Trap net fishery

A Traditional Way of Fishing

In the Archipelago Sea, trapnet fishery represents the traditional way of catching herring. Although also new types of floating trapnets have been tested to some extent, most trapnets are anchored by ropes, weights and stakes on traditional nearshore locations, usually on or close to herring spawning beds. The intensity of trapnet fishery has decreased during last two decades, and the amount of trapnets has diminshed dramatically, as well as total trapnet catch. The catches per unit effort, however, have not been decreasing.

Herring trap net catch
Photo: Johannes Sahlsten

Trap Net Fishing

In the Archipelago Sea, spawning herring are mainly caught by trap nets, which are placed on the spawning beds or in their vicinity in spring. Trap net fishery largely follows the natural rhythm and behavior of spawning: the trap nets are set when herring is expected to start spawning, and the fisheries are continued as long as there is fish enough to make fishing economically profitable. Because the shores are different as to the bottom profile, quality, depth etc., the trap nets are usually made to measure for a specific location and are used in the same place over years. Traditionally, herring trap nets are fixed to the bottom with long wooden stakes, but also floating trap nets have been brought into use in some areas recently. The largest trap nets used in Airisto may hold 15-20 tons of herring at a time, but most nets are smaller.

In the herring trap nets in Airisto, the mesh size is 12-13 mm (knot-to-knot measure). There are no studies on the selectivity of the herring trap nets, but unlike gill-nets or trawls, commercial trap nets are considered as non-selective gear. They catch herring irrespective of size, age, sex and reproductive stage and even small herrings (ca 10 cm in length) are retained inside the trap.

In the early 1980’s, there were about thousand trap nets in the Archipelago Sea. The traditional method of fishing spawning herring has largely been abandoned since then and the trap net catch of herring has decreased as a consequence of this. However, the catch per unit effort (CPUE; tons/trap net) shows no decreasing trend but basically seems to be dependent on the size of the spawning stock in the Bothnian Sea (ICES subdivision 30, see map).

Photo: Johannes Sahlsten

The data set maintained by our herring project has been largely collected in close collaboration with local fishermen, from who we have annually received samples of spawning herring. As a result of the decrease in commercial trap net fishery, in 2019, the Archipelago Research Institute acquired two trap nets, which are now being deployed near known spawning sites in the inner archipelago. The trap nets are smaller than the ones used commercially, but are still able to catch quantitative samples for research purposes. The trap net project was funded by the Southwestern Finland ELY-centre and European Maritime and Fisheries Fund (EMFF), Archipelago Sea Fisheries Action Group (Saaristomeren kalatalouden toimintaryhmä). The aim of this project was to continue to collect spawning herring samples from the Archipelago Sea as well as to transfer knowledge and skills from professional fishermen to students in Livia college.

Related publications and reports

  • Jari Hänninen & Johannes Sahlsten (2020): Rysäkalastuksen tekniikka ja ammatillinen tietotaito silakan tutkimuksen käyttöön-pilottitutkimus 2019-2020 – väliraportti
    [Hankenumero: 93977] (In Finnish).
  • Vahteri, P. & Vuorinen, I. 2001. The crash in Baltic herring (Clupea harengus membras L.) reproduction – a case study in the northern Airisto spawning grounds, Archipelago Sea S-W Finland. Baltic Sea Science Congress 2001, Abstract Volume. Stockholm University, Stockholm, Sweden.
  • Griffin, F.J., Pillai, M. C., Vines, C. A., Kääriä, J., Hibbard-Robbins, T., Yanagimachi, R. & Cherr, G. N. 1998. Effects of salinity on sperm motility, fertilization, and development in the Pacific herring, Clupea pallasi. Biol. Bull. 194: 25-35.
  • Kääriä, J., Rajasilta, M., Kurkilahti, M. & Soikkeli, M. 1997. Spawning bed selection by the Baltic herring (Clupea harengus membras) in the Archipelago Sea of SW Finland. – ICES J. mar. Sci. 54: 917-923.
  • Rajasilta, M., Eklund, J. Hänninen, J., Kurkilahti, M., Kääriä, J. Rannikko, P. & Soikkeli, M. 1993. Spawning of herring (Clupea harengus membras L.) in the Archipelago Sea. – ICES J. mar. Sci. 50: 233-246.
  • Rajasilta, M., Eklund, J., Kääriä, J. & Ranta-aho, K. 1989. The deposition and mortality of the eggs of the Baltic herring, Clupea harengus membras L., on different substrates in the south-west archipelago of Finland. – J. Fish. Biol. 34: 417-427.
  • Rajasilta, M., Eklund, J., Kääriä, J. & Ranta-aho, K. 1989. The deposition and mortality of the eggs of the Baltic herring, Clupea harengus membras L., on different substrates in the south-west archipelago of Finland. – J. Fish. Biol. 34: 417-427.