Northeast Multispecies: How Discards Are Calculated
During fishing operations, fish are occasionally discarded at sea. The amount of the discarded fish must be estimated so that we can account for all of the catch.
Discards for Observed Trips
If 100 percent of the hauls are observed, the actual observed discards are applied to the trip:
Landings + Discards = Total Catch (for the trip)
If less than 100 percent of the hauls are observed, the discards from the observed hauls are used to estimate the amount of discards for the unobserved hauls.
Discards for Unobserved Trips
Discards are estimated for trips that are not observed. The discard estimation process is performed at the stratum level. A stratum is made up of trips by members of the same sector, or members of the common pool, with the same gear type/mesh and in the same stock area. In other words, a stratum is a way of grouping similar trips.
The basis for calculating discards on unobserved trips is the ratio of discards-to-total landings from observed trips within a stratum. This is called the discard ratio. (A ratio compares the size, or magnitude, of two quantities.) For example, suppose there were on observed trips within the stratum 100 pounds of discards out of a landings total of 10,000 pounds. The discard ratio would be: 100/10000 = 1/100 = 0.01
We use a cumulative method of estimating discards. This means that as more data are reported during the fishing year, discards are re-estimated with updated discard ratios using the most current year-to-date totals.
The method to determine the discard ratio for unobserved trips is one of the following:
The method is determined by how many observed trips have occurred, as explained in the following sections.
In-season Discard Rate (A "Year-to-Date" Discard Rate)
- An in-season discard rate is used when there is sufficient contemporary observer data in the stratum.
- The discard rate is the cumulative sum of all observed discards for a stock divided by the cumulative sum of all observed kept species on observed trips in a stratum.
- The discard rate will change as more data become available over time.
Transition Discard Rate
- A transition discard rate is used when there is some contemporary observer data, but not enough for an in-season discard rate. It is a tool for industry and managers to monitor in-season discards, but will not be the primary determinant of annual discard rates.
- A statistical method is being developed that will transition between assumed discard rates at the beginning of the year and the in-season discard rates. The statistical method will ensure that the use of the assumed discard rate will decrease rapidly as new in-season data are collected.
- The statistical method will be used to assign relative weights to the assumed rates and in-season discard data. As more in-season observed data becomes available, more weight will be assigned to the in-season discard rate and less weight to the assumed discard rate.
- For strata that receive sufficient observer coverage, the transitional rate will have negligible or zero influence of the overall total estimate of discards.
Assumed Discard Rate:
- An assumed discard rate is used at the beginning of the fishing year, before there are contemporary observer data to calculate in-season discard rates.
- Assumed discard rates are based on last year’s observer data.