Fisheries effects on the Baltic ecosystem 

Fish community relationships, trophic interactions?

Fish perform several important roles in the ecosystem: they act as essential consumers of plankton (especially the pelagic species herring and sprat), they serve as a food for marine top predators (for mammals and fish-eating birds) and they may substantially facilitate pelagic-benthic coupling.

The trophic interactions between cod, herring and sprat may periodically exert a strong influence on the state of the fish stocks in the Baltic, depending on the abundance of cod as the main predator on herring and sprat. On the other hand herring and sprat prey on cod eggs. In the Central Baltic cod and sprat spawn in the same deep basins and have partly overlapping spawning seasons. The prey-predator relationships can be quite complicated. A possible cannibalism of the eggs and seasonal and inter-annual variations should be taken into account (Köster and Möllmann 2000, Köster et al., 2003a).

Hydrographic-climatic variability (i.e., low frequency of salty inflows from the North Sea, water temperatures) and high fishing pressure on cod during the past 20 years have led to a shift in the fish communities from cod to clupeids (herring, sprat) by first weakening cod recruitment and subsequently generating favorable recruitment conditions for sprat. This kind of shift in fish communities can represent a profound change in the marine ecosystem, also called as regime shift (e.g., Alheit et al. 2005), and can be accompanied by shift in zooplankton and phytoplankton. On the other hand the regime shifts can be explained at least partially to be caused by eutrophication. Recent studies implicate that also some local species as perch and pike have suffered from recruitment failures along the Swedish Baltic coast (Nilsson et al., 2004; Sandström and Karås, 2002).

The ecosystem effects of fishing activities

The impact of fishing effects mainly concern the commercially important fish stocks, but also the benthic invertebrate and fish communities, marine mammals, seabirds and the abiotic environment. The main ecological effect of fishery is the removal of large quantities of target species. Fishing decreases densities and stock biomasses of target species and alters predator-prey interactions. Fishing removes bigger individuals increasing the survival and growth of the smaller individuals of the population.

Bottom trawls have an impact on marine biota in several ways: by reducing structural biota, altering the relative abundance of species and changing the structure and ecology of the seabed and biota. There are only a few studies available in the Baltic region on bottom trawling impacts on the marine ecosystem. It has been documented that bottom trawling may cause damage to several species of thin-shelled bivalves and starfish, whereas solid shelled bivalves or even more fragile Macoma baltica suffer little or survive well (Rumohr and Krost 1991).

Unwanted by-catch is one of the main environmental effects of fishing. Discarding is a waste of marine resources and should be reduced to the lowest level and ultimately eliminated, if possible. In March 2007 the European Commission issued a "Communication on a policy to reduce unwanted by-catch and eliminate discards in European fisheries". In the Communication it was stated that the discards and by-catch rate of fish in the Baltic fisheries is presently unknown, but some studies indicate that the major Baltic commercial fisheries for cod, herring, and sprat have low discards and by-catch rate compared to other areas e.g. the North Sea.

The discard estimates for the two Baltic cod stocks are available since 1996 when the sampling was commenced. The western and eastern cod discards have fluctuated this period (1996-2007) between 5.0-26.6 and 3.7-23.3 million of mainly juvenile cod individuals, respectively. However, the discard estimates are relatively uncertain (ICES 2008a). The HELCOM Baltic Sea Action Plan urges that all caught species and by-catch which can not be released alive or without injuries should be landed and reported by 2012. Furthermore it urges the adoption of measures to minimize by-catch of undersized fish and non-targeted species by 2012.

By-catch of seabirds and mammals

Several studies have shown that the set net (gillnet) fishery in the Baltic Sea can in certain places cause high bird mortality. The by-catch problem is of special relevance where gillnet fishery is exercised in the areas with high concentrations of resting, moulting or wintering seabirds. The conflicts are usually seasonal. Lost gillnets kill unnecessarily many Baltic cod and flounder and are also dangerous for birds and mammals like harbour porpoises.

Piscivorous birds (divers, grebes, mergansers, auks, cormorants) and benthophagic ducks may get entangled and die in fishing gear. Set nets also cause mortality of long-tailed ducks (Clangula hyemalis), velvet scoters (Melanitta fusca), eiders (Somateria mollissima), and black scoters (Melanitta nigra). There are also reports of guillemot and razorbill (Alca torda) mortality in the driftnet fishery for salmon (HELCOM, 2003).

The size of the Baltic harbour porpoise population is uncertain, estimates being between 200 and 1000 individuals. Being so low any loss of porpoises to fishery in the Baltic marine area may be too high to sustain the population. The highest densities of porpoises can be found in the Skagerrak and Kattegat, in the southern part of the Baltic, in the inner Danish waters, in the Kiel and Mecklenburg Bays and Polish waters. In the Swedish part of the Skagerrak and Kattegat the annual by-catch rate of harbour porpoises was estimated to amount to 25 porpoises in bottom trawls and 89 porpoises in gillnets, trammel nets and pelagic trawls (ASCOBANS, 2008). In the German (Siebert et al. 2006) waters most of the 105 recorded by-caught porpoises of the years 1990 to 2001 were reported from bottom-set gillnet fisheries or stranded with characteristic net marks A total of 45 by-caught animals were reported from Polish waters between 1990 and 1999 (Skóra & Kuklik 2003). In Latvia, two porpoises were found entangled in fishing nets in the Gulf of Riga in October 2003 and in January 2004, respectively (ASCOBANS 2004).

It has been estimated that at least 300 grey seals, 80 ringed seals and 7–8 harbour seals are captured as by-catch annually in the Baltic Sea (ICES 1995). Further estimates are available for the Swedish Baltic Sea coastal fisheries where in total over 400 grey seals and 50 ringed seals were by-caught in 2001 (Lunneryd et al., 2005). The added mortality of seals does not appear to restrain the seal populations from increasing (Helander and Härkönen, 1997).

Fishing activities can affect the seabird community also positively through the discarding of unwanted catch and fish offal. There are estimates that over 50% of the fish offal discarded in the Baltic marine area will be consumed by seabirds (ICES, 2000).

In the Baltic Sea Action Plan it was agreed to increase knowledge on and protection of Baltic marine habitats, communities and species by the establishment an effective monitoring and reporting systems for all by-caught birds and mammals.

References

Alheit J, Möllmann C, Dutz J, Kornilovs G, Loewe P, Mohrholz V, Wasmund N (2005). Synchronous regime shifts in the Central Baltic and the North Sea in the late 1980s. ICES J Mar Sci 62:1205–1215.

ASCOBANS (2004). Information on bycatch of harbour porpoises in Latvian waters 2003/2004. ASCOBANS Document AC11/Doc. 20(S). Availalbe at : http://www.service-board.de/ascobans_neu/files/ac11-20.pdf

ASCOBANS (2008). Annual National Reports 2007: b) Sweden. Document AC15/Doc. 15 (P). Availalbe at : http://www.service-board.de/ascobans_neu/files/ac15-15b.pdf

Helander, B. and Härkönen, T. 1997. Säl och havsörn. Bottniska viken 1996 - årsrapport från den marina miljöövervakningen. pp 21-23. Umeå Marina Forskningscentrum.

HELCOM, 2003. The Baltic Marine Environment 1999–2002. Baltic Sea Environment Proceedings No. 87.

ICES (2000). Report of the Working Group on Ecosystem Effects of Fishing Activities. ICES CM 2000/ACME:02. Ref.: ACFM+E.

ICES 2008. Overview report on Baltic Sea. http://www.ices.dk/committe/acom/comwork/report/2008/2008/8%201-8%202%20Baltic%20ecosystem%20overview.pdf

Köster, F.W., Möllmann, C., Neuenfeldt, S., Vinther, M., St. John, M.A., Tomkiewicz, J., Voss, R., Hinrichsen, H.H., Kraus, G. and Schnack, D. 2003. Fish stock development in the Central Baltic Sea (1976-2000) in relation to variability in the physical environment. ICES Mar. Sci. Symp., 219: 294-306.

Köster, F.W. and Möllmann, C. 2000a. Trophodynamic control by clupeid predators on recruitment success in Baltic cod? ICES J. Mar. Sci., 57: 310-323.

Lunneryd, S.G., Hemingsson, M., Tärnlund, S., Fjälling, A. (2005). A voluntary logbook scheme as a method of monitoring the by-catch of seals in Swedish coastal fisheries. ICES CM 2005/X:04, 5 pp.

Nilsson, J., Andersson, J., Karås, P., Sandström, O. (2004) "Recruitment failure and decreasing catches of perch (Perca fluviatilis L.) and pike (Esox lucius L.) in the coastal waters of southeast Sweden" Boreal Environment Research 9: 295-306

Rumohr H. and Krost P. (1991) Experimental evidence of damage to benthos by bottom trawling, with special reference to Arctica islandica. Helgoländer Meeresuntersuchungen 33:340–345.

Sandström A., and Karås P. 2002. Effects of eutrophication on young-of-the-year freshwater fish communities in coastal areas of the Baltic. Env. Biol. Fish. 63,89–101.

Siebert, U., Gilles, A., Lucke, K., Ludwig, M., Benke, H., Kock, K.H., Scheidat, M. (2006). A decade of harbour porpoise occurence in German Waters – Analysis of aerial surveys, incidental sightings and strandings. Journal of Sea Research 56: 65-80.

Skóra, K.E., Kuklik, I. (2003). Bycatch as a potential threat to harbour porpoises (Phocoena phocoena) in Polish Baltic Waters. NAMMCO Scientific Publication, 5: 303–315. 

Last updated 4 December 2008