I once saw a jewel fish, during such an eveningtransport of straying children, perform deed which absolutely astonished me.

during such an evening transport ofstraying children这是一个在see+ 宾语+ 宾补结构中插入的一个时间状语。

【文意】我有一次看到宝石鱼在晚间转运迷路的小鱼的过程中做出了一个使我大吃一惊的动作。