There are many roads to the Ibera Marshes, but some become real marshes when it rains. Although you can come in from Paso de los Libres or Gobernador Virasoro, the best road is the one that connects Carlos Pellegrini with Mercedes and the paved road from there to Bella Vista, that is located on the paved highway from Parana to Corrientes. Most buses to Mercedes come along that highway.
Bella Vista is a beautiful spot on the Paraná river, where dorados (of the salmon family) and surubis (very similar to pikes) can be fished. Bella Vista is located between Empedrado and Goya, two other excellent fishing destinations. From north to south I may mention the following fishing paradises: Paso de la Patria and Isla del Cerrito (near Corrientes), where the Paraguay river runs into the Paraná. You can go from Paso de la Patria on the regular motorboat service or (if it is not running) on the regular bus services from Resistencia (Chaco) that take about an hour and a half to get there. Buses from Corrientes to Resistencia run about every 20 minutes (one hour travel). You will do better fishing at Isla del Cerrito (and reduce your costs) if you cross the river on the motorboat that runs twice a day to this island, since there you can fish dorados from the shore. The Paraguay river (nearly a mile wide) runs into the Parana (two miles wide at this point) forming a huge whirlpool where dorados are waiting for their food. Live bait is allowed, mainly morenas (morays) or even eals, although the Paraguay river, unlike the Paraná, generally brings clear water, so you could use fly-fishing (not the tiny flies you use for trout, some flies for dorados are six inches long) or large spinners. You will need a steel leader, because those dorados have sharp teeth and otherwise they will cut your nylon. Also, where both rivers meet, there are many submerged trees, so you should always fish with a floating line, and if you use live bait, without any sinker. You will loose many yards of nylon, many leaders, many hooks, so go prepared.
I usually take a small fishing rod for fishing bogas (similar to shads) and found that salami is an excellent bait for bogas (that weigh normally two to four pounds) because it does not rot. You fish them near the shore, and fishing them is pure fun. Then you cut the boga in pieces of 3 to 5 inches long, and use those pieces for fishing the dorados. I have fished dorados weighing 22 pounds from the shore using this modality.
Itati and Ita-Ibate: These have been two traditional fishing places, but after the construction of the Yacyretá dam (the largest hydroelectric project in Argentina) that also has navigation locks, fishing possibilities are unpredictable. There is also intense commercial fishing on the Paraguayan side that has reduced drastically fishing possibilities. And there have been some cases of "pirate" stealing in the river from the Paraguayan side, that in most cases have consisted in stealing the motor of the boat. There is a Coast Guard in the area, but how effective it is, nobody knows. When the cruising ships go up the river from Corrientes to Iguazu, the locks are first opened and then closed, modifying substantially the flow of the river and therefore the fishing possibilities. I would no longer recommend this area. Flow is more steady at Paso de la Patria and Isla del Cerrito, because there are no dams on the Paraguay river, and there will always be a constant flow of water, specially at Isla del Cerrito.
Newspapers always inform the flow of the river and if it is swelling or decreasing its flow. For example, this site will tell you if the level of the river is increasing or dropping: http://184.108.40.206/alturas/. Generally speaking, if the level of the river at Corrientes is under 300 centimeters (10 feet) , fishing possibilities increase. Empedrado: a beautiful fishing spot, but you will need to hire a fishing guide (a normal boat will cost far less than a motorboat). There are very nice riverbanks eroded by wind and water where you can take some photos. I had my first Parana river fishing experience there in 1970, and was really thrilled. There is a very nice hotel for tourists (Hotel de Turismo) overlooking the river. It is not expensive.
Bella Vista and Goya. I have never gone fishing to these two places, but have read fishing reports and fishing is excellent. The Guarapo lodge is reasonably priced, offers an all-inclusive deal including transportation from the airport, navigation to the lodge, sleeping in tents and all meals. Check their site: www.guarapo.com.ar I find their deals interesting. Esquina: the Corrientes river, that comes down from the Iberá Marshes, meets the Paraná at Esquina. There is good surubi fishing in this small river; dorado fishing depends on the time of the year. Normally dorados are up north in the winter (Isla del Cerrito and Paso de la Patria) and will come down the river by September/October. The dorado fishing season ends in October, while surubis can be fished during the summer months. I have gone fishing in Esquina, and my results were poor, but the sons of a friend went at the right time and enjoyed beautiful fishing of dorados up to 30 pounds both in Empedrado and Esquina. Check at the hotel’s front desk who are the best (and affordable) fishing guides in the area. But I feel that the Guarapo deal is interesting, you fish inside a private property, catch and release, and are exposed to less hazards, since they are experts in this.
Another page you might want to consult (if you understand Spanish) is www.tumejorpesca.com. This service belongs to Florida Travel Service of Buenos Aires, they have fairly low prices and visit the main fishing destinations from Concordia, all the way up to the Amazon in Brazil. Just now they are featuring a fishing tour to Esquina for 330 dollars (3 days and 2 nights, including transportation from Buenos Aires, 2 nights at a lodge, all meals, motorboat and gasoline). By what I read, fishing dorados in Esquina is allowed all the year round. As a general rule, fishing is better when the river is shrinking, because the dorados wait at the outlet of lagoons for their prey. The city of Corrientes is another nice place you will want to visit. Fishing possibilities there are minimal, but you are only half an hour away from Paso de la Patria, an hour and a half away from Empedrado, two hours from Bella Vista, and a longer distance from Goya and Esquina.
Corrientes is a provincial capital, and has some 150,000 inhabitants. The pacú is a delicious fish to eat. It may seem curious, but the best bait for this fish is fruit... And bogas can save the day. It is real fun to fish them. They are generally near the coast and I prefer to fish them if possible from a pier. If you use a floater, it will start popping up and down. Do not try to hook them at that time, they are only sucking the bait but they have not swallowed the hook. Once they sink the floater, hook them firmly, remembering that they have a very delicate mouth, that could break and you would loose the fish. If you are fishing with a 3 or 4 ounce sinker, you will feel the bogas if you keep a finger on the nylon. But wait until the line takes a 30 degree inclination before hooking the fish. Keep the line at all times away from the pier. These rascals turn around the beams of the pier and if they are successful, you will hook the pier, not the boga. Average size of bogas is some 2 pounds but 4 to 6 pound bogas are very common in the upper Parana river. I believe you will find this information very interesting.