 On the second visit,  a number of fish is captured and one observes that the ratio of native-to-labeled is 10:1.
<clarity>  On the second visit,  a number of fish is captured and one observes that the ratio of native-to-labeled is 10:1.
 On their second visit, a number of fish is captured and one observes that the ratio of native-to-labeled is 10:1.