Rashi writes that the kosher chagavim (grasshoppers) don't need shechita because they are mentioned in the pasuk after fish which don't need shechita. The Rashba (chulin 27a) when the gemara talks about fish not requiring shechita explains that the default is that nothing requires shechita unless the Torah says it does. The only reason that the gemara 27a searches for a source that fish don't require shechita is because of the pasuk of הצאן ובקר ישחט להם אם את כל דגי הים יאסף להם, which implies that fish require some positive act of "asifa" in place of shechita. The gemara concludes that asifa- gathering is actually to the exclusion of shechita. In short, Rashi and the Rashba seem to argue whether we need a source that grasshoppers don't require shechita or whether without any source the default is that no shechita is required.
The language of the gemara באסיפה בעלמא סגי להו in the context of fish, is somewhat ambiguous. Do fish require a positive act of "asifa" which would demand that they be gathered from the sea by a person while they are still alive, or is it just a way of saying that they don't require any act and can be eaten even when found dead? The Kesef Mishna (Hil. Shechita 1:3) cites the opinion of Rav Sadya Gaon who says that fish require "asifa" and if they are found dead they are forbidden to be eaten (this would be similar to nechira prior to the requirement of shechita, which required the animal to be killed but would not permit a neveila). Rav Hai Gaon disagrees and says they can even be eaten when found dead. The Kesef Mishna understands that the Rambam holds like Rav Hai Gaon, not Rav Sadya Gaon, but in truth the language of the Rambam is very cryptic. The Rambam begins by implying that fish require a positive act of "asifa", but in the end says that even if they die in the water they can be eaten.
רמב"ם - דגים וחגבים אינן צריכין שחיטה אלא אסיפתן היא המתרת אותן, הרי הוא אומר הצאן ובקר ישחט להם ומצא להם אם את כל דגי הים יאסף להם ומצא להם, אסיפת דגים כשחיטת בקר וצאן, ובחגבים נאמר אוסף החסיל, באסיפה לבדה. לפיכך אם מתו מאליהן בתוך המים מותרין, ומותר לאכלן חיים
The Rambam doesn't say that fish and grasshoppers don't have a "matir", rather he says that the gathering of them is the matir in place of shechita. This would imply that they would need to be gathered from the sea while still alive, yet the Rambam concludes "therefore if they die by themselves in the water, they are permitted". Aside from the first statement seemingly contradicting the second, the Rambam connects them using the term "therefore"! Although the kesef mishna tries to explain that once shechita isn't necessary, there is no longer any method demanded to kill the fish, thereby permitting even fish that are found dead; if this were the Rambam's intention his language should have been that there is no matir necessary for fish and grasshoppers?