Weaver et al.: Influence of nutrients from carcasses of Petromyzon marinus on growth of larval conspecifics 
147 
tion for the simulation of subsidized populations 
with the incorporation of growth feedback. Sub¬ 
sidized populations of total spawners were vari¬ 
able for 100 years, then began to stabilize at a 
population size of approximately 5500 individuals, 
indicating a 12% increase in adult returns (Fig. 2). 
We simulated changes in the distribution of 
age classes when larvae achieved higher growth 
rates from the input of nutrients from adult car¬ 
casses. When we assumed no feedback, >95% of 
all metamorphosed larvae were 8 years or older 
(Fig. 3). When we assumed nutrient feedback, 
larval populations shifted toward younger age 
classes. Approximately 30% of all metamorphosed 
larvae were younger than 8 years. The shifts in 
age at metamorphosis toward younger individu¬ 
als correspond with the manipulations of age at 
length in the Von Bertalanffy growth model (Fig. 
4). Results from the model indicate higher growth 
rates among the subsidized populations that al¬ 
lowed larvae to reach the minimum length at 
metamorphosis (160 mm TL) at an earlier age 
in contrast with results for unsubsidized popula¬ 
tions that reached the minimum length at meta¬ 
morphosis at relatively older ages. 
The results of our sensitivity analysis 
indicated that age-1 larval mortality was 
relatively sensitive to change (IS I >1.00; 
Table 2). Generally, we found that increases 
in larval mortality resulted in lower returns 
of adult spawners and, conversely, that re¬ 
duced larval mortality resulted in increases 
in the number of total spawners. Total mod¬ 
eled spawner abundance was inversely in¬ 
fluenced by increases in mortality related 
to larval metamorphosis and the juvenile 
phase in the ocean; however, these param¬ 
eters were less sensitive to change. Con¬ 
sistent with the role of the 2 constants in 
the Ricker function, a was positively associ¬ 
ated with larval recruitment and (3, which 
delineates carrying capacity, was nega¬ 
tively related. Neither of these parameters 
was deemed sensitive on the basis of our a 
priori sensitivity threshold (1.00), although 
both sensitivities approached the measure. 
Among subsidized populations, we found 
that changes in y and 5 parameters resulted 
in larger annual fluctuations in the number 
of total spawners and potentially prolonged 
the time necessary for populations to stabi¬ 
lize, but we did not find these parameters to 
be highly sensitive with respect to spawner 
Results abundance in the stabilized population. 
The population model was adjusted to simulate unsub¬ 
sidized populations of total spawners at approximately Discussion 
4800 individuals (Fig. 2), which is a reasonable estimate 
of spawner carrying capacity in a small, third-order We constructed a deterministic model to illustrate the 
stream. This number was used as the starting popula- complex life history of sea lamprey, and the manipula- 
o 
in 
o 
sz 
Q. 
6 
E >. 
CO 0) 
0) a 
E E 
-= tS 
CO — 
o 
£ to 
_ m 
o 
c 
CD 
o 
CD 
CL 
30 i 
25 - 
20 
15 - 
10 - 
5 - 
d Growth feedback (“subsidized”) 
I I Baseline (“unsubsidized”) 
0 i-1-1-1-r 
1 2 3 4 5 6 7 8 
Age 
10 11 12 
Figure 3 
Percentage of total number of metamorphosic sea lamprey ( Petro¬ 
myzon marinus ) for each age class in baseline unsubsidized model 
populations (gray bars) in which there are no effects of nutrients 
from carcasses of sea lamprey on larval growth, and corresponding 
changes in metamorphosic sea lamprey for each age class in sub¬ 
sidized modeled populations (black bars). The model for subsidized 
populations manipulates growth to account for the effect (or growth 
feedback) from nutrients released from carcasses. ' 
CD) 
c 
c 
S 
a) 
Ql 
co 
o 
a 
E 
Figure 2 
Number of spawning sea lamprey (Petromyzon marinus ) that 
return to freshwater from the ocean 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 spawner abundance from subsidized 
model populations (solid line). The model for subsidized 
populations manipulates growth to account for the effect (or 
growth feedback) from nutrients released from carcasses. 
