Both plant and animal life needed to recover after the flood. Much of this life was not on the ark, so how was it able to survive?

Most of the fish died in the flood, but some fresh, brackish, and salt water individuals would survive in various pockets/gradients of water having appropriate salinity. It is a mistake to think that the flood waters would be fully mixed to a homogenous salinity state. It is also true that most fish can withstand some change in the salinity level of their water.

The waters of the flood were covered with large floating mats of vegetation stripped from the land surfaces. Some plants would continue to grow on these thick mats. Some birds, and maybe some insects, amphibians and other small creatures may also have survived on these mats. Some of the mats would wash up onto the new shorelines to reseed the land, but most were buried to form the large coal and oil deposits we now find. The buried seeds of many plants would also become exposed during the erosion events at the end of the flood period (the erosion at the end of the flood was significant, and caused large geologic structures such as the Grand Canyon to form).

What did the animals from the ark eat after they left the ark? It is interesting to note that God kept them on board for three months following the first appearance of dry land. This would give time for some plants to grow. Other food sources would have included seaweed, fungi, carrion (the meat of dead animals exposed by erosion), fish, insects, earthworms, and rodents.