Scale Geochemistry - A Record of the Life History of Salmon
Professor Martin Palmer
School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH
North Atlantic salmon stocks have fallen dramatically over the last 30 years, so that conservation and management of fish stocks is a high priority. Although it is imperative that policy is based on sound science, the basic information is often lacking or based on few observations. For example, the Atlantic salmon migrate from their birth rivers to the open ocean, remain there until reaching sexual maturity, and they then return to their natal river to spawn. Despite that fact that this is a commercially important fish to many nations, and there has been a precipitous decline in stocks, little is known of its oceanic behaviour, migration routes, and summer feeding locations of specific stocks. Direct observation is very expensive, and high marine mortality rates mean that tag experiments are inefficient (~0.1% recovery of tagged fish).
Fish produce two incrementally grown, biomineralised tissues; otoliths and scales. The trace element composition of otoliths (the ear bones of fish) and scales is controlled by ambient water chemistry, hence they contain a record of the trace element composition of the water inhabited by the fish. The chemistry of scales and otoliths thus presents a record of life history. Scales have the advantage over otoliths as they can be removed non-lethally, or with no effect on the commercial value of the carcass when harvested from commercially caught fish. Many nations compile historical libraries of scales, extending back more than 50 years, and may thus provide a record of ocean chemistry and fish migratory behaviour, that can be linked to records of sea surface temperature, phytoplankton abundance and fisheries landings.
Southampton Oceanography Centre is ideally placed to study this problem as we have a wide range of state-of-the-art analytical tools and individuals with wide experience in oceanography, fish biology, geochemistry and river water chemistry.
Starting in September 2004, we will commence two pilot projects to test whether salmon scales can indeed be used to reconstruct their life history. One project will be centred on the River Frome catchment and the other will be based around the Hampshire Avon. In both cases the projects will follow the format.
- River water samples would be collected from ~10 sites within the each drainage area. Samples will be collected during summer low flow and autumn/winter high flow conditions. These samples will be analysed for a wide variety of elements and isotope ratios.
- Salmon scales will then be selected from the CEH historical archive at Winfrith (Frome drainage) and the Environment Agency (Avon drainage) and from fish taken in the Mudeford nets.
- All the scales will photographed and a digital image archived for possible future use.
- Laser analyses will be performed on the scales to determine the chemical composition of the different growth rings.
- The river portion and sea portion of the scales will then be dissected out and used for isotope analyses.
These analyses will allow us to test the following hypotheses.
- Whether the chemistry and isotope composition of the river portion of the scale reflects that of the river water.
- Whether all returning salmon caught within the River Frome and Avon drainages originated from these rivers.
- Whether there are changes in the chemistry of the river portion of the scale that reflect variable run off rates and/or anthropogenic contamination.
- Whether the major/trace element composition of the sea portion of the scale provides a record of where the salmon travel to in the oceans.
- Whether the stable isotope composition of the sea portion of the scales provides a time resolved record of the marine diet of the fish.
- Whether some or all of the above parameters are linked to changes in the population size and population structure of salmon returning to the two rivers.
As well as these two river based projects, we also have scales taken from salmon caught at sea in the Greenland and Faroes fisheries. We will carry out similar analyses of these scales as described above. However, in this case we will be able to directly test whether the chemistries of scales from the two areas are distinct from one another. If this is the case, we may then be able to determine the area of the North Atlantic where River Frome and Avon salmon migrate to when they leave their respective rivers.
These initial projects will take about one year to complete. Should they prove successful, we plan to extend this study to many more areas of the UK and possibly across all of the NW European range of Atlantic salmon.
AGM 2005 Prof Martin Palmer gives an update of preliminary findings of the scale project.
