Ecological hypotheses for a historical reconstruction of upper trophic level biomass in the Baltic Sea and Skagerrak

Abstract

Fish and marine mammal populations in the Baltic Sea and Skagerrak have undergone major fluctuations over the past five centuries. We summarize how these fluctuations may have depended on various forms of predation (e.g., cannibalism, fishing, hunting) and environmental processes. The best-documented long-term fisheries in this region are the herring (Clupea harengus) fisheries near Bohuslän, western Sweden, and in the Øresund. These fisheries have been important since at least the 1200s and appear to be partly climatically driven. However, in the rest of the Baltic, information about fisheries for herring and other fish species is rare until after 1900. During the 20th century, while the Baltic underwent eutrophication, the biomass and landings of three fish species (cod (Gadus morhua), herring, and sprat (Sprattus sprattus)) all increased, whereas the biomass of marine mammals (grey seals (Halichoerus grypus), ringed seals (Phoca hispida), harbour porpoises (Phocoena phocoena)) decreased. The relative roles of exploitation, marine mammal predation, and environmental variability (e.g., eutrophication, major inflows of saline water, climate change) on the long-term dynamics of key fish species is not clear and requires increased collaboration among historians, fisheries and marine mammal ecologists, oceanographers, and climatologists.

Contrasting effects of food quality and quantity on a marine top predator

“…Favourable conditions led to a cod Gadus morhua boom in the late 1970s (Eero et al 2011), resulting in high catches and investments in the cod fishery in the 1980s. However, overfishing and unfavourable reproductive conditions led to a collapse of the cod stock, which allowed their main prey, sprat Sprattus sprattus, to increase dramatically (MacKenzie et al 2002, Möllmann et al 2009). Warmer winters since the end of the 1980s were at the same time beneficial for sprat reproduction (MacKenzie & Köster 2004, Möllmann et al 2008.…”

Diatom and stable isotope records of late-Holocene lake ontogeny at Indrepollen, Lofoten, NW Norway: a response to glacio-isostasy and Neoglacial cooling

“…Diatom-conductivity models have proven to be useful tools in the study of the response of coastal areas to hydrographic and climatic transitions (Holm and Bager, 2001;MacKenzie et al, 2002;Ryves et al, 2004).…”

Consequences of artificial deepwater ventilation in the Bornholm Basin for oxygen conditions, cod reproduction and benthic biomass – a model study

“…Such a high salinity is found in the Bornholm Basin and, east of this basin, only close to the bottom and then frequently associated with toxic hydrogen sulfide, which is produced during anoxia. MacKenzie et al (2002) summarized the volumes of water in the main cod spawning areas of the eastern Baltic and Bornholm basins where salinity and oxygen conditions could allow successful fertilization and hatching. From the mid-1980s, this was almost non-existent in the Gdansk and Gotland basins, and favorable conditions were only found in the Bornholm Basin.…”

“…Re-oxygenation of the deeper basins in the Baltic will undoubtedly have a positive effect on the recruitment and food availability for cod, which would be beneficial for the fisheries in the long run. Today the total annual catches of cod are only on the order of 50 000 tons, whereas the potential might be 400 000 tons, as it was in the 1990s (MacKenzie et al, 2002).…”