Multinational team studies salmon populations in Spain and England

Multinational team studies salmon populations in Spain and England Hatchery restocking programs may be counter-productive

By Quentin Dodd

A team of British and Spanish researchers has warned that climate change and human interference through hatcheries and other programs are damaging the distinct genetic characteristics of Atlantic salmon (Salmo salar) populations of five rivers in the Spanish Principality of Asturias.

The team, whose British members were led by Dr. Jamie Stevens from the University of Exeter’s School of Biosciences, has expressed concern that disruption of the species’ migratory behaviour and strong homing instinct could have long-term consequences. Their report in Global Change Biology, noted that part of their task was to “disentangle” the individual threats to native salmon populations, going beyond climate change and genetic interruption. Perhaps most importantly, they focused on the changes recorded over 20 years to runs of Atlantic salmon in northwest Spain, which is about the southern limit of its natural range. That makes these runs particularly vulnerable since farther south rivers are just too warm for egg survival.

Thames connection

Dr. Stevens, the paper’s co-author, has recently published a study on the salmon-restoration program for England’s historic but polluted River Thames, that tracked the origins of tagged and untagged fish ascending the Thames between 2005 and 2008. The researchers wanted to know whether it was more likely that the salmon were the descendants of hatchery-bred fish stocked in peaty rivers in Scotland or Ireland, or were strays from other rivers.

Using 12 microsatellite loci in their analysis, the authors sampled 10 tagged and 16 untagged salmon taken from the Thames; and compared them to genetic data from 3,730 salmon from 55 rivers in the southern part of the European range of Atlantic salmon.

All but one of the tagged fish originated from rivers north of the Thames. (The exception came from northern France). In contrast, all but one of the untagged fish came from two rivers in the south coast of England that Stevens said run through chalk, and have similar characteristics to the Thames.

From these findings, says the paper, the hatchery-based Thames restoration program can be considered a failure.

It began in 1979 and continued sporadically until 2011, when the last group of hatchery-raised fish were planted out in Thames tributaries. But its demise has been anticipated for years: the numbers of salmon recorded in the Thames rose slowly to a peak of more than 300 in the early 1990s, then gradually dwindled to zero in 2005.

Connecting the dots

Stevens tied the Spanish and English studies together by explaining that wild salmon develop distinct population “structures” because of their ability to return to their natal rivers, and “can quite quickly (in evolutionary terms)” develop genetic profiles that become defined by the characteristics of a particular river system. Such characteristics can include “a whole bunch of things.” Stevens mentioned river chemistry and temperatures, and behavioural factors such as run time to the sea and return time to spawning grounds. Over time those characteristics – which relate to the fish having adapted to the unique conditions found in each particular river – become increasingly developed and passed down through future generations. But that hereditary process can be disrupted by human intervention, which includes so-called enhancement programs.

That is particularly the case with hatchery-based programs that use fish from distant rivers with very different characteristics, Stevens said, which was certainly borne out with the River Thames restoration, that primarily used fish from hatcheries on peat-streams in Ireland and Scotland.

Natural tendencies

Stevens told Hatchery International that we can be grateful for the salmon’s natural tendency to stray to non-natal rivers – usually from comparatively nearby similar rivers with much the same characteristics. It’s that tendency, he suggested, that hold the most hope for recovery of salmon runs in the Thames.

Stevens suggests that his research shows that the few salmon now beginning to show up on the boundaries of the huge London conurbation are more likely to be strays spawned in nearby rivers than descendants of fish from the hatchery-based programs. For the future, increased emphasis is expected to be placed on reducing pollution – both from London and other towns along the river – and also on enhancing fish habitat and spawning areas.

Back to Spain

With regard to the Asturias study in Spain, Stevens and his colleagues don’t want to see the population characteristics of the so-called “vulnerable” Spanish rivers disrupted by human intervention. Those populations have a range of adaptations that can give the fish advantages within their particular rivers.

Between 1988 and 2007, the team analysed 924 tissue samples taken from adult salmon returning to the five rivers in the study area, and found high levels of mixing between the local populations. But, Stevens said, the study identified two distinct periods: one prior to 1992 when many “foreign” hatchery-bred fish were introduced to the rivers; and the other after that. The research shows that a high number of fish stray between the rivers, mating with native fish, but that the rivers’ runs continue to have problems. There is still no major trend toward the populations re-developing distinct genetic differences, which, Stevens said, is a sign of the rivers having healthy populations. It’s the “re-” part of re-developing that is now most important, he suggested.

There is some indication in the rivers of northwest Spain that even as the genetic impact of the hatchery programs declines, other things are causing disruptions, not the least being climate change. Previous studies have already suggested that increased water temperature – which is happening in that part of Spain – is linked to an increase in fish straying between rivers, and to a breakdown of population structures.

Stevens also said that without more study it will not be possible to determine the exact moment, or life stage, at which the individual population structures are eroded.