Stocked fish or native invader?

Anyone with an interest in rivers or lakes and the life within them, be it from a conservation, management, or angling perspective (and of course those three are not mutually exclusive) will be aware of invasive non-native species (INNS), the impacts they may cause in certain situations, and the importance of biosecurity. Ecologists with a particular interest in invaders differentiate between non-native (i.e. those species introduced beyond their original distribution range, a pertinent example being pink salmon) and native invaders (referring to species that add to existing or establish new populations within their native range). The artificial stocking of salmonids into waterbodies is one such example of the deliberate introduction of native invaders, to allegedly enhance commercial and recreational fisheries, as well as for conservation purposes in some instances. The evidence accruing as to the benefits of this approach makes interesting reading.

In 2016, Buoro and colleagues used the global-scale introductions of salmon and trout as a robust biological model (i.e. lots of studies with plenty of data to analyse) to investigate the ecological effects of changing intraspecific (within species; e.g. stocking farmed brown trout in to rivers containing wild brown trout populations) and interspecific (between species; e.g. stocking rainbow trout into lakes with Arctic char) diversity. The enormity of the dataset collated from the literature allowed them to look at various levels of organisation which introduces a lot of complexity. However, the take home message was that, overall, introduction of native invaders resulted in stronger ecological effects than those associated with changes in interspecific diversity caused by non-native species.

Sifting through the results revealed that physiological traits (mainly associated with stress response) were most impacted by native invaders, and probably the basis of the initial response of for example, wild brown trout facing introduced farmed brown trout. Furthermore, the study also revealed significant impacts of native invaders on growth and condition, suggesting that there may be evolutionary consequences further down the line. This is aside from the more direct impacts at the genetic level resulting from interbreeding and the introduction of less ‘fit’ genes if diploid (fertile) native invaders are stocked.

Buoro et al highlighted that actually, there is a significant knowledge gap regarding the wider ecological consequences of introduced native salmonids on recipient communities and ecosystems. A paper published this week by Hasegawa and colleagues has taken up that challenge using the introduction of hatchery chum salmon fry on top of the wild masu salmon fry population, as well as their benthic invertebrate prey, and the algal food of those invertebrates.

They assessed the impacts of a single stocking event, the release of 3 million fry into the Mamachi stream, Hokkaido, northern Japan. As was revealed in the previous meta-analysis (above), the foraging efficiency and growth of wild masu salmon fry was reduced following the introduction of the chum salmon fry (the native invader). There was a marked decrease in abundance of various mayfly and caddis species via predation pressure from the fish. This induced an apparent ‘trophic cascade’ across the various links in the food-chain: Lots of fish induced high predation pressure on the invertebrates, reducing their overall number and therefore reducing the grazing pressure on algae at the bottom of the chain. As a consequence, algal biomass increased although the extra nutrients recycled via stocked fish excretion has also been shown to promote algae.

Together, these findings highlight the need to consider the more subtle ways that introduced native organisms like stocked trout and salmon can potentially change the ecology of a waterbody, as well as the more obvious detriment caused to wild populations of salmonids. The Wild Trout Trust always urges caution and consideration when asked about stocking: our position paper and an ever increasing number of case studies from clubs that have ceased stocking are available on our web pages. Click the green links above.