Releasing fish is not always beneficial

One of the ways non-native species spread is through their release by owners into the environment. This is particularly true for aquarium fish. It’s worth noting that from a legal standpoint, this is not releasing fish but abandoning them. Aquarium fish released into urban ponds usually live briefly, exposed to unsuitable conditions. Exceptions occur when conditions turn out to be suitable, and such a release becomes the beginning of a biological invasion.

Effects of the “Free the Carp” campaign

Releasing fish involves not only aquarium specimens but also those intended for consumption. In Poland, as a form of protest against the inhumane tradition of keeping carp in stores and transporting them in inappropriate containers during the pre-Christmas period, there is the “Free the Carp” campaign. As a result, fish are released into the environment, which in Poland are a non-native species. In this way, individuals that have lived their entire lives under artificial feeding conditions, often in ponds with specific thermal properties and unnatural species compositions, are introduced into a completely foreign environment, where they either acclimate or not.

In extreme cases, they are released into the sea, where conditions usually prove fatal. Fortunately for the ecosystem, their invasive potential in Poland’s current climate is negligible. The campaign also has a relatively small reach, so the introduction of individual specimens does not significantly impact local rivers or lakes. It’s easier to encounter escapees from fish farms or those lakes treated as ponds by fisheries.

Releasing fish by the Chinese

A completely different scale involves the ritual release of fish practiced in Chinese Buddhism and Taoism. This is especially common in regions like Hong Kong and Taiwan, as well as among the Chinese diaspora, e.g., in Singapore. There, it also involves fish primarily bred for meat, some of which are given a chance to live in the sea. China is among the leading countries breeding freshwater fish from various carp and crucian species, but in coastal regions, marine aquaculture is developing. Among them, species from the grouper genus (Epinephelus) hold a high position. Some of them live in the South China Sea, so their release in this area should not have undesirable effects.

What is TGGG?

In aquaculture, due to the decline in native species, a dominant role is played by a grouper hybrid. Its parent species are Epinephelus fuscoguttatus and Epinephelus lanceolatus, known in English as Tiger Grouper and Giant Grouper, so their crossbreed is commonly referred to by the acronym TGGG. The first of them currently has the status of an endangered species. The second, with a threatened status, is still insufficiently studied, although its population is considered declining.

The natural range of both species extends from the coasts of East Africa to Polynesia. Hong Kong lies at their northern limits, so they themselves are not invasive in this region and have their ecological niches. However, they usually do not occur together, and their hybrids are not common in nature. Like many other serranid fish, they are predators with a generalist and opportunistic feeding strategy. They hunt various species of fish, crustaceans, and mollusks, usually those that are the most numerous and easiest prey at a given time.

One might expect that the hybrid would exhibit intermediate traits. However, TGGG is an example of a phenomenon known as hybrid vigor. It gains weight faster and reaches larger sizes than its parent species. It is valued in farming and reduces fishing pressure. Everything would be fine if TGGG remained farm animals living in closed breeding. Despite naturalists’ warnings, with the increasing popularity of this species in trade, the number of its religious releases is growing.

TGGG as a new element of food webs

TGGG breeders are familiar with the utility parameters of this species, but its behavior in the wild is unknown. Therefore, two researchers from the University of Hong Kong decided to study the diet composition of individuals released into the sea. For comparison, they also analyzed the stomach contents of four other Epinephelus species (different and more typical for this region than TGGG’s parent species). The catches were made in the harbor area, where fish release ceremonies are particularly frequent.

In many cases, the stomach contents were already digested to a degree that made visual identification impossible, but the DNA of the prey remained. Based on this, the researchers found that contrary to the assumption of opportunism, all studied grouper species are quite selective, and their food bases overlap to a small extent. This is particularly evident in the case of TGGG. This means that the fish, cephalopods, and crustaceans that became prey for the hybrids are generally avoided by their relatives. Thus, TGGG, appearing in the ecosystem, not so much competes for prey with related species as it constitutes a completely new element of the food web. At the same time, it is one of the largest predators in the ecosystem.

Moreover, not only does TGGG exhibit quite low diversity in prey selection, but individual specimens also turn out to be inflexible. Quite a few cases had empty stomachs. This may indicate that those raised in captivity are poorly adapted to the natural diversity of prey. Among them, species from shallow rocky shelves dominate, which may resemble feeding conditions in farming.

Ultimately, the introduction of TGGG into natural ecosystems disrupts food webs. The scale of this phenomenon has not yet been estimated, but it shows that behaviors rooted in the noble idea of unity with the ecosystem and good deeds for nature can have a negative impact on it.

main photo: sanjiv nayak / Uns
Source: Chung Arthur, Schunter Celia. Distinct resource utilization by introduced man-made grouper hybrid: an overlooked anthropogenic impact from a longstanding religious practice. Rev Fish Biol Fisheries (2024). https://doi.org/10.1007/s11160-024-09907-6