174 
Fishery Bulletin 115(2) 
Table 2 
Contributions of prey taxa to the diets of winter flounder (Pseudopleuronectes americanus) and summer flounder {Paralichthys 
dentatus) expressed as frequency of occurrence (%F), numeric percentage {%N), volumetric percentage (%V), and the index 
of relative importance (%IRI, Eq. 1). Mean values (and standard errors [SEs]) were calculated by using a cluster sampling 
estimator (Eqs. 2 and 3). Size-independent estimates of Levins niche breadth index {B; Eq. 4) and Schoener dietary overlap 
index (a; Eq. 5) are also presented. Winter and summer flounder were collected from the Seekonk and Taunton Rivers during 
2009-2015. 
Winter flounder Summer flounder 
Prey taxon 
%F 
%N 
%v 
%IRI 
%F 
%N 
%v 
%mi 
Crustaceans 
Amphipoda (amphipod) 
81.2 (SE 0.3) 
27.9 (SE 0.3) 
47.7 (SE 0.3) 
48.3 (SE 0.3) 
41.9 (SE 0.4) 
14.8 (SE 0.3) 
15.6 (SE 0.2) 
15.0 (SE 0.3) 
Isopoda (isopod) 
Anthuridae (anthurid isopod) 
8.4 (SE 0.3) 
1.3 (SE 0.1) 
2.3 (SE 0.1) 
1.3 (SE 0.1) 
Idoteidae (idoteid isopod) 
3.4 (SE 0.1) 
0.4 (SE 0.01) 
0.7 (SE 0.03) 
0.4 (SE 0.02) 
0.2 (SE 0.02) 
<0.1 
0.2 (SE 0.01) 
<0.1 
Tanaidacea (tanaid) 
- 
- 
- 
- 
0.3 (SE 0.02) 
<0.1 
<0.1 
<0.1 
Cumacea (cumacean) 
0.3 (SE 0.08) 
<0.1 
<0.1 
<0.1 
1.8 (SE 0.1) 
0.3 (SE 0.01) 
0.4 (SE 0.02) 
0.2 (SE 0.02) 
Calanoida/Harpacticoida 
(copepod) 
66.4 (SE 0.4) 
44.5 (SE 0.4) 
19.4 (SE 0.2) 
25.6 (SE 0.3) 
14.5 (SE 0.3) 
15.5 (SE 0.5) 
3.7 (SE 0.1) 
4.0 (SE 0.1) 
Ostracoda (ostracod) 
8.9 (SE 0.2) 
0.6 (SE 0.01) 
0.4 (SE 0.01) 
0.1 (SE 0.01) 
1.1 (SE 0.1) 
0.2 (SE 0.01) 
<0.1 
<0.1 
Mysidacea 
(mysid shrimp) 
1.3 (SE 0.1) 
<0.1 
0.2 (SE 0.01) 
0.1 (SE 0.01) 
70.8 (SE 0,5) 
51.6 (SE 0.5) 
49.1 (SE 0.6) 
54.4 (SE 0.6) 
Decapoda(decapod) 
Crangon septemspinosa 
(sand shrimp) 
<0.1 
<0.1 
<0.1 
<0.1 
29.6 (SE 0.5) 
10.4 (SE 0.2) 
20.1 (SE 0.4) 
18.6 (SE 0.4) 
Palaemonidae (grass shrimp) 
- 
- 
- 
- 
0.1 (SE 0.01) 
<0.1 
0.1 (SE 0.01) 
<0.1 
CalUnectes sapidus (blue crab) 
- 
- 
- 
0.2 (SE 0.01) 
<0.1 
<0.1 
<0.1 
Grapsidae or Xanthidae 
(shore or mud crab) 
0.8 (SE 0.03) 
0.3 (SE 0.01) 
0.4 (SE 0.01) 
0.3 (SE 0.01) 
0.8 (SE 0.03) 
0.2 (SE 0.01) 
0.4 (SE 0.01) 
0.2 (SE 0.01) 
Pagurus longicarpus 
(longwrist hermit) 
<0.1 
<0.1 
<0.1 
<0.1 
Unidentified crab 
(zoea life stage) 
<0.1 
<0.1 
<0.1 
<0,1 
Unidentified crab 
(megalope life stage) 
0.9 (SE 0.03) 
0.3 (SE 0.01) 
0.2 (SE 0.01) 
0.2 (SE 0.01) 
0.2 (SE 0.01) 
<0.1 
<0,1 
<0.1 
Unidentified crustacean 
0.3 (SE 0.02) 
<0.1 
<0.1 
<0.1 
0.2 (SE 0.01) 
0.1 (SE 0.01) 
<0.1 
<0.1 
Insects 
Chironomidae (midge larvae) 
22.8 (SE 0.5) 
6.9 (SE 0.2) 
8.9 (SE 0.2) 
6.3 (SE 0.2) 
0.2 (SE 0.01) 
<0.1 
<0.1 
<0.1 
Coleoptera (beetle) 
<0.1 
<0.1 
<0.1 
<0.1 
- 
- 
- 
- 
Diptera (pupae life stage) 
<0.1 
<0.1 
<0.1 
<0.1 
0.4 (SE 0.03) 
<0.1 
0.1 (SE 0.01) 
<0.1 
Formicidae (ant) 
<0.1 
<0.1 
<0.1 
<0.1 
- 
- 
- 
- 
Unidentified insect 
<0.1 
<0.1 
<0.1 
<0.1 
0.7 (SE 0.05) 
0.1 (SE 0.01) 
<0.1 
<0.1 
Arachnids 
Acarina (mite) 
<0.1 
<0.1 
<0.1 
<0.1 
Aranae (spider) 
- 
- 
- 
- 
0.1 (SE 0.01) 
<0.1 
<0.1 
<0.1 
Table continued 
niche dietary breadth was more variable across lengths 
and riverine sites, and moderate and high diet diver¬ 
sity was common for summer flounder >115 mm TL 
(Fig. 4B). 
Ontogenetic effects on intraspecific diets 
Hierarchical cluster analyses revealed distinct dietary 
groups for winter flounder and summer flounder, and 
these groups corresponded with 4 broad size categories 
for each species (Fig. 3). After accounting for the size- 
dependent effects on diet, the corrected cluster sample 
sizes for winter and summer flounder {n^) equaled 242 
and 157, respectively (i.e., 1 seine haul resulted in >1 
cluster when multiple size classes were present). Small 
winter flounder (<39 mm TL) had a 94.9% similarity 
in diet (SIMPROF: ii=0.16, F=0.83; Fig. 3A) and fed 
predominantly on copepods (%/i?/=76.5%), polychaetes 
(%li?l=11.8%; unidentified and Fabricia sabella), and 
amphipods (%/J?J=11.4%) (Fig. 5A). Winter flounder 
