Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This movie is based on two of Steinbeck's short novels Cannery Row and its sequel, Sweet Thursday. While Cannery Row is concerned largely with Doc and his neighbors the Bums, Sweet Thursday introduces Suzy and her romantic relationship with Doc. The movie then adds a new character, the trumpet-playing Seer, and a baseball-pitcher back story for Doc that is not found in either novel, because Hollywood just can't leave well enough alone. Worse than that, the movie leaves out my favorite character in Cannery Row - the gopher. He was worth a sequel all by himself.
The frog section in the movie is taken from Cannery Row and I have also seen it printed separately as "The Frog Hunt." It's a short passage, worth reading, even though it describes a horrible practice and turns it into comedy. A few people have put up an excerpt of the story online. If this link is no longer good, try searching for another.
Doc, played by Nick Nolte, is a biologist living on Cannery Row in Monterey in the 1930s, where Mack, played by M. Emmet Walsh, and several other bums live a life free of money, work and worries. They sleep outdoors under debris from a cannery that was abandoned after the sardine population crashed from over-fishing and the entire industry stopped being profitable. They see Doc romancing Suzy, played by Debra Winger, a vagabond who recently came to town so desperate for work that she took a job at the local brothel, and they decide they need to throw a party for Doc. They have no money to buy food and alcohol for the party, but Doc often pays them to catch animals that he sells through his biological supply business, so they ask him if he needs any turtles - he once gave them a quarter for every live turtle they found. (This entire business is a sad look at the fomer practice of harvesting great numbers of local wildlife, including stray pet dogs and cats, and selling them for research. That cruel and unsustainable practice is now outlawed by licensing requirements and quotas for collecting herps and other wildlife but it still occurs all around the world due to collection for the commercial pet trade.) Doc doesn't need any turtles, but he has an order for several hundred frogs and offers them 5 cents per frog they bring.
The bums steal a truck owned by the local shopkeeper, named Joseph and Mary, and drive out to a wetlands at night. John Huston's voiceover narration describes the traditional relationship between frog and hunter which the frogs accept, and the confusion experienced by the frogs when the bums change the rules and use a brand new method. We see them herd hundreds of frogs into shallow water with garden rakes and tennis racquets and scoop them up into bags.
When the bums return, Doc is away on business, but they are impatient to get paid. They make an offer to Joseph and Mary to sell them to him at 4 cents per frog and he accepts. Instead of paying in dollars, they develop a system of barter where the frogs are the currency, but the bums are disappointed to discover that the price of goods keeps getting inflated as the worth of a frog is devalued. They finally throw the party for Doc, but things quickly get out of hand when a group of frat boys show up, and the ensuing brawl destroys Doc's place and all of the frogs escape.
During the frog hunt we hear a loud chorus of Pacific Treefrogs calling, but the frogs we see are all leopard frogs (which are not native to the Monterey area.) When we see a frog close-up we also hear a single Pacific Treefrog call. This is common practice in movies where it is assumed that the audience thinks that all frogs make the "ribit" sound made by the Pacific Treefrog. Hundreds of live leopard frogs are used in the movie. Most of them appear to be juveniles. They were probably purchased from a biological supply house similar to one Doc was going to sell them to, but I suspect they were bred in captivity. There was no disclaimer at the end of the movie explaining that no animal was injured during the production because many frogs must have been killed or injured. At one point, we see the character Hazel fall down on a bunch of live frogs. That must have taken out a few frogs.