Survey methods should aim to be consistent so that data is comparable over time. Some strategies include sampling:
using the same vessel and gear
at the same locations
at the same time of day
for the same duration
at randomly selected sites from the same gridded area
Repeated surveys (for example, annually) provide a time series of data that can show how stocks change over time.
Surveys can be:
fixed: sample the same unit (place, individual) over time
random: equal chance of sampling any unit in the stock
random stratified: equal chance of sampling any unit within subsets of the stock
Locations for fishery-independent sampling are often selected using a random stratified approach: a geographic area is divided into sections (known as strata) based on certain criteria (for example, square kilometres, depth). Strata can be randomly selected to more accurately reflect the entire stock in the data.
