Monitoring practices for sea trout spawners and parr are described on a general level in the HELCOM Monitroing Manual in the Sub-programme: Migratory fish.
Specific guidelines are under development.
The monitoring activities relevant to the indicator which are currently carried out by HELCOM Contracting Parties are described in the HELCOM Monitoring Manual in the Monitoring Concepts table.
Sub-programme: Migratory fish
Monitoring Concepts table
The intensity and period during which monitoring has been carried out varies between countries (ICES 2008). Some countries started monitoring only in recent years, whereas very long data series exist for a few streams.
Sea trout is monitored by all Baltic Sea coastal countries by electrofishing for parr in the natal streams, giving a good index measure of recruitment. Parr densities are measured by regular electrofishing surveys in early autumn (August-September). One river can be surveyed annually or at 2-7 years intervals. Electrofishing usually takes place at fixed sites to allow for comparison of the densities between years. There are usually several electrofishing sites in one river.
In a couple of countries sampling of parr densities is used to calculate the smolt production by a relation of parr to smolt survival either developed in the same stream or in different streams (ICES 2008). In most countries (not in Denmark or Poland) this is supplemented with monitoring of smolt escapement by trapping and counting smolt numbers in one or more streams. In total, smolt production estimates exist for nine rivers in the entire Baltic area, but the time series are not complete for all years.
In only one river (Åvaån in Sweden) is the number of spawners monitored by trapping and inspection of the ascending sea trout. In Lithuania, the spawning run is estimated by test fishing in a couple of rivers. In nine rivers (eight in Sweden, one in Poland) is the number of spawners monitored by automatic fish counters. Determination of species is possible in these, but the exact size, sex, etc. cannot always be determined. In three rivers, the total run of salmonids is determined with an echo sounder, however this technique does differentiate between sea trout and salmon.
An indication of spawning intensity by count of redds is collected from a number of streams in Poland, Lithuania and Denmark (ICES 2008). In a couple of streams in Denmark the catch in sports fisheries has also been used to estimate the development in the spawning run. Catch numbers from sports fishery in rivers are available also for some Swedish rivers.
Tagging and marking are used as methods to obtain quantitative and qualitative information on trout populations.
There are too many sea trout rivers and brooks to monitor them all. It is recommended that monitoring of sea trout is carried on the main stocks and expanded to stocks which are poorly known.
Part of the monitoring of sea trout parr takes place in connection with monitoring salmon populations, which results in less precise estimates of sea trout recruitment because of differences in habitats used by the two species. More electrofishing sites should be established in smaller rivers and streams, e.g. tributaries of salmon rivers, to ensure sufficient coverage of sea trout nursery areas.
Sea trout rivers and brooks are also within the focus of EU Water Framework Directive (WPD). Regular monitoring of selected of rivers, and additionally more rigorous inventories at 5-10 years intervals, would fit as a part of national WFD programmes.