Guidelines for fish monitoring sampling methods of HELCOM

Background
   Monitoring strategy
   Programme design
Monitoring methods
   General
   Choice of gear
   Description of the gear
       Passive gear
   Sampling strategy and localities
   Fishing performance
       Passive gear
Data collection
       Passive gear
References

Background

Fish are to an increasing extent studied in environmental science. Standardized techniques for long-term monitoring and predictions of the size and productive capacity of fish populations, as well as continuous control of their health in a wide context are thus required. Since the guidelines for the HELCOM coastal fish monitoring programme was last updated in 1999 (Neuman et al. 1999) and some additional gear has been included in monitoring programmes, this document was produced to describe new guidelines for the fishing methods used, the use of the gear and the variables monitored to study coastal fish populations in the Baltic Sea and Kattegat. Country-wise information about this are listed in tables. The guidelines of this document could also serve as a platform for an integration of biochemical/physiological data and contaminant concentrations with basic ecological data.

Monitoring strategy

The monitoring strategy is designed to prove the impact of exposure to eutrophication, habitat alteration, climate change, toxic substances and overexploitation. There are well-developed models predicting the reactions of the Baltic coastal freshwater fish community to these factors (Hartmann 1977, Neuman and Sandström 1996), but for marine fish, however, there is still a considerable lack of understanding.

When monitoring the impact of toxic substances, an integrated approach has been suggested by many reviewers (Owens 1991, Sprague 1991, Munkittrick 1992). Species should be selected according to well specified criteria, and monitoring should be performed on different levels indicating the health of the population. A tiered strategy is often recommended for the analysis of observed deviations, including biochemical endpoints to support interpretations of possible toxic influence (Neuman and Sandström 1996).

Two fish species have been approved for Baltic coastal monitoring: perch (Perca fluviatilis) and eel-pout (Zoarces viviparous). Both species are stationary, large enough to allow for biochemical and chemical sampling, generally abundant in their respective habitats, and with a biology allowing far-reaching analyses of reproductive impacts. Perch may be monitored in most sheltered habitats in the Baltic, while the eel-pout more represents the open coasts of the Baltic and the Kattegat. Flounder (Platichthys flesus) could potentially be used as an alternative to eelpout and perch as a sentinel indicator species. Flounder is, for example, the preferred indicator species for contaminant analyses and time trends of POP’s and metals in Danish monitoring programmes.

Programme design

Responses at the community level are monitored by analysing changes in the abundances of populations, where cyprinids are generally acting as sensitive indicators of coastal eutrophication in the Baltic. In monitoring population characteristics like age structure, growth and reproduction is also added to the abundance studies.

The programme is designed for truly coastal species, which are mainly demersal. Most Baltic coastal areas are dominated by warm water adapted freshwater species. In the Kattegat, the eel-pout is one of few typical non-migratory coastal species available for monitoring of the whole life-cycle. Pelagic coldwater species like herring (Clupea harengus) often occur in coastal areas, but they are mainly migratory members of open sea communities and should therefore not be included in the present monitoring programme.

Established and standardized methods for fishing with gill nets and fyke nets are used for population monitoring as well as for sampling of fish for analytical purposes. A detailed description of the principles behind the system is given by Neuman (1985).

Other measurements on the individual level, being outside the scope of the basic programme described here, such as contaminant analyses, biomarkers, physiology, pathology etc, can easily be included (see Neuman 1985, Larsson 2006). The basic programme can be (and in some countries already is) applied both in reference areas (i.e. areas without local anthropogenic influence), and in hot-spot monitoring. Moreover, monitoring of fish diseases could also be included in the basic monitoring programme (Thulin et al. 1989).

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Monitoring methods

General

An important objective in fisheries management and nature conservation is to retain a natural abundance and species composition of the fish community. Most methods for monitoring changes in fish abundance provide catches of several species and information on changes in the species composition of the community could thus also be extracted. In abundance studies the absolute density of a species can not usually be measured. Instead, studies are made of the changes in the relative measure catch per effort and in the species composition.

Abiotic ambient factors play an important role in the behaviour and metabolism in fish. Activity, for example, normally increases with increasing temperature, something that could potentially influence the catches in passive nets. Activity may also be influenced by changes in the wind conditions, currents, salinity and water transparency. Moreover, since fish are poikilotherm organisms, their metabolism, and thus growth and survival, is strongly influenced by temperature. Growth capacity has for example a strong positive temperature dependency up to an optimum temperature depending on the species and size. Furthermore, survival during the first year of life is both directly and indirectly, via food uptake and growth, linked to temperature. Consequently, when analysing data from fish monitoring, it is essential to include temperature. Variation in other important abiotic factors should also be registered since they are of importance for the interpretation of the catch data.

Choice of gear

The choice of nets is governed by the species composition in the community to be studied and the desire to obtain reasonably accurate estimates of size- and age-distributions. Net sets have been extensively used by fishery biologists to study fish communities. The basic unit in the recommended programme is a link of four nets with different mesh sizes, set in a locality – ”station”– with uniform (hydrographical) conditions. In the northern Baltic, however, depths and substrates often show a considerably small-scale variation, making it difficult to find uniform areas large enough for a representative use of four nets. A multi mesh-size coastal survey net was developed to allow a representative sample of fish to be collected in such variable conditions. Coastal survey nets are consequently widely used in the Gulf of Bothnia and along the Finnish coast of the Gulf of Finland. In all other parts of the Baltic, net sets should be used. The Nordic coastal multi-mesh gillnets is a more recently developed multi mesh-size net, which since 2001 is used in Sweden, Åland and Finland (Appelberg et al. 2003, Söderberg et al. 2004, Söderberg 2006). In Denmark, a modified version of the Nordic coastal multi-mesh gillnets in combination with fyke nets are used in the monitoring of coastal fish populations (see tables below for further description).

Description of the gear

Passive gear

The coastal survey net consists of 3 m (10 feet) deep bottom gill nets. The height in the water is about 2.5 m and the length is 35 m. The lower net-rope (main line) is 10 % longer than the upper net-rope (=38.5 m). The nets are made up of five parts, each 7 m long. These have different mesh sizes and are placed in the following order: 17, 22, 25, 33 and 50 mm (mesh bar). The nets are made of green monofilament nylon of 0.20 mm diameter in the two largest mesh sizes and 0.17 mm in the others. The upper net-rope for coastal survey nets is net-rope and the lower is plastic net-rope (weight = 3.2 kg/100 m).

Table 1: The coastal survey net

* Pilot studies with the use of coastal survey nets (mesh size 6-48 mm) were made in Vistula Lagoon in 2007 (June, August and October) to collect ichthyologic material from very shallow waters.

The set of nets used in July-August consists of bottom set gill nets which are 1.8 m (6 feet) deep and made of spun green nylon. A net consists of a 60 m long stretched net bundle which is attached to a 27 m net-rope (35 cm between floats, buoyancy 6 g/m) and a 33 m lower net-rope (weight 2.2 kg/100 m). A set of nets is composed of four nets with mesh sizes 17, 21.5, 25 and 30 mm. Yarn thickness is no. 110/2 for all mesh sizes, according to the Tex-system (e.g., 110/ 2 means 2 filaments each weighing 110 g per 10 000 m).

Table 2: The set of nets used in July-August

The set of nets used in October consists of bottom set gill nets which are 1.8 m (6 feet) deep and made of spun green nylon. A net consists of a 60 m long stretched net bundle which is attached to a 27 m net-rope (buoyancy 6 g/m) and a 33 m lower net-rope (weight 2.2 kg/100 m). A set of nets is composed of five nets with mesh sizes 21.5, 30, 38, 50 and 60 mm. Yarn thickness is no. 210/3 for mesh size 60 mm, no. 212/2 for 50–38 mm and no. 110/2 for the other sizes, according to the Tex-system (e.g., 110/ 2 means 2 filaments each weighing 110 g per 10 000 m).

Table 3: The set of nets used in October

The Nordic coastal multi-mesh gillnets consists of 1.8 m (6 feet) deep bottom gill nets with a length of 45 m. The lower net-rope (main line) is 10% longer than the upper net-rope (=38.5 m). The nets are made up of nine parts, each 5 m long. These have different mesh sizes and are placed in the following order: 30, 15, 38, 10, 48, 12, 24, 60 and 19 mm (mesh bar). The nets are made of transparent monofilament nylon of 0.15 mm diameter in the seven smallest mesh sizes, 0.17 mm in mesh size 48 mm and 0.20 in mesh size 60 mm. The upper net-rope has a buoyancy of 6 g/m and the lower net-rope weigh 22 g/m.

Table 4: The Nordic coastal multi-mesh gillnets

*Pilot studies with use of Nordic coastal multi-mesh gillnets (mesh size 10-60 mm) were conducted in Szczecin Lagoon in 2006 and 2007 (June, August, September and October). Main target was to compare data obtained with these gears with data obtained with bottom trawl. 

The fyke nets are 55 cm high with a semi-circular opening and a leader or wing that is 5 m long. They are made of 17 mm mesh in the arm and 10 mm in the crib of yarn quality no. 210/12 in twisted nylon.

Table 5: The fyke nets

Sampling strategy and localities

Coastal nets and net series The smallest geographical unit is a station at which either a net set or two coastal survey nets are placed. A group of neighbouring stations with similar conditions (depth, exposure, etc.) and similar influence of environmental disturbance forms a section. An area is a denominated geographical area within which there may be one or more sections. The recommended number of stations and the number of visits per station may vary depending upon the morphometric characters of the area and the abundance of fish. To select stations for trend monitoring a predesign study has to be made. A large number of stations (>20) are visited once to provide a mapping of spatial variability. About 10 stations are then selected for a continued three year evaluation period. Based on these experiences, the number of stations may be further reduced after performing statistical tests of homogeneity. Monitoring of abundance trends, using net sets or survey nets, is generally possible by sampling a minimum of six stations per area.

Exceptions

Estonia uses fixed stations only in Hiiumaa and in the cold water (October) monitoring of the area Küdema. In all other areas random sampling inside the section(s) are conducted. The number of stations in most areas is at least 20. In Matsalu the number of stations has been at least 40. Near the tiny island Vaindloo, which is in the central part of Gulf of Finland 26 km from mainland, only 5 stations were monitored. But the abundance of fish has been very high there.

Nordic coastal multi-mesh gillnets The sampling strategy is based on depth-stratified random sampling using approximately 45 net stations distributed in different depth intervals (Söderberg et al. 2004). The smallest geographical unit is a station at which one Nordic coastal net are placed. A group of stations within the same depth interval (0-3 m, 3-6 m, 6-10 m or 10-20 m), forms a section. An area is a denominated geographical area within which there are a number of sections (depth intervals). The recommended number of stations is at least 45 but it may vary depending upon the morphometric characters of the area and the abundance of fish.

Fyke nets The smallest geographical unit is a station at which two fyke nets joined leader to crib are placed. A group of neighbouring stations with similar conditions (depth, exposure, etc.) and exposed to the same influence of environmental disturbances, forms a section. Within a section the bottom depth at the nets must not differ more than 2 metres between stations. An area is a named geographical area within which there may be one or more sections. The recommended number of stations and the number of visits per station may vary depending upon the morphometric characters of the area and the abundance of fish. To select monitoring stations a predesign study has to be made. A large number of stations are visited once to provide a mapping of spatial variability. About 20 stations are then selected for a continued three year evaluation period according to the routines described above. Based on these experiences, the number of stations may be further reduced after performing statistical tests of homogeneity.

Fishing performance

Passive gear

Fishing techniques

Coastal nets and net series (July-August) are set lightly stretched from an anchored buoy kept at a fixed position during the fishing period. The direction of the net (the set) should be constant when fishing in shallow water. A main rule is that the nets are set parallel to the shore. Before the fishing is started each station must be carefully documented with regard to the type of bottom and position (longitude, latitude). Occasional broken meshes are tolerated. Checks must be made on every occasion when the nets are emptied.

Coastal nets and net series (October) are set lightly stretched from an anchored buoy kept at a fixed position during the fishing period. Before the fishing is started each station must be carefully documented with regard to the type of bottom and position (longitude, latitude). Occasional broken meshes are tolerated. Checks must be made on every occasion when the nets are emptied.

Nordic coastal multi-mesh gillnets are set lightly stretched. The direction of the net should be constant between years when fishing in shallow water. Before the fishing is started each station must be carefully documented with regard to the depth and position (longitude, latitude). Occasional broken meshes are tolerated. Checks must be made on every occasion when the nets are emptied.

Fyke nets are set tightly stretched at right angles to the shore. The fyke nets are placed in pairs with leader to crib. Stones with buoys are attached with short lines to the inner leader and the outer crib. Before the fishing is started each station must be carefully documented with regard to the type of bottom and position (longitude, latitude). Occasional broken meshes are not tolerated in fyke nets. Checks must be made on every occasion when the nets are emptied. Before the fyke nets are used, they must be checked on land to ensure that during stretching all parts should be extended.

Exposure

The nets are set between 14 and 16 hrs. They are collected on the following day between 7 and 10 hrs. Within each area the times for setting and lifting should vary as little as possible between fishing efforts. The time when the nets are set and collected during October could differ from the time in August due to shorter day-length in October.

Exceptions

In Estonia the nets are set between 18 and 21 hrs and collected between 8 and 11 hrs.

Fyke nets are emptied daily between 7 and 10. They are replaced immediately after being emptied.

Fishing period

The nets Fishing is done during the period July 25 – August 15, if possible within a 14-day period. Areas to be compared should be fished with as short time difference as possible.

Exceptions

In Estonia some areas (Kihnu, Vilsandi, Kõiguste) are fished during the first half of July and areas in Gulf of Finland in the second half of August. In Sweden, some test fishing with nets is conducted later than August 15, but not later than August 31. In Denmark the period between 15 July and 15 September is recommended for the monitoring of coastal fish populations. The recommended period for contaminant monitoring in flounder and biological effects monitoring in eelpout is October – November in Denmark.

Fyke nets Fishing is done during the period October 15–November 15, if possible within a 14-day period. Areas to be compared should be fished with as short time difference as possible.

Exceptions

In Estonia fyke nets are used parallel to nets during the summer monitoring to collect data about the eel (Anguilla anguilla). In Denmark the period between 15 July and 15 September is recommended for the monitoring of coastal fish populations. The recommended period for contaminant monitoring in flounder and biological effects monitoring in eelpout is October – November in Denmark.

Frequency

Coastal nets and net series At least six fishing efforts are conducted at each station yearly. All stations within a section are fished on the same day. If all sections cannot be fished on the same day, the fishing is continued in the remaining sections before returning to the first section.

Exceptions

In Estonia except in Hiiumaa and cold water fishing in Küdema (October), fishing is conducted in 5 to 40 random stations.

In Sweden and Åland the fishing effort are reduced from six nights to three nights from year 2006.

Nordic coastal multi-mesh gillnets One fishing efforts are done at each station each year. In Denmark the frequency for monitoring coastal fish populations is once every sixth year per station. For contaminant monitoring in flounder and biological effects monitoring in eelpout, however, the frequency is once per year.

Fyke nets At least six fishing efforts are conducted at each station. All stations within a section are fished on the same day. If all sections cannot be fished on the same day, the fishing is continued in the remaining sections before returning to the first section. In Denmark the frequency for monitoring coastal fish populations is once every sixth year per station. For contaminant monitoring in flounder and biological effects monitoring in eelpout, however, the frequency is once per year.  

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Data collection

In the following tables every measured parameter (data about the station, ambient data and catch data) are marked with a cross (x) if measured.

Passive gear

Table 6: Coastal net, net series and Nordic coastal multi-mesh gillnets (x = measured)

This type of gear is not used in following countries: Poland

Table 7: Fyke nets (x = measured)

This type of gear is not used in following countries: Finland, Åland, Estonia, Latvia, Lithuania and Poland

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References

Appelberg, M., M. Holmqvist, and G. Forsgren, 2003. An alternative strategy for coastal fish monitoring in the Baltic Sea. ICES CM 2003/R:03. 13 pp.

Hartmann, J. 1977. Fischereiliche Veränderungen in kulturbedingt eutrophierenden Seen, Schweiz. Z. Hydrolog. 39(2):243–251.

Larsson, Å. 2006. Hälsotillstånd hos kustfisk – biologiska effekter på subcellulär och cellulär nivå. (Health status in coastal Fish – biological effects on subcellular and cellular levels.) Undersökningstyp i Naturvårsverkets miljöövervakning.
http://www.naturvardsverket.se/upload/02_tillstandet_i_miljon/Miljoovervakning/undersokn_typ/hav/fiskhalso.pdf. In Swedish.

Munkittrick, K.R. 1992. A review and evaluation of study design considerations for site-specifically assessing the health of fish populations. J. Aquat. Ecosyst. Health. 1:283–293.

Neuman, E. 1985. Fisk. In: Recipientkontroll vatten – metodunderlag. (Recipient control in water – methodological background.) Ed.: N.Brink. Naturvårdsverket Rapport 3075. 184 p. In Swedish.

Neuman, E., O. Sandström, and G. Thoresson, 1999. Guidelines for coastal fish monitoring. National Board of Fisheries, Institute of Coastal Reseach. 44 pp.

Neuman, E. and O. Sandström. 1996. Fish monitoring as a tool for assessing the health of Baltic coastal ecosystems. Proceedings Polish– Swedish Symposium on Baltic Coastal Fisheries Resources and Management, Gdynia.

Owens, W. 1991. The hazard assessment of pulp and paper effluents in the aquatic environment: a review. Env. Toxicol. Chem. 8:1511–1540.

Pedersen B. and M.L. Larsen. 2004. Miljøfarlige stoffer i fisk.  NOVANA Teknisk anvisning for marin overvågning, Chapter 6.2, the National Environmental research Institute / Ministry of the Environment, Denmark, 19pp. Link: http://www2.dmu.dk/1_om_dmu/2_tvaerfunk/3_fdc_mar/programgrundlag/TekAnv2004_2009/Del6/TA04_6_2_MFS_fisk.pdf

Strand J. and  I. Dahllöf. 2005. Biologisk effekt monitering - fisk.  NOVANA Teknisk anvisning for marin overvågning, Chapter 6.3, the National Environmental research Institute / Ministry of the Environment, Denmark, 19pp. Link: http://www2.dmu.dk/1_om_dmu/2_tvaerfunk/3_fdc_mar/programgrundlag/TekAnv2004_2009/Del6/TA04_6_2_MFS_fisk.pdf

Strand J.and I. Dahllöf. 2006. Fiskeundersøgelser i kystnære marine områder.  NOVANA Teknisk anvisning for marin overvågning, Chapter 6.1, the National Environmental research Institute / Ministry of the Environment, Denmark, 19pp. Link: http://www.dmu.dk/Overvaagning/Fagdatacentre/Det+Marine+Fagdatacenter/Tekniske+anvisninger+NOVANA+2004-2009/

Sprague, J. B. 1991. Contrasting findings from Scandinavia and North America on toxity of BKME. Introductory comments. Can. Tech. Rept. Fish. Aquat. Sci. 1774(2).

Söderberg, K., G. Forsgren and M. Appelberg, 2004. Samordnat program för övervakning av kustfisk i Bottniska viken och Stockholms skärgård. (Coordinated programme for monitoring of coastal fish in Bothnian Bay and Stockholm Arcipelago.) Finfo, 2004:7. In Swedish with English summary.

Söderberg, K. 2006. Provfiske i Östersjöns kustområden – Djupstratifierat provfiske med Nordiska kustöversiktsnät. (Test fishing in the coastal areas of the Baltic Sea – Depth statified test fishing with Nordic coastal multi-mesh gillnets.)http://www.naturvardsverket.se/upload/02_tillstandet_i_miljon/Miljoovervakning/undersokn_typ/hav/provfisk_osjon.pdf. In Swedish.

Thulin, J., J. Höglund and E. Lindesjöö. 1989. Fisksjukdomar i Kustvatten. (Fish disease in coastal waters.) Naturvårdsverket Informerar. Allmänna Förlaget, Stockholm. 126 p. In Swedish with English summary and figure notes.

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Last updated 4 December 2008