2.2. Fish collection and data collection
Field surveys at each site were carried out in the first
and second halves of the year, and repeated twice. Sampling at each site
continued until no new species were caught, which was at least three
days. Most of the fish were collected by commercial fisherman, and the
remainder by the authors. Field sampling practices followed the
“Monitoring manual of fish resources in the Yangtze River” (Chen,
2013) and the “Standards and specifications on the monitoring of alien
fish species” (Ministry of Agriculture and Rural Affairs, China). Fish
were primarily caught using gillnets and shrimp nets, and then
identified to species, weighed and counted, following the methods of Pan
(1991), Hu (2015), and Xu and Qiang (2018).
The following data were recorded for each river:
1) WT, total weight of all fish collected (kg);
2) NT, total number of all fish collected;
3) WNT, total weight of all non-native fish collected
(kg);
4) NNT, total number of all non-native fish collected;
5) W1-I, total weight of each fish species (kg), where
‘I’ represents the number of fish species represented;
6) N1-I, total number of each fish species collected,
where ‘I’ represents the number of fish species represented;
7) WN1-X, total weight of each non-native fish species
collected (kg), where ‘X’ represents the number of non-native fish
species;
8) NN1-X, total number of non-native fish species
collected, where ‘X’ represents the number of non-native fish species;
9) FO1-I, frequency of occurrence of the different fish
species;
10) T, the mean minimum temperature of the eight rivers. These data were
obtained fromhttp://lishi.tianqi.com; the mean
temperature over 31 days in January 2018 was recognized as the mean
minimum temperature of the coldest month for the analysis for each site,
after that the mean temperature of the main sites across each rivers was
recognized as the mean minimum temperature for each of the eight rivers.