It's strange that John mentions that the fish caught by Jesus' disciples in John 21 numbered 153. So what? And who counted them? And why remember it? And even if you remember, why put it in your Gospel account?
Richard Bauckham, who has noticed that the prologue to John contains 496 syllables and the epilogue 496 words (496 is both a perfect and a triangular number), and that the two mini-conclusions to John (20:30-31 and 21:24-25) both contain 43 words, suggests an explanation; here's his conclusion.
We have still not exhausted the significance of the figure 153. It seems also to be linked to the first part of the Gospel's conclusion (20:30-31) in the following way. That passage states that the signs done by Jesus have been recorded in the Gospel so that readers may believe that he is the Messiah, the Son of God, and through believing have life. That, of course, also describes what the catch of 153 fish in the narrative of chapter 21 parabolically illustrates: people coming to faith in Jesus and to new life as children of God. The keywords of 20:30-31 are: sign, believe, Christ, life. Each of these four words occurs for the last time in the Gospel in those verses. If we count the number of occurrences of these words in the whole Gospel up to and including this passage, the results are: sign 17, believe 98, Christ 19, and life 36. The sum of the last three of these numbers is 153. So the number 17 and its 'triangle' 153 are written into the whole Gospel in the form of these word statistics and are implicit in 20:30-31. (R. Bauckham, Testimony of the Beloved Disciple, 280-81)