Paleolimnology and Past Salmon Abundance
Paleolimnology is the study of lake sediments and their use as archives of lake and watershed history. Like tree cores and ice cores, sediment cores can be used to reconstruct records of past climate, lake productivity, pollutants, or other biological, chemical and physical changes that occurred in the lake or watershed.
Raincoast’s interest in lake sediments stems from their use as potential archives of past salmon abundance. As nitrogen – from the decomposition of spawned out carcasses – is washed downstream into a sockeye lake, the nutrients are cycled annually into the sediments on the lake bottom. Ideally, the enriched nitrogen isotopes in these sediments (δN15) can be used as a proxy for salmon abundance. Other sediment components such as diatoms and algae pigments help to build a picture of lake productivity and salmon dynamics over the centuries.
Salmon Nutrients Paleo Workshop
As attractive as all this sounds, reconstructing past salmon abundances is not without its challenges. To address this, Raincoast hosted a workshop of North America’s leading scientists and researchers in the field of fisheries science and paleolimnology. The workshop goal was to discuss the problems, techniques and interpretation challenges associated with salmon reconstructions and make recommendations for future work. The abstract, proceedings and video-taped presentations can be found here.
Raincoast’s paleo work
In 2004, sediment cores from Owikeeno Lake at the head of Rivers Inlet were extracted in the hopes of understanding salmon abundance prior to the start of the commercial fishery and examine the role that extensive fisheries exploitation might have played in the drastic decline of Rivers Inlet sockeye. Because Owikeeno Lake sockeye supported such a large grizzly population, the sockeye collapse in 1999 had lethal consequences for many of regions bear’s who wandered to the village looking for food when theirs failed to materialize. As such, the consequences of fisheries collapses extended far beyond the closure of a fishery. (For more information on Raincoast’s work on salmon and bears click here).
Preliminary findings from Owikeeno Lake sediment cores
Owikeeno Lake is a glacially turbid, oligotrophic nursery lake in Rivers Inlet, BC that supported a total sockeye population of one to three million between 1948 and 1974, and an average catch of one million 1884-1974. A decline in abundance began in the 1970s and collapsed to 3600 spawners by 1999. The population has not recovered above 150,000.
Sediment cores extracted from the less glacial upper basins in 2004 were analyzed for geochemical proxies. Diatom and zooplankton microfossils were surveyed but virtually absent. CN ratios averaging 18:1 indicate a strong terrestrial influence on organic matter. Despite this, both δ15N and δ13C show weak but significant correlations with fluctuating escapement trends over the 20th century and a decline from the 1980s onward. There is no obvious change in δ15N corresponding to the start of the commercial fishery. A positive correlation existed between sockeye escapement and Wannock River discharge and a negative correlation between escapement and air temperature. C:N ratios were negatively correlated with temperature and PDO. A mixing model suggests that prior to the mid 1980s, roughly 22% of the nutrients in Owikeeno Lake were salmon derived. While additional proxies are needed to reduce geochemistry ambiguities, the preliminary findings suggest a decline in lake productivity may have occurred in the last 25 years. Light however, may still be a larger limiting factor on lake productivity. The degree to which these and other climate conditions combine to impact extremely low salmon abundance requires expanded analysis and investigation.
The video-taped presentation on the findings from sediment cores can be found here.
To support this work click here,