Much like other Pokémon preserves in Kanto and Hoenn, the Great Marsh features a Safari Game in which Trainers pay 500 for 30 Safari Balls and a limited number of steps (500) to catch all the Pokémon they can.
The mechanics are similar to other Safari Zones: when a wild Pokémon appears, the player may not send out any Pokémon to battle it, but must instead try to catch the Pokémon without harming it. The wild Pokémon may choose to flee the battle at any time, thus, catching a Pokémon in the Great Marsh requires mainly luck.
There are four options shown in the battle screen:
Throw a Safari Ball - Throws a Safari Ball in possibility of capture. It is much the same as throwing a Poké Ball in an average Pokémon battle.
Throw bait - Throwing Bait makes a Pokémon harder to catch, but makes it less likely to flee from battle.
Throw mud - Throwing Mud does the reverse of throwing Bait, making the Pokémon easier to catch, but making the Pokémon more likely to flee from battle.
Run from battle - Like in an average Pokémon battle, running away from a Pokémon will cause the battle to end.
The area was redesigned in Pokémon Platinum to feature slightly upgraded graphics that allow the player to distinguish between deep and shallow mud. The park is still divided into six areas, but the individual areas have slightly different layouts.
Some Pokémon will not always appear in the Great Marsh. Every day, a random Pokémon from a set list will be chosen by the game to appear in each area of the marsh, in addition to the Pokémon that can always be found there. Each possible Pokémon has a different chance of being chosen, but whichever one is chosen has a 10% encounter rate in its area. The same Pokémon can be chosen to appear in multiple areas on the same day, and, prior to obtaining the National Pokédex, some of the Pokémon that can be chosen can already be encountered anyway. After obtaining the National Pokédex, the list of changing Pokémon changes.
The only way to tell which changing Pokémon is available in each area for the day is to use the lookout upstairs from the entrance. For 100, the player can get a brief glimpse of the Great Marsh, showing certain Pokémon in certain areas of the marsh. In each slide, a part of the park will appear with a Pokémon in the center of the slide that can be found in that area. However, the Pokémon that are shown are completely random, meaning that it is possible for the player to only see Pokémon that can always be found in each area anyway. Record mixing also changes the changing Pokémon of the day in the Great Marsh so that all players mixing records have the same Pokémon on the same day.
As with previous Safari Zones, Pokémon are made easier and harder to catch by modifying their catch rates. An escape rate is also given for each Pokémon that appear in the Great Marsh. The escape rates for Pokémon are the same for all Generation IV games, except that those who do not appear in the Great Marsh or the Johto Safari Zone do not have their escape rates defined in the relevant games.
At the start of an encounter, both rates are set to their species defaults. Throwing Bait will lower the escape rate by one stage, but will also have a 90% chance of lowering the catch rate by one stage. Conversely, throwing Mud will raise the catch rate by one stage, but will also have a 90% chance of raising the escape rate by one stage. When bait is thrown and the flee stage is not decreased the message "... is busy eating!" will be displayed. When mud is thrown and the catch stage is not decreased the message "... is beside itself with anger!" will be displayed. Stages for catch rates and escape rates work identically to stat stages: they may not be raised above +6 or below -6, where 0 is the starting stage. The stage multipliers applied to the catch rates and escape rates are also identical to that of stat stages.
At the end of each turn, a random number between 0 and 254 (inclusive) is generated, and if it is less than the or equal to the modified escape rate, the Pokémon escapes.
Due to a programming error, Whiscash does not have a flee rate programmed into the game, and thus only has a 1/255 chance to flee (Flees on a random number result of 0).
Exclusively throwing Safari Balls provides the best chance of catching every Pokémon in the Great Marsh.
Base flee rates
These are the base flee rates. The Pokémon actual chance of fleeing each turn is affected by bait and mud.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
The "rates" below are the chances for that Pokémon to be selected to appear in an area. All of these Pokémon will appear with a 10% encounter rate within the Great Marsh, or a 10% greater rate if they were already present. In Diamond and Pearl, the changing Pokémon's encounter rate is compensated (so that all encounter rates sum to 100%) by lowering the encounter rate of Marill and Quagsire in the same area (as seen above) by 5% each, while in Platinum, it is compensated by lowering the encounter rate of Wooper by 10%. The way this works in the games, this means that if Wooper is selected as the changing Pokémon in Platinum, there is no change to the tables shown above.
As a boy, James was exploring the Great Marsh when he found a Carnivine stuck in the mud, struggling to get free. James pulled it out and they became friends ever since. During Team Rocket's journey through Sinnoh, Carnivine traveled with James.
In A Skuffle with Skorupi, Diamond, Pearl and Platinum came here after Crasher Wake's assistant recommended Platinum to get a Grass-type Pokémon before challenging the Gym Leader. Platinum tried to catch various Pokémon, like Budew and Skorupi, with no success. At some point, the group got stuck in the mud. While they tried to struggle themselves free, a wild Carnivine attacked Chatler and Lax, who fought it and won. Chimler saved Platinum from drowning in the mud, after which the group left the Great Marsh empty-handed.
Pokémon in the Great Marsh Cards listed with a blue background are only legal to use in the current Expanded format. Cards listed with a silver background are legal to use in both the current Standard and Expanded formats.
In Diamond and Pearl, a man in the Great Marsh incorrectly states that both the Bait and the Mud make Pokémon more likely to run but easier to catch. In Platinum however, the man states that the Bait will make the Pokémon easier to catch and more likely to flee, and the Mud will make it harder to catch but less likely to flee.
In Platinum, if the player encounters a Pokémon when they would normally "pop" out of deep mud, a glitch exists where upon exiting the battle, it will seem that the player is still "stuck", despite being able to move around normally. The effects of this glitch are most obvious when the player attempts to move to a tile with dry land after performing the glitch.
Despite the Galactic Bomb's successful detonation, no damage caused by it can be found at the Great Marsh.