality 
Logistic growth 
Nominal value 
S 
a 
0.0014 
0.833 
P 
2.84 x 10- 9 
-0.958 
•^larvl 
0.74 
-1.895 
M)arv2 
0.07 
-0.208 
M)arv3 
0.05 
-0.062 
•^Marv4 
0.09 
-0.062 
^larv5 
0.12 
-0.062 
•^larv6 
0.10 
-0.062 
■^larv7 
0.10 
-0.062 
^larv8 
0.10 
-0.062 
■^larv9 
0.10 
-0.062 
-^larvlO 
0.10 
-0.042 
■^larvll 
0.10 
0 
(^larvl2 
0.10 
0 
0.60 
-0.521 
M JUV 
0.413 
-0.708 
y 
-4.0 
-0.563 
5 
1.0 x 10- 4 
0.273 
tion of model parameters allowed us to explore 
potential nutrient feedbacks between freshwater 
and marine ecosystems. Previous research has 
shown increases in freshwater productivity from 
carcasses of sea lamprey and assimilation of car¬ 
cass nutrients by larval conspecifics (Weaver et 
al., 2016; Weaver, 2017). Results from our model 
indicate the potential role of carcass nutrients in 
stimulating larval growth and lowering the age at 
metamorphosis as a result of higher productivity. 
This model clearly illustrates the critical impor¬ 
tance of nutrient exchange among anadromous 
fish between freshwater and marine systems, an 
exchange that is likely to result in population- 
level changes. 
We hypothesized that recruitment dynamics of 
sea lamprey are driven by changes in in-stream 
primary production, but recruitment may also be 
shaped by density-dependent factors (e.g., prey 
abundance and competition; Klanderud, 2010), 
independent processes (e.g., temperature or flow; 
Tonkin et al., 2017), or a combination of both 
(Jonsdottir et al., 2013). Juvenile sea lamprey 
in the ocean function as parasitic predators and 
their successful recruitment may, therefore, re¬ 
spond to an additional suite of biotic and abiotic 
factors, including less obvious factors related to 
the population dynamics of their hosts. 
We show that changes in productivity may influence 
population demography, but other life-history attri¬ 
butes may be affected as well. For example, the sex-ra¬ 
tio of larval populations in more productive waters may 
become skewed toward females (Johnson et ah, 2017), 
a shift that conceivably is a result of the addition and 
availability of carcass nutrients. Furthermore, one 
male may mate with several females (Beamish, 1980; 
Gardner et al., 2012), and recruitment dynamics may 
be further influenced by populations that are subsi¬ 
dized by carcass nutrients. Results from our model 
also indicate that population demographics may have 
an even farther reaching influence that could extend to 
other species and across ecosystems. High densities of 
larval sea lamprey may reduce individual growth rates 
and survival and extend the duration of the larval pe¬ 
riod and hence delay metamorphosis (Morman, 1987; 
Murdoch et al., 1992). Established populations that are 
at or near carrying capacity likely experience slower 
growth that is reflective of limited resources. Converse¬ 
ly, low densities of larvae, characteristic of populations 
facing impaired migratory access and thus fewer adult 
returns, may show higher growth rates among all co¬ 
horts (Purvis 7 * ; Torblaa and Westman, 1980; Morman, 
1987). Models that do not account for density-depen¬ 
dent growth may not accurately capture associated 
7 Purvis, H. A. 1979. Variations in growth, age at transfor¬ 
mation, and sex ratio of sea lampreys reestablished in chemi¬ 
cally treated tributaries of the upper Great Lakes. Great 
Lakes Fish. Comm., Tech. Rep.35, 36 p. [Available from 
website.] 
