Marine Institute

Sampling and Data Collection


A key objective of fisheries research is to provide good information on the state of fish stocks. This information is collected through various samplingScallop dredges 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 Blue Whiting samplingsea 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 are the data used?

Plaice OtolithThe 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.

Catch Sampling

The demersal catch sampling programme is based on a stratified metier approach. This means sampling trips are divided into separate groups, made up of all vessels that fish with similar gears and catch similar species. The programme commenced in 1993 and has one of the longest time series of discard data collection in Europe. Since 2002 it has been mandatory for the Marine Institute to collect catch sampling information as part of the Data Collection Framework and now it is carried out as part of DCMAP (Data Collection Multi-Annual Programme). At sea catch sampling is essential to quantify the size and age structure of all species in the catch. Without accurate catch information, assessments become more uncertain and advice on sustainable fishing levels becomes more conservative, leading to lower quotas. Sampling at sea is also important for monitoring catches of non-quota species (including protected, endangered and threatened species). To date, catch samplers have sampled 1,266 commercial fishing trips covering numerous fishing grounds including Stanton Bank, back of the Aran Islands and the Smalls.

The catch samplers 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.

Marine Institute staff are based in the following ports around the country:

    • Greencastle - contact (041) 9889788
    • Dunmore East- contact (051) 385071
    • Castletownbere - contact (027) 71761
    • Ros a Mhíl - contact (091) 572584
    • Killybegs - contact (074) 9741926

We host a dedicated site for all information relevant to the DCMAP (Data Collection Multi Annual Programme) funded by the EU, which can be found at