Blackbass Barren — poster presented at Joint Conference of ASI &12th IPFC at Taipei
| posted on June 19, 2025
The following is English translation of a document distributed with the poster presentation. Get Original
BLACKBASS BARREN WHAT IS BLACKBASS BARREN? Since the 1970s, the frequent release and spread of blackbass (largemouth bass (Micropterus nigricans), Florida bass (M. salmoides), and smallmouth bass (M. dolomieu): the basses) has eventually destroyed the native freshwater fish fauna in Japan. It was unimaginable that almost all of the once abundant species of bitterlings and minnows would be listed as endangered. There is no doubt that basses are responsible for much of the severe decline in small fish such as bitterlings and minnows. There are many water bodies throughout Japan where basses are almost the only fish present, and native small fish such as bitterlings and minnows have disappeared. I call them "blackbass barrens". HOW BLACKBASS BARREN ESTABLISHES When bass are released into lakes and marshes, such as Lake Biwa and Izunuma-Uchinuma, as well as reservoirs and other water bodies, they appeared to coexist with small fish such as bitterlings and minnows for a while, depending on the size of the water body and the amount of bass introduced. In Lake Biwa, Shiga Prefecture, perhaps because of its large area (670 km2), it took 10 years after the first record in 1974 before the rapid increase of the bass and the sudden crash of native fish in 1984 occurred (Maehata 1987, 1990). One year before the sudden change, in the spring of 1983, I also witnessed a complete upset of the situation from the spring, when there were almost no bass in aggregation of small native fish, to the disastrous situation in the fall of that year, when there was only one or less native fish over 100 young-of-year bass. Perhaps an explosion of the bass reproduction occurred in the early summer in 1983. The countless numbers of juvenile bass that were born at that time probably fed on the fry of cyprinids and other small fish (Takahashi 2002, Fig. 2), and by the autumn the native fish had almost completely disappeared, leaving only young-of-the-year basses. In Hachirogata Lagoon, Akita Prefecture, which is less than 1/10 the size of Lake Biwa (about 48 km2), the diversity of fish species caught in the smelt fishery declined over about 10 years after the 1983 bass invasion (Sugiyama & Jinguji 2005, Fig. 3). The smelt fishery seems to rather stable due to their constant stocking, but other small fish have almost disappeared. In 1992, 220 kg of the bass were suddenly caught in Lake Izunuma-Uchinuma (4 km2), Miyagi Prefecture, and four years later, in 1996, the catch increased to 700 kg. The number of small fishes, including the rare native species zenitanago (Acheilognathus typus), has decreased dramatically since 1997 (Hagiwara 2009). It took five years from the bass invasion to the drastic decline of small fish. According to Takahashi et al. (2001), the number of fish caught per day per set net has decreased from about 3,500 in 1996 to about 250 in 2000 (Fig. 4). The number of fish caught regarding in autumn is less than 1/100 (2,370 to 23). The reason why the difference is particularly conspicuous in autumn is probably due to the bass juveniles' explosive birth in the early summer, feeding on small fry, as occurred in Lake Biwa in 1983. It should be noted here that the catch (in weight) has decreased by only one-third at most during this period (Takahashi et al. 2001, Fig. 5). This trend is the same in other water areas. Fisheries damage caused by bass is often described as a decrease in tonnage of fish catch by research institutes in various regions. This may give rise to the argument that the economic impact of bass on the ecosystem is tolerable, and that the economic profits from the bass angling industry is more important than the reduction in the economic loss of the fishery since there is not much of a reduction. However, in fisheries stock assessment, everything cannot be measured by weight. Especially in resource management to prevent overfishing, the original data on how many fish are in the ocean (stock abundance) and how much can be caught (allowable catch) is calculated in terms of the number of fish. This is then converted into a weight and announced as, for example, 1 ton for 1000 fish weighing 1 kg (e.g., http://abchan.fra.go.jp/digests2022/simple/K2022_01.pdf). Resource management is also linked to environmental conservation. Since even the fishing industry is based on the number of fish, it is even more wrong to talk about conservation in terms of the weight of living animals, but absolutely in terms of numbers. In this respect, the record of the severe decrease in the number of fish caught in Izunuma-Uchinuma (Takahashi et al. 2001, Fig. 4) provides valuable information. The reason why the total catch has not decreased significantly in spite of the increase in the number of basses can be explained by skewness of the captured fish size to larger. Using the Izunuma-Uchinuma example (Fig. 5), estimating the catch in 1996 was halved in 2000, and the number of fish caught decreased from 3,500 to 250 during the same period, the weight of each fish caught in 2000 was seven times bigger than that in 1996. The increase in average weight does not mean better growth of the fish, but rather that the small fish are being eaten up, leaving larger fish that cannot fit in the bass's mouth. Basses are also being released into dam reservoirs. In the Miharu Resorvoir (3.6 ha) in Fukushima Prefecture, a massive release of the bass in 1996 is suspected, just after its completion, and the number of bass increased dramatically. In 2007, the results of a survey showed that crucian carp (Carassius auratus) were all large, which clearly illustrates what must have happened in Izunuma (Fig. 6). Here, the size of the crucian carp is a little more than a half the maximum size of the bass (46 cm), although there are few smaller crucian carp species around 25 cm in length. The size of the crucian carp fits well with the experimental results that basses are capable to eat fish of a half their own size (Tabata 1977). In a small body of water such as agricultural ponds, the effect can be more severe in a shorter period of time. In a pond (smaller than 0.3 ha) upstream in the Izunuma-Uchinuma catchment, it was estimated that about 9,000 small fish that existed before the bass invasion were exhausted in about 40 days (Fujimoto et al. 2009, Fig. 7). The devastation in ponds has also been reported in Hiki Hills, Saitama Prefecture (Maezono & Miyashita, 2003), Ikoma City, Nara Prefecture (Takuma et al., 2004), and Oshu City, Iwate Prefecture (Tsunoda et al., 2008). In some of these ponds, bluegill, which, like the bass, are native to North America, have also invaded. In 15 ponds in the Hiki Hills, more than a half of the ponds with the bass were almost completely devoid of small fish, while those without the bass contained large numbers of minnows (Pseudorasbora parva) and gobies (Rhinogobius sp.). Out of the 31 ponds in Ikoma City, 12 ponds with the bass or bluegill had either no other fish at all or only one species. Those with no bass or bluegill contained up to six fish species, including rare species such as the Japanese fire belly minnow (Hemigrammocypris neglecta), a bitterling (Acheilognathus tabira tabira), and medaka (Oryzias latipes). This must have been the situation before the bass/bluegill invasion. In 12 ponds in Oshu City, eight without bass contained many bitterlings and minnows, while three of the four ponds with bass contained few small fish (Tsunoda et al. 2008). At the remaining sites, many small native fish were observed despite the presence of basses. My collaborators and I have been surveying 63 ponds in the Izunuma-Uchinuma catchment for several years. In 10 of them, there was only one species of small fish other than the bass, and small shrimps had disappeared. Seven of them had some small fish and shrimps along with the bass. Three of these seven sites appeared to be the bass stocked very recently; one site was connected to a river through an agricultural canal from which both the bass and small fish regularly migrated in. At the other three sites, the bass and small fish seemed to coexist, at least for a while. However, I believe that the examples of Oshu City and some ponds in the Izunuma-Uchinuma catchment do not provide evidence for stable cohabitation of the bass and small fish. Tsunoda et al. (2008) did not conduct a survey across years. Even if they were able to coexist for a while, they would not be able to reproduce constantly like a machine, and once a low birth cohort of small fish occurred by chance, they would not be able to recover with the bass, though they would be able to work resiliently without basses. We now know that the invasion of the bass will sooner or later destroy the ecosystem of even a lake as large as Lake Biwa, but what happens to a destroyed ecosystem over time? Will the bass, having consumed all the small fish they eat, starve to death and disappear, allowing the original ecosystem to recover? Unfortunately, we know that this will not happen. Although the bass are carnivores, they do not feed exclusively on small fish. They also feed on their juveniles in addition to aquatic insects, crayfish, and small shrimps. In ponds, 20 to 60% of the bodies of bass over one year old are made up of young basses that have been cannibalized (Yasuno et al 2020). When the situation becomes critical, even large fish (over 60 cm) begin to feed on plankton such as Daphnia (Maehata 1990). Even if the small fish that have been consumed by the bass are able to reproduce again, any chance of recovery is quickly killed by the bass juveniles, which love to feed on the small fish fry. All that remains are carp and crucian carp that are too large to fit in the mouths of bass. The blackbass barren is hence established (Fig. 8). Naturally, large carp and crucian carp cannot reproduce because their fries are eaten up by the bass juveniles. They will eventually reach the end of their life span and die out. However, bass fry feed on Daphnia, etc., and are constantly born and grow, so it is unlikely for the bass to see food deprivation. Furthermore, where bluegills are present, they have evolved to coexist with the bass and do not disappear like other small fish, so they are also a source of food. Blackbass barrens are therefore sustainable. It is the same in its native habitat (Dassow et al 2018). Barrens have been there for 30 years or even longer. RECOVERY FROM THE BARREN How can we eliminate the blackbass barrens and restore the lively wetlands to its former state? The barrens are sustainable, so leaving them will not solve anything. In addition, illegal release by bass anglers (bassers) with low awareness of laws (Miyazaki 2009, Tsunoda et al. 2011, Taniguchi et al. 2020, Funaki et al. 2021, Saito et al. 2021) may lead to further expansion of the barrens. Catching bass in the barrens and transporting them to other areas for release is something easily imaginable that any outlaw bassers who like to create their own bass fishing grounds. The only way to get rid of the bass is to do it one-by-one. In this direction, as establishing a barren, the size and structure of the water body and the number of bass present in the water body have a bearing on the effort and time it takes to recover from the barren. In Lake Biwa, a bass extermination project began in 1999 and continues even today, but the bass biomass has been reduced by only a little more than a half in the 14 years from 2007 to 2020 alone, and has remained flat in recent years (Taguchi 2022, Fig. 9). In large bodies of water, the rate of decline has been back-and-forth over a period of more than 20 years. In dam reservoirs, too, eradication measures are limited and seem to be on a long road. At the Miharu Dam, the water level is lowered during the bass spawning season to force the eggs laid to dry up (Kikuchi and Matsuzaki 2017). The key data on whether the number of basses in the dam lake has decreased is not available, but since the dried-up eggs are seen every year, it does not appear that the number of basses has decreased to the point where it is difficult to see them. The Lake Izunuma-Uchinuma, which is smaller than Lake Biwa, is showing positive results. A full-scale bass extermination project began in 2004, after Lake Biwa, and it is estimated that in the first six years, the number of bass was reduced to about one-fifth of that in 2003 (Fujimoto et al. 2021, Fig. 10). I participated in the eradication activities. The number of bass then continued to decrease steadily, and last year (2024), I saw almost no bass when I was netting during the time when the fry were emerging. Instead, the rare bitterling, which once became extinct around 1999 (Hagiwara 2009), possibly flow down from the upstream ponds and was found here and there in the lake from 2015 (Saitoh et al. 2016). In 2020, natural reproduction was observed, indicating that the fish had come back (Fujimoto et al. 2021). Last year, the fishes caught by the scoop nets used to remove the bass fry were replaced by of small fish juveniles. The situation has changed from one scoop with a lot of bass fry to catching a lot of small fish juveniles. About 100 rare bitterling juveniles were caught in all. However, this does not mean that the bass have disappeared completely. A few bass fry continue to be caught, and the angling results of the basses continue to be uploaded to social networking sites and YouTube (for example, https://anglers.jp/areas/3554 and https://anglers.jp/areas/3555). Therefore, we have to continue the bass extermination in Izunuma-Uchinuma for the time being. If we stop the eradication, the surviving bass will reproduce and eventually return to the barren. In smaller reservoirs, and in agricultural ponds created by filling shallow basins in hillsides, it is possible to dry the ponds by draining them completely. Draw-down of ponds has been a traditional practice of farmers to maintain the pond structure and function along with catching fish for foods. It is a good idea to catch all the basses at that time. However, the ease of removal varies depending on the structure and size of the pond. The Inokashira pond (4.7 ha) in a metropolitan park in Musashino City, Tokyo, consists of four ponds connected by a short channel, and it was considered impossible to drain all four ponds simultaneously, so a plan was made to dry the ponds three times over several years (2013, 2015, and 2017, Sugawara 2015). The plan was a large-scale project involving both the government and a private sector, and included civil engineering work such as the construction of temporary partitions by the Tokyo Metropolitan Government (the park administrator) between the pond to be dried and the others left inundated. After the first drainage in 2013, native organisms such as shrimps and native fish hardly recovered. After the second-round draw-down (2015), the number of native organisms increased noticeably (Yagi 2019, Fig. 11). the reason for the poor recovery of native organisms after one draining was that the bass and bluegill remained uncaught (Fig. 12). The number of basses, in particular, clearly increased after two years, indicating a rebound from the leftover. Nevertheless, by repeated draw-down of the pond, the basses were eradicated at the second time, and the bluegill at the third time. In Inokashira pond, the drying of the pond was conducted under the policy of eradicating all non-native species, including not only the bass and bluegill, but also American crayfish, domestic carp, and even domestic non-native species such as the tamoroko (Gnathopogon elongatus), which is native to western Japan. Perhaps because of this, not only native small fish, shrimp, and other animals, but also native rare water plants such as Inokashira stonewort (Nitella mirabilis var. inokasiraensis) were restored (Ito 2019, Fig. 13). If the pond has a simple shape and can be completely drained, bass control can be achieved by pond draw-down even in larger areas. At Hotokezawasawa pond (8 ha) in Misato Town, Akita Prefecture, all basses were removed by drying the pond three times between 1999 and 2003 (Sugiyama & Jinguji 2005). In smaller ponds, it is possible to remove the bass by pond draw-down only once (Takahashi 2006). In a 3-ha pond in Osaki City, Miyagi Prefecture, we dried the pond immediately after a bass infestation was confirmed in 2002, and were able to protect the rare species shinaimotsugo (Pseudorasbora pumila). Subsequently, several other nearby ponds were also dried and exterminated in the same manner. As a result, we were able to remove the bass from the surrounding ponds and streams and achieve a bass-free area. Subsequently, we have succeeded in restoring habitat not only for the shinaimotsugo, but also for the zenitanago (Takahashi 2017). Similarly, in the ponds surrounding Lake Izunuma-Uchinuma, the bass have been removed by pond draw-down, and native fish have returned by themselves (Asayama et al. 2020, Fig. 14). In Terukoshi pond (2.1 ha) in Kurihara City, Miyagi Prefecture, thorough drying of the pond resulted in the successful removal of bass (Saitoh et al. 2021, Fig. 15). Not only bass control, but also drying the pond itself has been shown to have a positive effect on the ecosystem. The restoration of waterweeds in Inokashira pond is one such effect. When the pond is drained and the mud at the bottom is washed away, the waterweeds die once, but they revive by germinating seeds that have been lying dormant in the mud (Ito 2019). The mud also becomes oxidative through exposure to the air, and the number of bivalves that serve as spawning beds for bitterlings increases. This allows the bivalves to reproduce more actively, which has a positive effect on bitterling (Matsuba et al. 2007). From a birds eye view, a short-term disturbance (pulse, Odum et al. 1995) in the ecosystem can have a positive effect on the growth and development of the fish. FUNDAMENTALISM OF POND DRAW-DOWN The know-how of pond draw-down has been accumulated through the experiences of private organizations such as "Society for Shinaimotsugo Conservation" in Osaki City, Miyagi Prefecture and "Namazu no Gakkou" in Kurihara City, Miyagi Prefecture. In short, the question is how to make a one-shot resolution. There are many ponds that need to clear bass, so we cannot proceed if we do things halfway. If there are several ponds in a basin, we must first make sure that there are no bass in the ponds upstream of the one we are drying, and if there are, we must start from the upper ponds. However, this simple statement is not always easy to do. Every ponds has a manager, right-holders or an owner. Coordination is necessary, starting with whether or not to dry the ponds and what the schedule will be. In the past, it was not possible to coordinate the upper ponds, so we started with the lower ponds. In such cases, we know that even if the lower ponds can be cleared, the effort will be fruitless because the water will flow down from the upper ponds. We had to give up drying that lower pond and choose another site. There are many other ponds with bass waiting to be cleared. After checking for bass and making arrangements with the relevant parties, it was time to dry the ponds. This reminds me of something my microbiology professor used to say when I was in a university:  — There are only two things in the world of cell culture. Sterile and the other. When a student says, "largely sterile" or "almost sterile," it's no longer fine. On the other hand, there is no need to say "perfect", because there are only sterile and the other. The principle of pond draw-down is the same as above: the extermination and the other, that is, zero and the other. Strictly speaking, if the sexes are not matched, even if there are a few leftovers, they will disappear at the end of their life. However, it would be easier to catch them all than to do it so nicely. On the other hand, if you leave even a small number of basses with both sexes present, the bass juveniles will emerge the next year and feed on the native fish fly recovering, and in a few years you will have lost everything. Recently, I often see TV shows dealing with all the water drain out from the pond, but as far as I watch it, it is not all the water out. I think the programs are good for lowering the hurdles in people's mind for drying ponds, but I wonder if the extermination of invasive alien fish is being done properly. I worry about if it was "largely exterminated" or "almost exterminated". If we were doing "the other", we will have to draw the same pond down over and over again, leaving many other ponds waiting to be cleared. We must be committed to the "fundamentalism for pond draw-down". In order to do this, we must drain the pond completely by opening the "bottom flume," a drain plug installed when the pond was built, and draining it to the bottom of the pond. If the bottom plug is old, it is buried in mud or broken, and water cannot be drained to the bottom. A drainage pump or other equipment is required. Even if you feel to drain water to the bottom of the pond, water will seep in from around the pond. The amount of the seeping water is not small. It is enough for the fish to survive if there is any leftover. To facilitate drainage, and to make it easier for the leftover fish to gather, find, and catch them, a trench is dug at the bottom of the pond (Fig. 15). Digging water pathways was also performed in the drying process at Inokashira pond (Sugawara 2017). Even so, this does not guarantee that there will be no fish left in the pond, so quicklime is spread to kill any remaining fish just in case. Although it is best not to use lime because it changes the pond environment to alkalic, it is unavoidable for the purpose of bass extermination, in my opinion. If there are plants such as the watershield that grow in acidic environments, lime cannot be applied, but I have no alternative idea but to be more thorough in catching the bass. Then, for further perfection, the pond is left to dry for a week or longer. If possible, it is better schedule the pond draw-down so that the days before and after the drying will be sunny. If it rains, even if it is only a light rain, the water will not be able to drain out, whatever by plugging off, digging water trenches, or using a pump. What should we do with the few remaining small native fish, that are caught along with bass and bluegill during the draw-down process? You may put them in a bucket or alike to protect them. It's just human nature. If not, the small fish will die in large numbers when the pond is dried out, and if lime is applied, the fish that are barely left will die even more. My opinion, however, is that the "protection" should be minimal. Mass mortality is natural (Saitoh 2013), since drying out (and rapid inundation) can be regarded as an ecological pulse, such as a naturally occurring drought or flood. The native fish that survive in the barrens are usually do not die out for short periods of time in the wet mud at the bottom of drained ponds. The effort to protect native fish would better allocate to draining the barrens completely, catching all the bass, and so on, so that the extermination efforts are less likely to fail. In the case of the Izunuma-Uchinuma catchment, where native fish are still present in the surrounding area, especially in the ditches and ponds upstream of the ponds, protection is not needed in the first place. Moreover, do not care them. At the drying site, the fish are probably crammed into a small container for temporary protection. Have you ever had to evacuate the fish to a nearby pond in a hurry because the fish would die if you were too slow? That's just human nature. But can you find and put every bass and bluegill out from the muddy fish packed in the container? What happens if you miss a bass or a bluegill? Drying ponds is hard, muddy work, and when you are tired, your judgment is impaired. Some of the volunteers participating in the pond draw-down cannot distinguish between crucian carp and bass. Even more so with minnows and bitterlings. I am not exaggerating. Furthermore, in the case of bluegill, during the fall and winter months when the ponds are dried, there are so many young-of-the-year fish that the smallest are no larger than a fingernail, making them even more likely to be mixed in with the small fish they are protecting. I have actually found one or two very small bluegills among the full of minnows caught upon pond draw-down. I advised the leader there to give up on protecting the minnows and kill them. But his response was, "I can't do such a pitiful thing". The destroying power of "it's a pity" sentiment is almighty. Unfortunately, the minnows were moved to a pond upstream, with the possible bluegill in them. In the end, the eradication actions in that pond were abandoned because repeated drying of the pond did not succeed in eradicating the bluegill. We should be careful not to waste all the hard work in the mud and spread non-native fish by being too emotional. Some people sneak into the "pity" sentiment of the people and suppress pond draw-down, hiding the real purpose to enjoy bass angling sustainably (Ikeda, Editorial Office of the Tsuribito — a Monthly Anglers' Magazine 2019). Rather than feeling "sorry" for the life in front of me, I think more of the life that can no longer even be born. The pulsed ecosystem in drawn-down pond is a "world with death as usual". I would rather cherish the life that will be born and recover than give unnecessary consideration to moribund fish in front of me. If we do things half-heartedly, the bass will rebound and kill the buds of life that will recover. Isn't that "pity"? However, in places like urban ponds, where native fish are not expected to come from nearby, consideration must be given to the native fish that are caught in the dried pond. However, this should be minimum, and limited to the number of fish that can be safely kept in a container that is clearly visible from the side, such as a glass aquarium for home use. Furthermore, if the fish are kept for a while and watched every day, the introduction of non-native fish will be eliminated. BATTLE AGAINST OUTLAW ANGLERS In 2005, the so-called Invasive Alien Species Act was enacted, prohibiting the keeping and release of bass and bluegill without special permission. Despite this, illegal release of bass continues (Miyazaki 2009, Tsunoda et al 2011, Mukai 2016, Taniguchi et al 2020, Funaki et al 2021, Saitoh et al 2021). On the other hand, eradication is not progressing well. In the Terukoshi pond, bass eradication once seemed to be successful, but two years later, a large number of juvenile bass appeared. When we counted backward from the number of fry at that time (7,000), we estimated that the number of basses left over when drying of the pond would not have been so large (8 large adults or c.a. 90 yearlings leftover). These estimated leftovers are unlikely unless we had been very careless. This is a result that leads us to believe that the fish were illegally released more than one year after the pond draw-down. This example of suspected illegal release in this pond may be at the level of a personal action bringing basses angled in another site and releasing there. However, there are suspicions of large-scale illegal releases that go beyond the individual level (Mukai 2016, Fig. 16). In Nakaike pond (9.5 ha), Seki, Gifu Prefecture, the pond was drained every year since 2009, and despite the removal of blackbass, nearly the same number of blackbass continued to be caught the next year and the next next year as before the removal. Since large-scale and repeated releases were suspected, a "boobie-trap" was considered to seize the site. However, once the trapping was discussed, the releases suddenly stopped. What does it mean? Because outlaw anglers seem sneaking here and there, it is very difficult to seize the site of illegal releases. So now we have: Barrens increase due to illegal activities > Barrens decrease due to extermination — This is the situation we are facing now. Somehow we would like to reverse this inequality. There have been many efforts to lower the left-hand side of the equation by promoting public awareness and education campaigns. To facilitate these actions, in the first part of this article, I have summarized the negative effects of bass and how blackbass and barrens can be made sustainable. In the second half, I summarized how to raise the right side of the equation, how to accelerate recovery from barrens, and as a way to do so. I summarized the know-how of evading the "other" action from extermination by pond draw-down. This is the principle of fundamentalism for pond draw-down. I hope that these things will work like the two wheels of a cart, and that the number of basses will decrease and the good-old-days', lively wetlands with abundant native fish will return. |
