Tagging Basics
Historical
ledgers of fish tagging date back to the early 1900's. They primarily
focused on the movements of cod, haddock and pollock, but there are also occasional
entries of other species such as monkfish and sharks. Since that time
scientists have refined their methods and an entire industry of tagging related
products has been established. Many of the products available to researchers
originated in other industries and were modified to fit the needs of fishery
biologists. For example, deer are commonly marked with an ear tag which
are used to identify cows on dairy farms. These are also used on the fins
of fish (such as sharks) to give the animal an individual identity or number.
Another example, and one of the most common types of fish tag, is the 'T-bar'
tag; these are nearly identical to the plastic tags holding the price tag onto
the clothing you purchase. Once it has been determined that tagging will
meet a program's objectives, the proper tag must be selected.
Animal tagging, or 'mark and recapture', programs use methods
ranging from simple marks (ex. a painted spot on a clam) to high tech and sophisticated
satellite tags which give precise location information. Scientists must
choose from a variety of marking methods, based on their program's objectives
and budget. A researcher must carefully consider many different factors
(animal size, tag size and color, number of animals to mark, area of the study,
effects to the animal, cost, etc...). The table below shows a comparison
of several common methods of marking fish as well as some of the advantages
and disadvantages of each method:
Tagging Program Objectives
Fish and Stock Movements
In fisheries the most common objective to any tagging program
is fish movement. By comparing location information at the time of release
and from the reported recaptures, we begin to see patterns of movement.
Often times these movements are related to environmental conditions (such
as temperature, salinity or current) but they may also be cued by biological
factors such as following their food supply or reproduction.
In
some cases, a species of fish is divided into stocks which are managed separately.
For example, off the northeast coast of the United States, three different
stocks of yellowtail flounder have been identified and each stock area is
managed as a spearate unit using different regulations. By tagging fish
in each of these stock areas, we are able to examine if and how they move
and intermix. Since they are treated as independent stocks of fish,
the tagging program can provide more detailed information with regards to
the habits and cycles of the fish in each area. Currently this is one
issue we are addressing in the Cooperative
Yellowtail Flounder Tagging Project.
Exploitation Rate
Exploitation
is an estimate of the proportion of a stock removed by fishing activities.
Using scup as an example, we treat the tagged fish as a sub-set of the total
population. After the first complete year of collecting recapture reports
we will be able to determine the proportion of tagged scup removed from the
overall number released. Considerations must be made to the recapture
number for occurrences such as unreported tags, unseen or lost tags, tag
shedding (a tag which falls off the fish - see the Double
Tagging page) or fish which are recaptured and re-released with the tag
intact. The numbers are evaluated using a mathematical model which
will calculate the rate of exploitation. For more
information on modeling and fisheries statistics, please use the links at
the bottom of this page.
Fishermen Participation
Cooperative research combines the expertise of fishermen,
researchers and managers, to collect information which will be used in the
assessment of a fish population. During the inshore residency period of scup
(when waters are warm; spring through fall) they live in and around underwater
structures such as reefs and wrecks. Following their offshore migration in
late fall, scup are found near the edge of the continental shelf and on parts
of the continental slope. The NEFSC bottom trawl survey does not sample close
to underwater structures because of the potential for net snags, nor does
it intensively sample the edge of the continental slope because of excessive
depth. Since the trawl survey may not provide a sufficiently accurate or precise
index of the true abundance of scup, a tagging program was initiated. The
Cooperative Scup Tagging Program is a way of addressing these problems and
collecting additional information which is based directly on the commercial
and recreational scup catches.
The
ultimate goal of the Cooperative Scup Tagging Program is to improve the stock
assessment of scup by providing scientists with the most accurate and detailed
information possible. In order for this program to succeed, we need cooperation
from both the recreational and commercial fishermen who recapture our tags.
We still have much to learn regarding scup movement and biology; since fishermen
are in the field almost daily, they represent a great opportunity to improve
scientific research by providing expert knowledge, collect information used
in the stock assessment, and improve our general understanding of scup.
Statistics
and Fisheries Modeling Links
STATWeb:
A Guide to Basics Statistics for Biologists - web site offered by the
University of Durham, England.
STAT
Soft, Inc.: Elementary Concepts - gives and overview of basic statistical
concepts.
Program
MARK: A Gentle Introduction - a Windows-based program for analysis of
data from marked individuals.
Short
Course in Fish Population Dynamics - web site offered by the Department
of Fisheries and Wildlife at Oregon State University.
