Lough and O'Brien: Life-stage recruitment models for Gadus morhua and Me/anogrammus aeglefinus on Georges Bank 
135 
100 n 
Haddock 
2005 
Figure 8 
Yearly comparison of the various cod ( Melanogrammus aeglefinus ) 
life stage model predictions of recruitment) (triangles) with the 
Georges Bank haddock virtual population analysis (VPA) for age-1 
recruitment (squares). Life-stage model predictions of recruitment 
start from the annual survey spawning abundance of eggs (A), the 
survey hatching abundance of larvae (B), and the abundance of eggs 
derived from the VPA spawning stock biomass [SSB] method (C). 
Icht.hyoplankton survey data were used from the Marine Resource 
Monitoring, Assessment, and Prediction (MARMAP) study (1977-87) 
and U.S. Global Ocean Ecosystems Dynamics (GLOBEC) study 
(1995-99); egg data were lost for the 1977-78 seasons. For the 
exceptional year of 2003, the VPA-derived recruitment was 789x10 s 
fish, whereas the predicted recruitment derived from the modeled 
VPA SSB was 192x10® fish. 
early life stages, when the pelagic juveniles 
also were surveyed on Georges Bank and 
showed that events during the juvenile stage 
may still alter the course of recruitment. 
Hindcasting age-1 recruitment from the 
VPA SSB-derived seasonal egg abundance 
with egg, larval, and juvenile mortality prox- 
ies resulted in the general trend in recruit- 
ment predicted by the VPA, but less so for 
individual years. A comparison of cod and 
haddock egg production from ichthyoplank- 
ton surveys and VPA SSB-derived data did 
not correspond closely for individual years. 
VPA SSB-derived egg production is generally 
considered to be an overestimate of potential 
reproductive output, particularly if the re- 
duced age structure of a population includes 
a greater proportion of first- or second-time 
spawners that produce significantly fewer 
viable eggs and larvae (Murawski et al., 
2001). As cod declined on Georges Bank, a 
greater percentage of females matured at 
an earlier age (O’Brien et al. 1 ). Additional 
corrections were made in this study for the 
higher egg mortality estimates of the first- 
ancl second-time spawners. However, the few 
length-fecundity studies reported in the lit- 
erature that were applied to the entire time 
series may still vary considerably from year 
to year. Moreover, there is no estimate of 
the difference between potential and real- 
ized fecundity. After spawning there is addi- 
tional mortality before eggs are sampled by 
nets. Factors that may have influenced the 
spawning biomass estimates, including the 
differential mortality of cod and haddock egg 
stages, are discussed in Lough et al. (2008). 
Larval mortality rates generally increase 
with increasing temperature, and within a 
narrow temperature range, they increase 
with decreasing growth rates (Morse, 1989; 
Buckley et al., 2010). The increasing sea- 
sonal growth rate in larval cod on Georg- 
es Bank in the spring was most related 
in generalized additive models (GAMs) to 
the increasing photoperiod and less so for 
the increasing temperature (Buckley et al., 
2006). Growth rates were low in late-winter 
spring 1995, but comparatively higher in 
1997-99, corresponding with the increasing 
trend in prey abundance, especially that of 
PseucLocalanus spp. (Buckley and Durbin, 
2006), and decreasing larval mortality rates 
(Mountain et al., 2008). Larval cod and had- 
dock mortality rates were high in 1995 and 
1996 and low during 1997-99. The larger haddock re- 
cruited year class of 1998 had the lowest egg and larval 
mortality in the GLOBEC time series. A combination 
of favorable events appeared to have been associated 
with the 1998 haddock year class. The increased older 
spawners in the population benefited from good feeding 
conditions the previous fall leading to higher egg pro- 
duction and more viable eggs and larvae (Friedland et 
al., 2008). Likewise increased growth rates of larvae in 
the spring associated with higher prey abundance and 
