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722 
Analysis of diet and feeding strategies 
within an assemblage of estuarine 
larval fish and an objective assessment 
of dietary niche overlap 
Daniel J. Gaughan 
San C. Potter 
School of Biological and Environmental Sciences 
Murdoch University 
Murdoch, Western Australia 6 1 50, Australia 
Present address for senior author: WA Marine Research Laboratories 
Bernard Bowen Fisheries Research Institute 
Fisheries Department of Western Australia 
PO Box 20, North Beach, WA 6020, Australia 
E-mail address (for D, J. Gaughan): dgaughan@fish.wa.gov.au 
Abstract .—Fish larvae and zoo- 
plankton were sampled during seven 
consecutive months from four regions 
of Wilson Inlet, an estuary in south- 
western Australia. Mouth size, prey 
size, and dietary composition of larvae 
of the gobiids Afurcagobius suppositus, 
Pseudogobius olorum, and Favonigobius 
lateralis, the blenniid Parablennius 
tasmanianus, and the syngnathid Uro- 
campus carinirostris were determined. 
Dietary niche overlap (DNO) was calcu- 
lated for co-occurring species pairs, 
both with and without incorporating a 
measure of relative prey (zooplankton) 
abundance. Significance of DNO was 
assessed 1) objectively, with boot- 
strapping of the dietary data and 2) 
subjectively, by assigning significance 
to values >0.6. The diet of A. suppositus 
was dominated by harpacticoids, poly- 
chaete larvae, and the calanoid Gladio- 
ferens imparipes, whereas diets of the 
other species were dominated by cope- 
pod nauplii and postnaupliar stages of 
the cyclopoid Oithona simplex, the pro- 
portions of the latter increasing with 
growth of the larvae. Small numbers of 
large and small prey items were found 
in the stomachs of A. suppositus 
(mean=2.5), which had the largest 
mouth, whereas large numbers (mean= 
28.7) of small prey and no large items 
were found in the stomachs of P. 
tasmanianus, which had the second 
largest mouth. Between these ex- 
tremes, P olorum, U. carinirostris, and 
F. lateralis each ate mostly small and 
intermediate-size prey, supplemented 
by a few large prey. The data did not 
support the hypothesis that an increase 
in the difference in gape size between 
species would decrease the prevalence 
of significant DNO. The lack of a con- 
sistent relation between mouth size and 
DNO among the five species is evidence 
that interspecific dietary differences 
reflect differences in feeding behavior. 
With bootstrapping, the prevalence of 
significant (P<0.05) DNO between spe- 
cies pairs was 32.6% when prey data 
were included in the analyses and 
46.5% when prey data were not in- 
cluded. By subjectively assigning sig- 
nificance to DNO values >0.6, we ob- 
tained substantially less conservative 
estimates that indicated the prevalence 
of significant DNO was >53%. 
Manuscript accepted 31 March 1997. 
Fishery Bulletin 95:722-731 (1997). 
Starvation has been considered a 
major cause of mortality in larval 
fish (e.g. Hunter, 1984), although 
evidence from the field has been dif- 
ficult to obtain (Heath, 1992). If 
starvation occurs within an assem- 
blage of larval fish, competition for 
food is expected to contribute to that 
starvation. However, indications of 
any such competition may go unde- 
tected if the diet of only a single spe- 
cies is examined or if the influence 
of other planktivores (Fortier and 
Harris, 1989) is not considered. In 
the case of larval fish, relatively few 
studies have examined in detail the 
diets of several co-occurring species 
(e.g. Last, 1980; Laroche, 1982; 
Govoni et al., 1983; Watson and 
Davis, 1989). Furthermore, no 
study has objectively assessed the 
significance of dietary niche over- 
lap (DNO), where DNO refers to the 
amount of sharing of food resources 
among larval fish. Assessments of 
whether dietary overlap within or 
between species (including larval 
fish) is significant have been based 
on indices that range from 0 for no 
overlap to 1 for complete overlap, 
with values greater than 0.5, 0.6, or 
0.7 considered to be significant (e.g. 
Harmelin-Vivien et al., 1989; Cervel- 
lini et al., 1993; Vega-Cendejas et al., 
1994; Hartman and Brandt, 1995). 
However, because these cutoff 
points are arbitrary, they are not 
necessarily biologically significant, 
as in the case with fish larvae where 
individuals of co-occurring species 
may be confronted with large con- 
centrations of the same prey type 
and thus any similarities in diet 
may be due to chance encounters. 
Although dietary compositions of 
larval fish have been considered in 
the context of the abundance of the 
zooplankton prey of those larvae 
(e.g. Dagg et al., 1984; Jenkins, 
1987; Hirst and DeVries, 1994; 
Welker et al., 1994), no study of di- 
etary overlap between larval fish has 
incorporated data on their zooplank- 
ton prey. This analysis is necessary 
in order to assess whether there is a 
likelihood of competition for food. 
Differences in mouth structure of 
fish may lead to differences in feed- 
ing success on particular prey types 
(Lavin and McPhail, 1986). Co-oc- 
curring larvae of different species 
with similar-size mouths may 
therefore exhibit a higher rate of 
significant DNO than those with 
mouths of different size. Because 
fish larvae usually swallow prey 
