We used new tags for the population estimate a year later. If we assume the tags stay on well over the year, what changes the proportion of the old marked tags in the population and make the use of new tags more accurate? (Hint: look back at the assumptions made by the mark and recapture method).

Mark and recapture is a technique to measure the size of a population before fishing and hunting season;commonly used by fish and wildlife managers

It involves the marking of number of individuals in a natural population,returning them to that population followed by recapturing some of those as a basis for estimating the size of the population at the time of marking and release
Population size can be estimated as(N)=M*T/R

here,M=initially marked;T=total in sample second;R=marked recaptures

There are certain assumptions to this method

Population is assumed to be closed which means no new individuals were born or immigrated into the population and none died or emigrated from the population
The chance for each individual in the population to be caught are equal and constant for both the initial marking period and the recapture period
One of the assumption of this method is that sufficient time must be given between the initial marking period and recapture period for all marked individuals but time period should not be very long
Animals do not lose their mark

The tags in the given question were estimated a year there is a chance that old marked tags population might have died or emigrated(the longer the time interval the greater the chance) and hence it becomes an "open" population which violates the first law

If proportion of population changes then marked recapture method formula won't turn out be applicable