14 
Fishery Bulletin 107(1 ) 
Figure 1 
Commercial landings (in metric tons) of white trout ( Cynoscion nothus 
and C. arenarius combined), from 1976 to 2006. Data provided by the 
Office of Science and Technology, National Marine Fisheries Service, 
Silver Spring, MD, 2007. 
overlap between the species. Addition- 
ally, hybridization, regional differentia- 
tion, or a combination of both, may often 
confound the trademark diagnostics used 
to distinguish between the two species. 
In any event, the superficial similarity 
of these species indicates that morpho- 
logical divergence has been minimal 
and the concurrent bimodal timing of 
spawning indicates overlapping life his- 
tory parameters (Sheridan et al., 1984). 
Distributional data seem to indicate that 
these species exhibit some habitat par- 
titioning (primarily by water depth and 
distance from the shore) with the re- 
sult that silver seatrout are found more 
frequently in deeper water farther from 
shore (Ginsburg, 1931; Byers, 1981). 
These life history and distributional 
data yield a framework for devising hy- 
potheses to test the influence of niche 
overlap on historical associations be- 
tween sand and silver seatrout populations. In par- 
ticular, if hybridization between these species occurs, it 
is likely to occur in areas of contact such as nearshore 
marine waters used commonly by both species. Hy- 
bridization in the genus Cynoscion has been previously 
documented on the Atlantic coast of Florida (Cordes 
and Graves, 2003). In their initial examinations, Cordes 
and Graves (2003) characterized populations of gray 
weakfish using genetic techniques and in doing so also 
identified putative hybrids between gray weakfish and 
either sand or silver seatrout. This identification was 
accomplished by using four microsatellite markers and 
two nuclear intron gene regions (restriction fragment 
length polymorphisms, or RFLP’s). Although these 
markers were appropriate for identification of hybrids 
with gray weakfish, they were ineffective for determin- 
ing conclusively whether the second gametic contribu- 
tion was made by sand or silver seatrout. 
Here, both morphological and molecular (nuclear mi- 
crosatellites and mitochondrial restriction fragments) 
data are used to characterize populations of sand and 
silver seatrout from the nearshore Gulf waters outside 
of Galveston Bay, Texas. Three competing hypotheses 
regarding genetic associations between these species are 
evaluated. First, in the case of contemporary hybridiza- 
tion, hybrids would appear as proportionate admixtures 
of both parental forms in microsatellite assignment 
tests. Moreover, the directionality of hybridization could 
be assessed with the use of mtDNA haplotypes (Wirtz, 
1999), and hybrids would likely be found to be interme- 
diate for diagnostic morphological characters (Hubbs, 
1955; Campton, 1987). Second, in the case of historical 
association, such as lineage overlap during speciation or 
lineage admixture after speciation, mtDNA haplotypes 
might be shared between the species despite a mutu- 
ally exclusive assignment of microsatellite genotypes 
(with the assumption of no contemporary hybridiza- 
tion). In such a case, assignment based on microsatel- 
lite genotypes should be more reliable than assignment 
by means of mtDNA haplotypes if mtDNA lineages 
have not been sorted categorically into contemporary 
populations (species). Third, in the case of no gene 
flow between the species, microsatellite assignment 
should be conclusive, mtDNA haplotypes should sort 
conclusively by species, and specimens should not reveal 
morphological intermediates for characters previously 
described as diagnostic among species. Each of these 
competing hypotheses was examined in light of evidence 
from the three data sets. The morphological and genetic 
similarities and differences between the species were 
examined as evidence for hybridization, and as an aid 
for future species identification. Finally, aspects of the 
ecology and life history of each species are invoked to 
explain the patterns of genetic variability within and 
between these congeneric species. 
Materials and methods 
Sample collection and laboratory methods 
In July of 2007, whole fish were collected offshore of 
Galveston Bay, TX, during annual routine monitoring by 
the Texas Parks and Wildlife, Coastal Fisheries Division. 
White trout were collected with a 5.7-m otter trawl with 
38-mm nylon multifilament mesh stretched throughout. 
Trawl tows (n = 4) were conducted parallel to the fathom 
curve at a speed of three mph for ten minutes (Fig. 2). 
After collection, fish were frozen and transported to the 
Perry R. Bass Marine Fisheries Research Station in 
Palacios, Texas, for processing. 
The sample consisted of 60 young adult sand seat- 
rout and 60 young adult silver seatrout. A single re- 
searcher completed all morphological and meristic 
counts because considerable risk of extraneous vari- 
ance has been demonstrated in data collected by mul- 
