Clardy et at: Relative contribution of Scomberomorus cavalla stocks to winter fisheries off South Florida 
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Figure 1 
Winter mixing zone established for king mackerel ( Scomberomorus 
cavalla) off south Florida. The zone exists throughout the U.S. 
exclusive economic zone (EEZ) but fish are mostly found over 
the shelf (200-m isobath). All landings taken from the zone from 
November through March are attributed to the Gulf of Mexico 
stock. During the rest of the year, landings are attributed to the 
Atlantic stock. 
somewhat subjective stock assignment 
system was implemented in an effort 
to protect the GOM migratory group, 
which was estimated to be overfished. 
However, assessment and manage- 
ment are complicated by the presence 
of both Atlantic and GOM fish in the 
mixing zone during winter. 
Accurate estimation of the contribu- 
tion of each stock to winter landings 
is necessary for effective management 
and conservation. Several different 
techniques to distinguish these two 
groups have been explored in vari- 
ous studies. Tagging studies support 
the current two-stock management 
approach but have not resolved win- 
ter inter-stock mixing proportions. 
Likewise, although studies of popu- 
lation genetics have confirmed that 
genetically distinct Atlantic and GOM 
stocks exist, genetic divergence be- 
tween the two stocks is weak; thus 
differences are not robust enough to 
distinguish winter landings effec- 
tively (Broughton et al., 2002; Gold 
et al., 2002). 
Analysis of otolith shape has prov- 
en to be a useful technique for stock 
discrimination in several marine tele- 
osts (e.g., Atlantic cod, Gadus moi'hua 
[Campana and Casselman, 1993]; At- 
lantic salmon, Salmo salar [Friedland 
and Reddin, 1994]; and haddock, Melcinogrammus 
aeglefinus [Begg et al., 2001]). DeVries et al. (2002) 
demonstrated that otolith-shape parameters effectively 
distinguish Atlantic and GOM female king mackerel; 
classification accuracies from linear discriminant func- 
tion models ranged from 65.8% to 85.7%. They applied 
otolith-shape variables as natural markers to estimate 
the stock identity of female king mackerel landed be- 
tween Cape Canaveral and West Palm Beach, Flori- 
da, in winter 1996-97. A maximum likelihood model 
parameterized with stock-specific otolith-shape data 
revealed that Atlantic fish represented 99.8% of the 
composition of winter mixed fishery landings, thus cast- 
ing doubt on the management practice of attributing all 
winter mixing zone landings to the GOM stock. 
The objective of this study is to employ otolith-shape 
analysis to examine temporal and spatial variability 
in the Atlantic and GOM stock contribution to king 
mackerel landings around during winter around the 
southern tip of Florida. We build on the earlier suc- 
cess of DeVries et al. (2002) by examining sex-specific 
differences in otolith shape and by estimating the 
contribution of both Atlantic males and females to 
landings in the winter mixing zone. Temporal and 
spatial variability in stock mixing also is examined 
by estimating the Atlantic stock contribution to land- 
ings in three south Florida sampling zones distributed 
across the winter mixing area during two consecutive 
winters. 
Materials and methods 
King mackerel were sampled from recreational land- 
ings caught in eastern GOM and U.S. south Atlantic 
waters from April to October 2001 and 2002 when stock 
distributions did not overlap (Fig. 2); nearly all samples 
came from summer (June through September) months. 
Fish were measured to the nearest cm fork length (FL) 
and sex was determined by macroscopic examination 
of gonads. When possible, both sagittal otoliths were 
removed from fish, but for some samples only one sagitta 
was available. Once extracted, otoliths were cleaned of 
adhering tissue and placed in plastic vials for storage. 
Age of fish was estimated by examining whole otoliths 
for fish less than 80 cm FL and thin sections were 
prepared for aging larger fish (DeVries and Grimes, 
1997). Stratified random sampling was employed once 
all samples were aged to select up to 15 fish from each 
of five age categories: ages 2, 3, 4, 5, and 6 years. This 
age range was selected because winter landings typically 
are of small, young fish. 
King mackerel also were sampled from commercial 
and recreational landings from three different zones 
