Are these fish randomly selected from the entire population of fish of each species and sex?

Are these fish randomly selected from the entire population of fish of each species and sex? Or is the sample a convenience sample - i.e., the fish closest to the research station that are easiest to catch? In any observational study, you must be careful that the units measured are a proper random sample from the relevant populations. Are the factors to be considered fixed effects? Does it seem reasonable that if you were to repeat the survey, you would select the same sexes and species?