OTC and oxalinic acid (OXA) as well as for organo- chlorine pesticides (OCP) for both high-value and low- value fish commodities. OXA and OTC were the most common antibiotic residues found and methoxychlor for OCP from Luzon, Visayas and Mindanao. OXA in Penaeus vannamei sample from Mindanao was found to exceed the maximum residue limit (MRL, based on Japan Food Chemical Research) and Permissible Expo- sure Limit (PEL, based on the Occupational Safety and Health Administration based in the United States). In one sample of freshwater prawn Macrobrachium species from Luzon, the level of Endosulfan I (0.0144 ppm) was considered harmful based on PEL (0.00642) and
MRL (0.005). Endrin ketone (0.02582 ppm) was also detected from the same prawn sample, although no PEL and MRL is as yet established (Table 15).
4.2 Impact of Diversification of Culture
evolutionary pathway of native species by competitive exclusion, niche displacement, hybridization, introgres- sion, predation, and ultimately extinction (Mooney and Cleland 2001). Introduction of new pathogens along with the exotic species is also a risk of species introduc- tion (Joshi 2006). Introduced invasive species are con- sidered the second leading cause of species extinction and endangerment worldwide, the first being habitat destruction (Williams et al. 1989).
In the case of the golden apple snail Pomacea canaliculata whose introduction was as alternative pro- tein source for Filipinos, its introduction to the country has been blamed for the loss of the edible native snail Pila conica (Pagulayan 1997). The loss of most of the endemic cyprinids in Lake Lanao, the third-largest lake in the country, has been attributed to the introduction of the white goby Glossogobius giurus and the eleotrid Hypseleotris agilis (Juliano, Guerrero, and Ronquillo 1989). The introduction of the Thai catfish Clarias ba- tracus has resulted in the loss of the native catfish Clar- ias macrocephalus in many inland water bodies in the country. SEAFDEC/AQD implemented research and development activities to breed Clarias macrocephalus (Tan-Fermin et al. 2008) in the hope of restocking de- pleted inland water bodies, but difficulties in obtaining wild brood stock for induced spawning activities ham- pered efforts.
Introduced species have far-reaching adverse en- vironmental impacts. Cuvin-Aralar (2014) compared the fish biodiversity in an aquaculture and non-aqua- culture site in Laguna de Bay, the largest inland water body in the Philippines widely used for fish production.
Results showed that fish biodiversity was significantly lower in the aquaculture site compared to the non-aqua- culture site. There was a significantly higher predom- inance of introduced species for culture (Nile tilapia, bighead carp, Tra catfish) compared to native species in the aquaculture site. The non-aquaculture site had significantly higher relative dominance of native spe- cies. Indices of biodiversity such as Shannon-Wiener Index, Simpson Index, and Evenness all indicate signifi- cantly higher fish biodiversity in non-aquaculture sites
(Cuvin-Aralar 2014). Figure 38 illustrates the direct and indirect impact of introduced species on biodiversity.
4.2.2 Other Impacts of Introduced Species Aside from adverse impacts on biodiversity, introduced aquatic species have also adversely affected other envi- ronmental factors. Escapees from the ornamental fish trade like the South American sucker mouth catfish, locally known as janitor fish Pterygoplichthys pardalis and Pterygoplichthys disjunctivus, have become invasive in many areas in Luzon, including Marikina River and Laguna de Bay (Chavez et al. 2006; Jumawan et al. 2011) and Agusan Marsh in Mindanao (Hubilla, Kis, and Pri- mavera 2008). The fish with its hard armor-like cover- ing damaged the banks of the Marikina River due to its burrowing habit and damaged aquaculture fish cages in Laguna de Bay. Considerable expense has been incurred from a ‘bounty system’ type of approach to eradicate janitor fish wherein fishermen were paid to catch the janitor fish at PHP 5.00/kg, after which the caught fish are destroyed (Joshi 2006). The two introduced fresh- water cichlid species, the black chin tilapia Sarotherodon melanotheron and the Mayan cichlid Cichlasoma uroph- thalmus, have been caught in Manila Bay (Ordonez et al. 2015). It is to be noted that the black chin tilapia has been reported as an introduced species in Laguna de Bay (Cuvin-Aralar 2014). It is possible that Sarotherodon melanotheron found its way to Manila Bay via the Pasig River from Laguna de Bay, in addition to escapees from fishponds in adjacent areas. Both cichlids were reported to have had competitive interactions with other fish spe- cies in Manila Bay (Ordonez et al. 2015).
In the case of the golden apple snail (Pomacea canaliculata), its introduction caused considerable havoc not only to inland water bodies but many rice fields as well (Joshi 2006). To eradicate these snails, molluscicides valued at US$23 million were import- ed between 1998 and 2005, but had limited success;
the snails still remain a problem. The snails are also vectors of a rat lungworm that also affects humans (Joshi 2006).
Figure 39: Schematic diagram of direct and indirect impacts of species introduction on biodiversity
Allen species Predation
Reduction in native species;
endangering species; at times leading to extinction
Habitat destruction Competition with
native species Hybridization
Reduction in native population numbers;
inbreeding depression;
reduced fitness
New and unusual selection on native
species
Hybrid vigor;
increased fitness
Outbreeding depression;
reduced fitness Genetic changes
in native species Displacement of native species;
Findangering; extinction; loss of taxonomically distinct population/species Indirect impacts
Direct impacts
Introgression
Disease Ecological impacts Genetic changes
Source: de Silva et al. 2009.