stream thought

What is the latest science on trout stocking? 

 The Environment Agency (EA) - see link here) sought views on whether, in England and Wales, we should have mandatory implementation of stocking with non-fertile brown trout or local broodstock within 5 years, or whether we should voluntarily do this over 10 years. What is the latest science on the stocking of brown trout and what issues are thrown up?

 The science is comprehensively summarised in a recent paper by Professor Andy Ferguson, the expert asked by the EA to look into this. The essential points seem to be:

1. Wild brown trout are very genetically diverse, a feature that is probably very important to  the survival of the various species. Wide genetic diversity allows trout to respond to environmental fluctuations. Strategically, wild trout do not have all "their eggs in one basket", so to speak. Preserving genetic diversity is considered to be a good thing from an evolutionary point of view.
   
2. Many stocked trout from trout farms are less genetically diverse than wild brown trout. Most broodstock trout are descended from the trout that were caught from small areas in the 19th century, (Loch Leven for example), and hence the genotypes of these trout are a small subset of the genotypes in the wild. In addition, fishery management policies have probably lead to specific behavioural characteristics being preserved, and hence may have further narrowed down the genetic diversity of this subset.
   
3. There is some uncertainty over the extent to which farmed fish introductions into our rivers and lakes has reduced the genetic diversity of trout as a whole. The empirical research from Denmark, Austria, Spain and the UK seems to indicate a wide level of gene introgression, ranging from very low up to 90%. The results are very specific to particular rivers examined. It is unsafe to assume that past stocking has materially affected genetic diversity, but it is unsafe to assume that it will not do so in the future.
   
4. Farm-reared trout differ both genetically and, as important, phenotypically. That is, farm-reared trout behave, look, breed and feed differently. For example, farm-reared trout are less nervous of predators and are more likely to feed during the day. One study suggests that if wild brown trout are exposed to increased day-time predation, more of them will switch to night-time feeding. Farm-reared trout have a greater propensity to take prey at the surface (and are therefore more easily caught by anglers) and appear to be less successful at breeding (perhaps due to reduced homing to spawning gravels).

Andy Ferguson's advice is clear and is based on the need to preserve genetic diversity: the less we do to alter the genetic make-up of the population the better. The safest way of doing this is through habitat restoration, and the encouragement of native wild trout to do well . If stocking is to be used, then he advocates a form of stocking in which broodstock is taken from native wild populations each generation (to avoid the pitfalls of hatcheries and "domestication" of trout) and reared in a hatchery. An alternative (second best, if I read him right), is the introduction of triploid trout, which are sterile. All farm-reared trout should be physically marked. Anglers should remove farm-reared trout that they catch, but release wild trout.

On the question of using triploid fish, I think we should reserve our position until we know more about their affects on wild trout. The EA appear to have concluded that there are no impacts on wild brown trout when triploid fish are introduced, citing a draft paper by Chatterji et al . The problem is that we don't really understand the phenotypic affects (behaviour etc) of triploid trout on wild brown trout. The research cited by the EA is based on a very small field sample - it would be unsafe to use this as a basis for policy. The commercial pressure to use triploids may not help in reaching the right decision. Triploid fish have higher growth rates in culture compared to farmed diploid trout. The research seems to suggest that triploids overwinter better than introduced diploids. The higher growth rates of triploids may attract fishery managers, particularly as the evidence seems to suggest that triploids do not reduce the overall fishing experience (see here), and in terms of size of fish, enhance it for some.

What kind of fishing do we want anyway?

We clearly need to be cautious in any policies that affect trout genes at the population level.

The conservation debate over trout should encompass a wider remit than that driven by the angling lobby, but realistically the angling industry will be the dominant influence in this debate. There are over 430,000 trout anglers in England and Wales, over 750,000 takeable trout stocked each season. Add the fishery business interests ( who have had difficult times recently), the equipment businesses (who have been consolidating), riparian owners (searching for more income due to a declining agricultural sector), tourism and so on and you have a formidable set of commercial pressures and agendas.

 The small, wild, native brown trout is a jewel in our heritage. As an angler, I am not interested in fat, easy-to-catch trout. Far more rewarding is a shy, difficult to approach wild brown trout, respectfully released. So for me the EA policy should be as follows:

• Greater incentives and money applied to habitat improvements to clubs in the form of grants with a responsibility for each wild habitat angling club to demonstrate what work it is doing to improve and preserve the habitat.
  
• Supplemental stocking should only be with broodstock grown each year from the wild trout population.
• More research to be done on the effects of  triploid trout on wild trout success rates before a decision is made over triploids.

I will follow up with what was decided

Fishing journalists are particularly poor at summarising the answers that science has on the question of trout genetics and stocking policy. Here is a very good summary of the basic scientific questions from Professor Andy Ferguson