A key objective of fisheries research is to provide good information on the state of fish stocks. This information is collected through various sampling programmes and the data are used to provide advice on the sustainable management of fish stocks, upon which the industry depends.
There are two main sources of data collected and used by fisheries scientists. These can be divided into fishery independent and fishery dependent data. The first usually involves monitoring the changes in distribution over time in the relative or absolute abundance of fish populations using fisheries surveys in a way that is not subject to the biases inherent in commercial fishery data (National Academy Press, 1998).
Fishery dependent information involves collection of catch and effort data (CPUE) as well as biological sampling from commercial fisheries. CPUE data are usually collected from logbooks. The underlying assumption is that changes in CPUE accurately reflect changes in the abundance of the fish stocks.
In addition to the above all age structured assessment models used by scientists require accurate estimates of the number, weights and age of all fish removed from the population by various fisheries. These should include both landed and discarded fish.
How is the sampling done?
Length distributions of landed fish can be obtained from samples taken in the auction halls, ports, quays side sampling or at sea on commercial vessels. Generally, the length sampling is carried out by area, gear and time to ensure that the estimated size distribution is accurate. In addition, Marine Institute scientists collect data on the age and weight-at-length from these samples. Sampling throughout the year allows scientists to monitor differences in growth and age over the year.
Where is the data used?
The sampled length frequency information for either landings or catch can be converted to ages using age-length keys derived from the relationship between age and length. Ages are estimated using a small bone found in the head of fish called the otolith. Counting the rings on these otoliths gives an estimate of the age of the fish (similar to the rings of a tree). The numbers of fish at different ages for various gears, areas, quarters and nations are combined to produce annual catch numbers-at-age for each assessed stock.
Discard sampling allows scientists to quantify that part of the catch that is not landed at the ports. Sampling of fishermen's catches at sea provides a reliable method of acquiring data on the quantity and species composition of discards. Collecting information on discards facilitates the re-construction of the whole catch for a particular species, which is extremely important in stocks where the discards are a substantial part of the catch. Without this, the data collected would be based solely on landings information from the commercial markets. Sampling the length and age of discards allows the estimation of the discard size at age composition, which is sometime quite different to the size at age composition of landings.
The Marine Institute Discard Programme has been in existence since 1993. In the past, funding for this programme has been under the guises of many EC projects but in 2002 with the establishment of the data collection (regulation EC No. 1639/2001) the monitoring of discards is now a mandatory part of a European fisheries sampling programme.
The Fisheries Assessment Technician (FAT) discard programme monitors all the major fleets in the main ports. Currently there are three FATs based in Clogherhead, Ros an Mhíl and Castletownbere. The discard work is also supplemented by Fishery Sampling Contractors.
Take a FAT to sea
The FEAS Fisheries Assessment Technicians and Fishery Sampling Contractors carry out very valuable work on assessing discard rates in the Irish fleet. They need to go to sea on commercial fishing vessels every month and for this we rely on the co-operation of the fishing industry. Without the support of fishermen this programme could not succeed.