While this could indicate that the population is down, the numbers tagged could be low this year because the conditions have not been good for tagging this year:

Nights have not been cool like they were last year. (Tonight's low will be 75F.) Monarchs cluster together tightly on cool nights (say 55F), so big groups of them are easy to capture. Clusters this year are small--say 15-- compared to the groups of 50,60 or even 100 on a cool night. What's more, after we swoop the net when it's warm, the butterflies are capable of flight. With cool temperatures, the butterflies aren't able to fly well so we can usually even catch them by hand when they fall.