Ancient Fish Bones Reveal Impacts Of Global Warming Beneath The Sea

Through the study of archaeological material, tax accounts, church registers and account books of monasteries, an international group of fisheries ecologists and fisheries/maritime historians have drawn a picture of marine life in the northern European seas (North Sea, Wadden Sea, Baltic Sea, and White Sea) as it looked in the past.

New historical documentation is increasingly becoming available. Its interpretation is providing a broader basis for understanding processes and mechanisms that lead to variations in marine populations and ecosystems. The studies in this special issue are important contributions to the establishment of new baselines for management of marine ecosystems including conservation strategies for overexploited living resources. They were conducted under the auspices (or as part of) the History of Marine Animal Populations, a project of the international collaboration, the Census of Marine Life.

108,000 fish bones from 7000 BC predict the future

Global and regional climate models predict that air and sea temperatures will rise by approximately 3°C during the next 70--100 years. In order to understand some of the processes by which global warming might affect marine fish species near Denmark, researchers have investigated the fish fauna during one of the warmest prehistoric periods (the warm Atlantic period: ca. 7000--3900 BC).

A total of 108,000 fish bones were identified, and amongst them were bones of many species, for example, anchovy and black sea bream, which we usually consider to be typical of waters much farther south and warmer like the Bay of Biscay and the Mediterranean Sea. When temperatures cooled after the warm period ended, most of these species disappeared from the archaeological record, suggesting that local abundances declined. However, many of those same warm-water species have recently reappeared in waters around Denmark as temperatures have risen in the last 10--15 years. The archaeological information can be an indicator of which species may become common as climate change progresses and warms.

There were also thousands of cod bones present together with the warm-water species. That result was surprising because investigators knew that the increase in sea temperatures since the late 1980s to the early 1990s has reduced the survival of young cod in the North Sea. How can these two findings be reconciled? The researchers believe that the difference is due to the much lower fishing pressure in the archaeological period. The message from their work is that sustainable cod populations can be maintained in the North Sea even during the climate change expected in the 21st century, but the fish mortality needs to be lowered.

The climate influence the Gulf of Riga fisheries in during the 17th century

The exploited fish population in the Gulf of Riga was very different in the beginning of the 17th century from what is seen today. Archival evidence reveal that the timing of the fishing season, and species composition and, to an extent, the amounts of fish catches in the Gulf of Riga are strongly linked to climate dynamics.

The time-period 1675-1696 belongs to the coldest period of the Little Ice Age. During that time, fishing in the Gulf of Riga took place at more than 20 localities along the whole coast of the basin and was therefore an important livelihood at these times. Herring, flounder and eelpout (considered to be cold-tolerant species) formed the majority of the catch while the importance of species such as perch, pikeperch and various cyprinids, which prefer warm-water and are nowadays relatively widespread, was less than 1% of the catches.

Herring were in the coastal areas from the end of March to November. The highest catch was in today's terms taken during the warm season - June and July. This should be considered as a direct effect of severe winters, due to which the ice-cover melted relatively late and therefore, massive herring migration to spawning areas near the coast was shifted later in the season.

During the period studied, two sub-periods were identified: 1675-1683 and 1685-1696. Winters were more severe during the second period, and as a result, herring fishing seasons started later due to a shift in the timing of herring spawning in the summer months. As a result, the window where fish were available for fisheries was a shorter period of time. This led to substantially lower herring catches, and this is direct evidence of a climatic impact.

Why did the fishery collapse in the Limfjord in 1830?

The Limfjord in northern Denmark is a shallow sound which has supported commercial fisheries for centuries. In the beginning of the 19th century, the fishery declined by 90%, and fishermen went bankrupt when all the fish disappeared. What happened to all the fish?

By reconstruction of historical data series for herring, eel and whitefish the question has now been answered. We now know that both nature and humans played a significant role in the collapse.

In 1825 a winter storm broke the narrow Agger Tange isthmus, which used to separate the Limfjord from the North Sea. That led to an increase of salinity in the western part of the Limfjord. The eel population declined due to the salt water intrusion, and resulted in a 15-year long crisis for the eel fisheries before it had fully recovered. The whitefish did not survive the salinity obstacle and has never returned to the area.

The commercially most important fishery was for herring. The collapse of the herring fishery was most likely due to unsustainable fishing practices, such as fishing on top of spawning areas, eventually destroying the production rate of new young herring. Today the herring have returned to the area, but not nearly in the same numbers as they were in the early 19th century.

The study of the Limfjord fisheries provides an example of how historical ecology may help 'shift the baseline' by revealing the previous existence of a marine ecosystem very different from that which is known from contemporary ecological research.

These findings are presented in a special issue of Fisheries Research "History of Marine Animal Populations and their Exploitation in Northern Europe," 14 papers starting from ca. 7000 BC to present. The volume is edited by Henn Ojaveer and Brian R. MacKenzie.

Reference: Fisheries Research "History of Marine Animal Populations and their Exploitation in Northern Europe Volume 87, Issues 2-3, Pages 101-262 (November 2007)