1. What Does Japan's National Law Say About Releasing a Caught Bass?
Largemouth bass and bluegill are both native to North America. In Japan they are classed as invasive alien species, which means non-native animals that can harm local nature. Japan's Ministry of the Environment says that anglers may still fish for them. What the national law forbids is taking such a fish home alive to keep it, or moving it somewhere else and letting it go. Putting the fish back right where it was caught is not forbidden. Killing it on the spot and taking it home to eat is also fine [1].
The Ministry adds a warning in the same answer: some prefectures (Japan's regional governments, like US states) ban catch-and-release of non-native fish by their own ordinances [1]. Shiga Prefecture, where Lake Biwa lies, is one of them. It bans putting a caught bass or bluegill back into the lake. Its own website explains that the national government leaves this decision to each local government, and that the national guideline for controlling bass says a release ban should be considered case by case, depending on conditions in each body of water [2].
So the national rule and the local rule do not contradict each other: the national law does not forbid putting a fish back where it was caught, while Shiga's ordinance forbids it on Lake Biwa. Why is Lake Biwa's problem so bad?
2. Why Is Lake Biwa Protected More Strictly Than Other Waters?
Lake Biwa is the largest lake in Japan, about 63.5 km long, and one of the oldest lakes on Earth, with an origin going back around four million years. More than 30 kinds of fish and shellfish found there live nowhere else, and more than 10 of them are fish [3]. Over a very long time, some species in an old lake can evolve in ways that make them unique to that lake, so each species lost from it is lost from the whole world.
A research paper by scientists at the Shiga Prefectural Fisheries Experimental Station tells what happened next. Bluegill were first found in the lake around 1965 and largemouth bass in 1974. Bass numbers rose explosively in the 1980s, and native crucian carp and small fish called bitterlings disappeared from the shallow water along the shore. Bluegill then increased as well, and by the time of the paper (2009) they made up over 90% of the fish in the lake's southern basin. The catch of native shore-living fish had fallen to about half of what it was in the 1970s and 1980s [3].
The authors link this to what the newcomers eat: bluegill feed on plants, tiny animals, fish eggs and shrimp, while bass eat mostly fish and shrimp. They describe the native fishes' decline as "probably caused" by the alien fishes, which is careful wording, because proving the cause in a whole lake is hard [3]. Shiga's website (page updated in 2021) says that in recent years most of the fish near the shore have been these two species, so even a small reduction is worth pursuing [2].
That is the reason for the strict local rule. Every bass or bluegill put back is one more hunter or egg-eater in a lake that is trying to recover. But if you cannot put the fish back, what are you supposed to do with it?
3. Where Does a Caught Invasive Fish Go, if Not Back in the Water?
Shiga Prefecture set up collection tanks and pens at the lakeshore where anglers can drop off these fish. The collected fish are used as compost [2].
There is a catch, and it comes from the national law. The law bans moving a live invasive fish to a place that is not part of the same body of water. Shiga's reading is that carrying a live fish to a collection tank inside the lake area, including its fishing harbors, is fine, and so is keeping it alive in a bucket while you fish. But driving it past the road that circles the lake to some other place would break the law [2]. The idea behind the rule is easy to see: the fish got into new lakes in the first place because people moved them.
So the same fish can be handled in three different ways under three different rules: put back (allowed nationally, banned on Lake Biwa), dropped in a collection tank (encouraged on Lake Biwa), carried alive to a different body of water (not allowed anywhere in Japan). On Lake Biwa, the collection tank is the way to go. In other places, check the local rules.
What about a place far from Japan? The same reasoning applies, but the exact rules do not. State, provincial and national agencies in other countries each have their own lists of species and their own rules about releasing or moving fish, so an angler needs to read the local rules for the water they are standing at. We have not covered any one country's rules here.
4. What Else Do Anglers Leave Behind That Can Harm Wildlife?
Fish are not the only thing. Lost or thrown-away fishing line harms animals too, and the harm shows up at a very different scale: one bird at a time.
Audubon Florida, part of a US bird conservation group, describes it from the field. Fishing line, whether nylon, fluorocarbon or braided, is made of plastic and does not break down naturally; if left in the environment it can last hundreds of years. It is made to be thin and hard for fish to see, which also makes it hard for birds to see. The group's staff member in Sarasota, on Florida's Gulf Coast, described a black skimmer chick only three weeks old, too young to fly, whose legs were badly damaged by line; it did not survive. An adult skimmer was found with a hook in its wing and about 10 feet (3 m) of line trailing behind it, and the loose line had caught a second bird, a royal tern, so that the two were stuck together. Both were rescued and freed at a local seabird rehabilitation center [4].
A rehabilitation center in the same area counted 108 cases of fishing-line entanglement in 2021, affecting 13 kinds of birds [4]. These numbers come from one center in one area, so they show how common the problem can be locally, not how many birds are hurt everywhere.
The advice is simple: avoid fishing close to places where many birds gather, and put used line and tackle where it will be collected. In many parts of Florida, the state wildlife agency runs a program with collection bins for used line [4]. This is similar to the problem with invasive fish: in both cases, the harm comes from an ordinary angler's ordinary action, and in both cases the fix is for the angler to decide what happens next instead of leaving it to chance.
5. How Can You Find the Fishing Rules for Your Own Local Water?
If you or someone in your family fishes, try this with an adult. Search for the fishing regulations from the wildlife or fisheries agency of your state, province or country. Look for three things: whether any species must not be released, whether live fish may be moved, and whether there are bins for used line. If you find a rule that surprises you, see if the agency's page explains the reason. The Shiga page does, and a rule with a reason behind it is much easier to follow than one without.
The sources below are worth reading in their own words. Pay special attention to the research paper on Lake Biwa: it contains graphs of catches over time that are far more striking than a summary can be. If you find a stray hook or tangled line while out, tell an adult before touching it, since hooks can hurt people too.