Marine

The many considerations around stocking wild salmon

September 25, 2014 by Marine Directorate Communications No Comments | Category Collaborations, Marine Directorate Science, Publications

Although young salmon may look very much like one another, it is well established that their metabolic rates when resting are highly variable among families.

But what does that mean?

A recent publication from an international team, including John Armstrong of Marine Scotland Science (MSS) and Simon McKelvey of Cromarty Fisheries (Robertsen et al., 2014) found that salmon families with high metabolic rates grew and survived relatively well in some streams whereas those with lower metabolic rates performed best at other sites. Although it is not yet clear exactly what features of the local environment favour different levels of metabolism the study, using a large scale field experiment in northern Scotland, provides further insight into the likely importance of local adaptations.

Previous work at MSS conducted by Dave Stewart, Stuart Middlemas and Alan Youngson provides a good example of local adaption. Salmon are renowned for homing to the stream in which they were born. This process allows local evolution to occur, effectively fine-tuning the fish to their home stream. Dave showed that salmon from the upper river Tay catchment are quite different from those lower down the river in that they have evolved earlier “run timings”. Salmon grow in the river for typically 2-4 years before smolting into the marine form and leaving during spring. Dave’s experiment showed that by leaving earlier in the year, upper tributary fish arrive at the sea nearer the same time as those from lower in the river, and hence probably contribute to a large focussed run that reduces losses to predators.

Such potential local adaptations in metabolism, run-timing and many other biological characteristics, have important implications for fisheries management. Adding the wrong types of spawning salmon (technically termed “mixing up the locally evolved gene pools”) can be expected to result in offspring that are maladapted. Hence, they may leave as smolts at an inappropriate time and suffer poor growth and survival. The consequence of this approach can be a reduction in production of returning adults, and decline in the fishery.

The study of Grethe Robertsen and colleagues shows that even among streams within the upper River Conon catchment, there is the potential for significant differences in adaptation of metabolic rate. This result suggests that even stocking salmon from neighbouring streams may damage local gene pools. The growing consensus is that such stocking is truly a method of last resort in conservation of local populations and even then may not be a practical option. Further research from MSS in this area of interest is currently submitted for publication.

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