2.3. Data analysis
The index of relative importance (IRI)
was used to represent the
dominance of different fish species. This index is a composite measure
that reduces bias in descriptions of animal dietary data, but has been
proposed as a standard method for studying fish species diversity and
community ecology (Paul & Haefner, 1990; Hart et al., 2002; Hill et
al., 2017). The IRI value for each species combines its frequency of
occurrence, percentage of total biomass (PW =
W1-I / WT), and percentage of total
numbers of prey consumed (PN = N1-I /
WT):
IRI = (PW + PN) FO1-I *
104.
A species for which the value of IRI is >300 can be
recognized as a dominant species; in this study, an IRI value of
>1000 was used to denote an absolute dominant fish species.
A Bray–Curtis similarity measure was also computed. This was composed
of a similarity coefficient matrix based on the ratio of non-native fish
to the total number of fish (RN = NN1-X/ NT), as well as the ratio of different non-native
species in the total number of non-native fish species catch
(RNT = NN1-X / NNT). The
RN and RNT were square-root transformed
in the analysis of the similarity matrix across the different rivers.
After that, the similarity indices across the different rivers was used
to perform a clustering analysis with Primer software (Clarke et al.,
2006).
The numbers of non-native fish
species (X) in the data from the Hainan Island rivers and the Continent
coastal rivers was compared using one-way analysis of variance (ANOVA).
The ratios of the most-common non-native native fishes
(RNT = NN1-X / NNT) in
the Hainan Island rivers and the Continent coastal rivers were compared
using a nonparametric test (two independent samples). Linear regressions
were applied to determine the relationship between mean minimum
temperature in the coldest month (data for January 2018) and abundances
of the most widely distributed non-native fish species. All statistical
analyses were performed with SPSS version 16.0, and the results were
considered significant at p ≤ 0.05.