**4. Can escapees establish a natural population?**

According to CABI [52], the species itself is not considered a major threat to biodiversity and does not appear to have formed breeding populations. Briggs et al. [29] added that despite the fact that the species has been widely introduced, a comprehensive study of the literature carried out for this report and the information available from other countries in Asia and in the Americas did not find any evidence of the shrimp becoming established in the wild outside of its range (i.e., it may not become an easily "invasive" species). However, there is a need for further field research, as there was insufficient information available on the natural breeding habits of the shrimp to make any further assessment of this issue.

Understanding the biology of *P. vannamei* is vital in order to know whether this animal once escape can establish a natural population that is crucial to the environment. However, there is limited information on the biology of this animal and is neglected during the last two decades. According to Dugassa and Gaetan [53], *P. vannamei* lives in tropical marine habitats, and the adults of this species live and spawn in the ocean. However, the larvae and juveniles are usually found in inshore water areas such as coastal estuaries, lagoons, or mangrove areas. The shrimp females grow faster than the male of this species. The matured female weighing 30–45 g can spawn 100,000–250,000 eggs. The shrimp life cycle is very complex, and it

*Spatial Variability in Environmental Science - Patterns, Processes, and Analyses*

to 2006. In fact, about 700 hectares of *P. vannamei* illegal farms in Luzon have been reported in 2003 [36]. The ban was lifted in 2007 after experimental trials and a series of public consultation and hearings [37]. In 2008, there were 38 grow-out farms engaged in *P. vannamei* farming, and it increased to 53 farms in 2013 [38]. Additionally, based on the [39] master list of shrimp farms as of December 2013, the shrimp has been polycultures with fish (17 farms, 642.7 ha) and *P. monodon* (4 farms, 33.0 ha). In 2014, there were 27 accredited hatcheries that continually support the demand of seedling requirements of the shrimp [40]. As of August 31, 2019, however, there were already 40 accredited hatcheries and 545 grow-out farms of the shrimp with a total productive area of 7382.08 hectares [41–43]. The boom and rapid expansion of the shrimp aquaculture are expected because of its demand for both the local and global markets. With this continuous expansion of *P. vannamei* farming, it is possible that shrimp can be found in the different estuary or brackish water rivers in the Philippines wherein the hatcheries and grow-out farm's operations are found due to escapes. The probability of escapes is higher because out of the total farms in 2019, 331 farms with an area of 4144.19 hectares were operating extensive grow-out practices wherein old and not properly designed ponds are usually utilized. A small percentage of escapes per operation cycle could translate to significant numbers of individuals entering the ecosystem. According to Panutrakul et al. [44], the shrimp can enter estuary at various life stages including post-larvae (produced from hatcheries), juveniles, and subadults (cultured in ponds). As of 2019, however, no studies have been conducted in the Philippines to verify the presence of the shrimp in

In Thailand, floods in Surat Thani and Pranburi in 2003, for example, led to several million *P. vannamei* escaping to the coastal environment. Not surprisingly, the shrimp, therefore, has been reported in fisherfolk's catches on Andaman and Gulf of Thailand coasts. No detailed information on catches is available, but numbers have not been reported as large [29]. Similarly, an incident of escape was reported from farms to the Bangpakong River in Thailand [30, 45]. The study showed that the numbers of the shrimp sampled in the river positively correlated with the location and area of shrimp ponds. Manthachitra et al. [45] used remote sensing and a geographic information system (GIS) to estimate the location and total area of shrimp ponds (active, inactive, and abandoned ponds) in the Bangpakong River watershed and found that most ponds were located within 5 km of the river. The presence of the shrimp was confirmed based on the survey of marine shrimp populations in the Bangpakong River during the same period conducted by Senanan et al. [30]. The mean proportion of *P. vannamei* relative to all penaeid shrimp per net per year (all stations combined) ranged from 0.005 (June 2005) to 0.16 (January 2006), with the highest abundance detected in 2006. The presence of the shrimp in the river may be a consequence of pond water releases during the intense farming activities of 2005. In the Southern Gulf of Mexico coast, there is a first report of the presence of *P. vannamei* [35]. During a shrimp monitoring program survey conducted in this area, seven specimens were collected in the Carmen-Pajonal-Machona lagoons near La

Azucena and Sanchez Magallanes in Tabasco, Mexico.

**3. Can escapees survive the natural environmental conditions?**

Pacific white shrimp *P. vannamei* is the most economically important species for aquaculture in extensive, semi-intensive, and intensive systems in many parts of the world [46, 47] due to several advantages compared with other cultured shrimps. These are largely associated with the ability to close the life cycle and produce broodstock within the culture ponds, rapid growth rate (at up to 3 g/week),

**146**

the wild.

usually takes around 1.5 years to complete the whole life cycle. The matured shrimp females spawn their eggs in the offshore waters [54], while fertilization occurs in the external environment [53]. However, the maturity of the shrimp escaped from farms to natural environments is an important factor in determining their ability to establish a feral population [22]. A study has been conducted to compare the histology of gonads of wild-caught and captive *P. vannamei* of known ages [55]. Captive individuals could develop mature gonads at 11 months after post-larvae 15 (ovaries contained 50% mature oocytes; testes contained 80% mature sperm cells). The result of the study showed that they did not find sexually mature individuals in the wild although some wild-caught males larger than 19 g contained a small percentage of mature sperm cells. However, the authors cannot conclude that escapees can establish a feral population because the study might have under-sampled sexually mature individuals due to inappropriate sampling sites and timing. This issue remains important for further investigation. A monitoring program in offshore areas may provide opportunities for us to obtain sexually mature individuals. Likewise, Panutrakul et al. [44] found no evidence that the shrimp present in the wild could reach maturation in the Bangpakong River although gonadal development has been observed. Moreover, Wakida-Kusunoki et al. [35] argued that it was not possible to find evidence of *P. vannamei* becoming established in the zone of the Mexican coast of the Gulf of Mexico. The low frequency of *P. vannamei* encounters in the monitoring program of artisanal shrimp fishing in lagoon system and the negative presence of the shrimp in surveys of the commercial shrimp catches of coastal waters near to the mouth of this lagoon indicate the absence of an established population. They suggested further sampling and monitoring are required to find evidence that confirms the establishment of a population of the shrimp in the Southern Gulf of Mexico. According to Panutrakul et al. [44], natural reproduction would require released adults and a high probability that the mature adults could find mates. In the Philippines, the population of *P. vannamei* in the wild therefore already exists, although it is still uncertain if this population is now breeding [29] until this time.
