Biological Sciences

Atlantic Salmon Research

There are a number of rivers in the south coast of England that support populations of Atlantic salmon (Salmo salar). However, in some of these rivers the populations have been in decline in recent years. The reason(s) for these declines in the southern chalk rivers are not always clear. A number of projects are investigating the biology of Atlantic salmon.

In some rivers in the historical range of the Atlantic salmon there have been reports that up to 80 % of the fish returning from their oceanic feeding grounds are females. The reason for this is not clear. As a basis for investigating the sex ratios of salmon at different stages of their life cycle, Cliff Cheung is working on a non-destructive way of determining the gender of immature salmon. The only definitive way of determining the gender of juvenile salmon is to examine their gonads. However, this often involves sacrificing the fish. Cliff is working on a technique to examine the gender of a salmon from a fin clip. The work involves identifying sex-specific molecular markers that can be used on small amounts of tissue taken from living fish.

Lucy Evans is examining possible agrochemical effects on salmon populations. Specifically, she is examining the effects of sediments and sediment-derived pesticides in the early life stages of salmon. Salmon eggs are laid in their freshwater breeding grounds and lie in the river gravel for many months before the alevins hatch. During this time they are vulnerable to sediment accumulating around them and they can suffocate. In areas such as the chalk rivers of southern England this sediment is derived from fields that may have had pesticides sprayed on to them. Lucy is at present surveying the river waters and sediments from a number of spawning grounds of salmon in chalk rivers to see what pesticides, and in what quantities, are present. Future laboratory work will examine the effects of these pesticides on the early life stages of salmon.

Many of these same rivers have quite a few trout farms situated on them. The effluent form these farms may have effects on wild populations of Atlantic salmon, especially at particularly vulnerable stages of their life cycle. Two such stages are the period when the salmon are smolting and preparing to leave their freshwater habitat and migrate to their oceanic feeding grounds, and their return (spawning) migration as adults when ready to breed. Neil Crooks is caging salmon smolts and sexually mature salmon downstream of the trout farm effluents and examining the effects on the salmon. Also, he is tagging adult salmon with acoustic tags on their return migration and examining their movements around the fish farm effluents.

Nicola Lower is exposing a variety of Atlantic salmon life stages, including embryos, smolts and sexually mature males, to a variety of pollutants known to be present in salmon rivers and examining a variety of biologically-relevant end points. The ultimate aim of the research is to enter the data into Atlantic salmon life history models to determine population level effects.