Tag Fate & Double Tagging

The Fate of a Fish and its Tag

In a study of exploitation (definition), we are tracking the life and death of the tag rather than the fish.  Usually they share the same fate (a tagged fish is recaptured and kept), but at times the fish and the tag follow different paths.  The most common examples of fish and tag fates are described here:

1. Recaptured fish is kept and reported - this is the most common occurrence and the easiest to deal with since both the fish and tag are treated the same.
2. Recaptured fish is released with the tag in place - often times undersized fish are re-released. These recaptures are used to examine the movements of fish, but since the tags are not 'dead' only a portion of them (dead due to tag induced mortality) can be used in the estimation of survival (see the Tagging Basics page).
3. Recaptured fish is released but the tag is kept - again, most commonly with undersized fish, sometimes the tag is retained but the fish is re-released.  This information is useful for mapping fish movements, but cannot be used in mortality and survival estimates since we do not know the natural fate of the fish.  A re-released fish may die naturally, or it might die due to tag related injuries, or even be taken by a fisherman at a later time.  Since the tag was removed, the end fate of the fish is unclear and the tag information is not used to calculate stock estimates.
4. Recaptured fish is not reported - unreported tags are the most common hinderance to all tagging programs.  When a tagged fish is recaptured, every fisherman makes a decision to either report or NOT report the tag.  Tags might be lost, a fisherman may simply forget he recaptured a tag or he consciously chooses not to report a tag for reasons of his own (link to the Why Report Tags? section).  We rely on commercial and recreational fishermen for the majority of our recapture reports.

When a tag is reported we collect information on the fates of the fish and the tag,  this allows us to determine how each report can be used.  We prefer to have both the fish and the tag kept at the time of recapture, however, we have tagged sub-legal sized fish and do not encourage fishermen to break the law.  The most important point is that each recapture is reported with the fates of both the fish and the tag.

Double Tagging  &  Holding Studies

One of the assumptions we make when conducting a tagging program is that a tagged fish acts the same way as an untagged fish, and there is no change in the fish's chance of survial. Obviously, that assumption is not completely true. Fish may die as a result of tagging or may alter their activity until they have adjusted to the tags presence.  To address these differences, we conduct additional experiments to give us a more complete understanding of fish behavior.

Two simple and common methods of investigating the differences in fish or tag survivial are to add double tagging and holding study components to the tagging program.  Double tagging is simply applying two tags to a group of the tagged fish (in the scup program we double tagged approximately every 10th fish, 10%).  When one of these fish is recaptured, we determine whether both tags are present or if one has fallen out.  We track the tag numbers of the double tagged fish, count how many were recaptured with both tags, count how many fish had only a single tag in place, and then find the difference between the two.  This proportion represents the amount of tag loss or tag shedding experienced in the program.  Since it is calculated based on the double tagged fish, the proportion is applied to the total number of fish tagged in the program and the recapture numbers are adjusted to account for these tag losses.  It sounds a bit confusing, but it is a way of accounting for the fish which were released with tags but later recaptured without one.

Holding studies address a similar problem and allow us to make adjustments for tag induced mortality; this refers to fish which die because of the tagging procedure.  A few causes of tag induced mortality may be handling the fish (bleeding from a hook wound, ascending too quickly from deep water, over-expansion of the air bladder) or by the actual tagging (bleeding at the tag site, extremely warm surface water temperature, fish does not descend quickly and is eaten by gulls).

While in the field, we collect a group of fish and bring them back to a facility with holding tanks.  Once they are placed back in the aquaria, we allow them to adjust to their new surroundings for 3 - 4 days before tagging.  The ones that die during this time are the result of handling issues and not from tagging.  Those that remain alive are tagged, fed each day and kept for several months to observe healing of the tag site, changes in behavior, or deaths.  Careful records are kept and after the wounds are healed we can determine the number of fish which died because of the tagging procedure.  If possible we repeat the study for each year of releases and will then have a larger set of data to determine our tag induced mortality estimate.  Similar to the double tagging experiment, the proportion of fish which died during the holding study (deaths from both handling and tagging procedure combined) are applied to the total number of fish tagged in the program, and adjustments are made before final analysis of the tag return data.  Both of these concurrent studies, double tagging and holding studies, are used to account for some the 'unknowns' within the tagging program.

Tag shedding - a tag falls out before the fish is recaptured; this can be due to improper tagging technique (such as shallow and skin tagging) or the tag might be biologically rejected by the fish.

Exploitation rate - a statistical estimate of the proportion of a stock which is removed due to fishing activities.  Basically, the number of fish killed in a year by fishermen of all types (recreational and commercial).

Tag induced mortality - tagged fish dying due to injuries caused by the tagging procedure.