4.3. Two factors most affecting distributions of the non-native
fish species
The eight rivers surveyed could be divided to two big assemblages,
the
Continent rivers and Hainan Island rivers, according to the amount
ratio, which reflects the geographical differences between continents
and islands (Fig. 3). The non-native fish species composition, numbers
of species, and dominant species differed between the two assemblages of
rivers (Tables 1, 2; Fig. 2). Of the 5-most-common non-native fish
species found in the rivers, the amount ratios of 3 species
significantly differed between the Hainan Island rivers and Continent
rivers (Fig. 5). The presence, survival, and spread of non-native fishes
in specific natural waters usually results from intentional
introductions, escapes of cultured fish, or disposals and ensuing
self-spread (Kolar & Lodge, 2001). This progression may be determined
by local aquaculture conditions, market requirements, and policy support
related to the geographical situation (Liu, Li, et al., 2014; Lin, Gao
& Zhan, 2015). Hainan Island has become one of China’s most important
hatchery and culture areas for many non-native fish species owing to its
relatively warm temperatures and similarities to the native habitats of
the species reared (Fishery Department of the Ministry of Agriculture
and Rural Affairs of China, 2019). As such, a greater number of
non-native fish species was found on the island than was collected in
the rivers on the continent, and several of these, namely the streaked
prochilod, sutchi catfish, redhead cichlid and blackspot barb, were
found only on Hainan Island (Table 2). The island’s isolation may block
the spread of cultured non-native freshwater fishes after they escape or
are discarded (Zambrano, Martinez, Menezes & Peterson, 2006; Albert &
Crampton, 2010). Thus, it seems that geographical isolation is an
important factor distinguishing the species composition between the
Hainan Island and the Continent rivers.
The ratio of non-native fish species to total fish species collected
allowed further grouping of the Continent rivers into Continent coastal
rivers (JJ and MYJ), the Pearl River system 1 (BJ and DJ), and the Pearl
River system 2 (XJ) (Fig. 3). Temperature also importantly affected the
distribution patterns of the two widely distributed and common
species—the Nile tilapia and redbelly tilapia (Figs. 6, 7). A warm
climate makes the river basins of Hainan Island and the Continent
coastal area important regions for Nile tilapia production (Zhu, Lu &
Huang, 2008; Zhou, Wang, Cao, Wan & Li, 2016; Yang, 2016; Fishery
Department of Ministry of Agriculture and Rural Affairs of China, 2019;
Gu, Yu, et al., 2019). Nile tilapia are indeed the most important
cultured freshwater fish in these basins: 84.29% of freshwater fish
production on Hainan Island constitutes tilapia farming, and tilapia are
also the most important cultured fish in the basins surrounding JJ and
MYJ, which produces ~18% of China’s total tilapia
production, while the Nile tilapia and its hybrids contribute to
>90% of the tilapia production (Zhu, Lu & Huang, 2008;
Yang, 2016). The same climatic conditions in these river systems that
promotes the culture of Nile tilapia also supports its establishment and
spread once it enters natural waters (Gu, Yu, Xu, Wei, Mu, et al.,
2018). Furthermore, the Nile tilapia is a strong competitor in the
warmer water because of its larger body size and faster grow rate (Gu,
Yu, Xu, Wei, Mu, et al., 2018; Gu, Yu, et al., 2019). Nile tilapia was
the most common species in both the Continent coastal rivers and Hainan
Island rivers, and could be recognized as an absolute dominant fish
species in those river assemblages (Figs. 3, 4, 6). However, Nile
tilapia is sensitive to temperature, with mortalities increasing at
temperatures below 12°C, thus its abundance is less in colder rivers
(Gu, Yu, Xu, Wei, Mu, et al., 2018) (Fig. 6). Although the
second-most-abundant species, the redbelly tilapia is not popular in
aquaculture owing to its small size and slow growth; however, it can
tolerate lower temperatures than the Nile tilapia, and was thus more
abundant in colder rivers and could be recognized as an absolute
dominant fish species in the DJ and BJ (Figs. 3, 7). Previously, Gu, Yu,
Xu, Wei, Mu, et al., (2018) similarly found that the distribution
patterns of these two most common invasive tilapia species were related
to temperature conditions. Likewise, Yu, Gu, et al. (2019) noted
similar circumstances for the distributions of the invasive mrigal carp
and native mud carp Cirrhinus molitorella .