The animal imagery that is used throughout the novel is used in particular to convey the characters of Lennie and Curley at the end of Chapter 3 in the fight scene. Curley is decribed as stepping over to Lennie when he sees him laughing "like a terrier". Terriers are small dogs who are renowned for snapping away ineffectually and going after something. This conveys that Curley's character isn't that strong in reality, although he may give an appearance of being strong. He is picking on Lennie because he feels the need to prey on those who he assumes to be weaker than him, especially after he has just tried to pick a fight with Slim. He is the typical figure of a bully - trying to threaten those who are weaker and smaller than he is to bolster his own sense of esteem.
With Lennie, however, despite his initial defencelessness, he has met his match. The animal imagery of comparing Lennie to a bear that was started in Chapter 1 when we first met Lennie is continued here, with Lennie covering his face with his "huge paws". The implied metaphor here reflects the bear conmparison. Later, this metaphor is repeated with Curley's fist being "lost in [Lennie's] paw."
Curley, in response to this attack, "was flopping like a fish on a line", and eventually his struggling becomes weak. Against Lennie's strength, he is shown to be truly weak, and it shows that his bravado and outer appearance of being a strong man was just that - an appearance. He is unable to free himself from Lennie's grasp and this simile reflects his weakness and also his certain defeat.