STOCK ASSESSMENT OF WHITE GRUNT FROM THE WEST ...

Similarly, point estimates of annual mortality rate for white grunt caught in the head-boat
fishery calculated using Robson and Chapman’s (1961) estimation procedure were 0.280 and
0.303 for spawning and post-spawning females, respectively, and 0.376 and 0.375 for spawning
and post-spawning males, respectively (Murie and Parkyn 1998). Hence total annual mortality
for female grunts based on Robson and Chapman’s estimates were within the 95% confidence
interval calculated using regression analysis, but for males the estimates based on Robson and
Chapman’s method were slightly greater than the upper confidence limit of the catch-curve
analysis.
Manooch (1976) estimated mean total mortality for white grunts from North Carolina and
South Carolina in the 1970’s to be between 37 and 51%, slightly greater than the total mortality
range for white grunts in the Gulf in 1998 (range of 28 to 38 %). The catch curve that Manooch
(1976) used to calculate total mortality for white grunts from the Carolinas was fundamentally
different than the catch curve for white grunts in the Gulf of Mexico (Murie and Parkyn 1998).
In particular with regard to total mortality estimates, Manooch (1976) did not have white grunts
fully recruited into the head-boat fishery until they were age 7, and the oldest fish that was aged
in his study was age 13.
Both of the methods used to estimate survival rate, and hence mortality rate, of white
grunts in the Gulf are based on the underlying assumptions that for the age classes in the
analysis: 1) survival rate is uniform with age, and hence mortality rate is uniform with age; 2)
mortality rate has not changed with time; 3) all ages represented in the analysis have been
sampled randomly (i.e., they are equally vulnerable to the sampling gear); and 4) recruitment is
constant for all year classes (Ricker 1975). It was readily apparent that white grunts may have
undergone significant changes in recruitment among year classes. This was especially evident
with age class 5, which was the 1993 year class, because it was poorly represented in both
females and males. Although the mechanism(s) involved in producing a weak or strong year
class are unknown and difficult to evaluate (i.e., spawning failure, differential recruitment,
density-dependent mortality, etc.), trends in year class strengths can be effectively assessed
through cohort survivorship curves by obtaining catch curves over a series of years or,
alternatively, by a tag-recapture program (Robson and Chapman 1961, Krebs 1989).
Annual estimates of average fishing mortality for white grunt (made using the modified
DeLury models) peaked during the early to mid 1990s on both coasts. On the Atlantic coast,
average instantaneous fishing mortalities peaked at 0.47-0.49 per year in 1992 then decreased
through 1995 but has increased since (Fig. 24). The range of estimates for instantaneous fishing
mortality in 1998 was 0.25-0.28 per year. On the Gulf coast, the peak in fishing mortality
occurred in 1994 or 1995 when F reached about 0.30 per year (Fig. 24). Since then fishing
mortality has declined reaching about 0.18 per year in 1998. Given an estimate of M equal to
0.2 per year, the Murie and Parkyn (1998) estimate of total mortality would represent
instantaneous fishing mortalities (F) ranging 0.08-0.23 per year for Gulf coast populations of
white grunt. Similarly, the Manooch (1976) estimate of total mortality for the Atlantic coast
grunt population would represent Fs ranging from 0.26-0.51 per year.
The estimated average F relative to F MSY (fishing mortality at maximum sustainable
yield) showed the same patterns seen for the absolute estimates of F (modified DeLury
estimates). The estimated ratios of F/ F MSY are much more precise than are the absolute
estimates of F made using the biomass dynamic model ASPIC (Prager 1994).
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